💻 C Programming

📖 1. Introduction to C Programming

1.1 What is C?

C is a general-purpose, procedural programming language. It is one of the most important programming languages for learning programming fundamentals and problem solving.

C provides low-level memory access through pointers while also providing high-level programming features such as functions, loops and decision-making statements.

1.2 History of C

C was developed by Dennis Ritchie at Bell Labs in the early 1970s.

1.3 Features of C

• Simple: C has a relatively small set of core language features.
• Procedural: Programs can be organized into functions and procedures.
• Fast: C programs can execute efficiently.
• Portable: C programs can often be moved between different systems with limited changes.
• Structured: Large programs can be divided into smaller functions.
• Memory Access: Pointers provide direct access to memory addresses.
• Extensible: Programs can be organized using reusable functions and libraries.

1.4 Why Should You Learn C?

C is particularly useful for understanding how programs work internally.

1.5 Applications of C

C is still used in many areas of software and hardware development.

• Operating Systems
• Embedded Systems
• Device Drivers
• Compilers
• Networking Software
• Database Systems
• IoT Devices
• System Programming

1.6 C vs Other Programming Languages

1.7 Your First C Program

#include <stdio.h>

int main()
{
    printf("Hello, World!");

    return 0;
}

Output

1.8 Line-by-Line Explanation

1.9 How Does a C Program Execute?

A C program is normally converted into executable machine code before it runs.

1.10 What is a Compiler?

A compiler translates source code written in a programming language into a form that the computer can execute.

1.11 Common Beginner Mistakes

• Forgetting the semicolon ;
• Missing braces { }
• Using incorrect format specifiers
• Forgetting & in appropriate scanf() calls
• Incorrect variable declarations
• Confusing = with ==

1.12 Quick Revision

1.13 Practice Questions

1. Who developed the C programming language?
2. What type of programming language is C?
3. What is the purpose of main()?
4. What is the purpose of printf()?
5. What is a compiler?
6. What is the purpose of #include?
7. What is a pointer?
8. Why is C called a structured language?
9. Give three applications of C.
10. Why is C important for learning Data Structures?

1.14 Beginner Programming Problems

1.15 Key Takeaway

🏗️ 2. Structure of a C Program

2.1 Basic Structure

A C program is generally organized into different parts. Each part has a specific purpose in the program.

#include <stdio.h>

int main()
{
    // Variable declarations

    // Input

    // Processing

    // Output

    return 0;
}

2.2 Main Parts of a C Program

2.3 Header Files

Header files contain declarations and information needed by the program.

#include <stdio.h>

The header file stdio.h provides declarations for standard input and output functions such as printf() and scanf().

2.4 main() Function

The main() function is the entry point of a hosted C program. Program execution begins from main().

int main()
{
    printf("Hello");

    return 0;
}

2.5 Curly Braces { }

Curly braces define a block of code. They indicate where a function, loop or conditional block begins and ends.

int main()
{
    printf("Hello");

    return 0;
}

The opening brace { starts the block and the closing brace } ends the block.

2.6 Semicolon ;

Most C statements end with a semicolon.

int age = 20;

printf("%d", age);

2.7 Comments

Comments are notes written inside the program for programmers. They are ignored by the compiler.

Single-line Comment

// This is a comment

int age = 20;

Multi-line Comment

/*
   This is a
   multi-line comment
*/

int age = 20;

2.8 Variable Declaration

A variable declaration specifies the data type and name of a variable.

int age;

float marks;

char grade;

A variable can also be declared and initialized at the same time.

int age = 20;

float marks = 85.5;

char grade = 'A';

2.9 Input Section

The scanf() function can be used to read formatted input from the user.

int age;

scanf("%d", &age);

2.10 Processing Section

Processing is the part of the program where calculations or logical operations are performed.

sum = a + b;

Here the values of a and b are added and the result is stored in sum.

2.11 Output Section

printf() is commonly used to display formatted output.

printf("%d", sum);

If sum contains 30, the output will be:

2.12 return 0;

return 0;

In the usual hosted C environment, returning 0 from main indicates successful program termination.

2.13 Complete Example

#include <stdio.h>

int main()
{
    int a, b, sum;

    printf("Enter two numbers: ");

    scanf("%d %d", &a, &b);

    sum = a + b;

    printf("Sum = %d", sum);

    return 0;
}

2.14 Sample Output

2.15 Line-by-Line Explanation

2.16 Input → Processing → Output

Most beginner programming problems can be understood using the IPO model.

Example

2.17 Program Execution Flow

2.18 Common Mistakes

• Forgetting ;
• Forgetting closing brace }
• Writing Printf instead of printf
• Writing Scanf instead of scanf
• Forgetting & in appropriate scanf() calls
• Using a variable before declaring it
• Forgetting to include stdio.h when using printf() or scanf()

2.19 Quick Revision

2.20 Practice Questions

1. What is the purpose of #include?
2. What is the role of stdio.h?
3. What is the purpose of main()?
4. Why are curly braces used?
5. Why is a semicolon used in C?
6. What are comments?
7. What is the purpose of scanf()?
8. What is the purpose of printf()?
9. What does return 0 mean when returned from main()?
10. Explain the Input → Processing → Output model with an example.

2.21 Programming Problems

2.22 Key Takeaway

🔤 3. Variables & Constants

3.1 What is a Variable?

A variable is a named object that represents a storage location used by a C program. Its value can generally be changed during program execution.

Every variable has a type, a name and, when initialized, an initial value.

3.2 Why Do We Need Variables?

Variables allow programs to store data and use that data in calculations and decision making.

int age = 20;

int nextYear = age + 1;

printf("%d", nextYear);

3.3 Variable Declaration

Declaration tells the compiler about the variable's type and name.

int age;

float marks;

char grade;

In these declarations, memory/storage requirements are determined by the type and implementation.

3.4 Variable Initialization

Initialization means giving a variable an initial value when it is defined.

int age = 20;

float marks = 85.5;

char grade = 'A';

3.5 Declaration vs Initialization

3.6 Assigning Values

The assignment operator = is used to assign a value to a variable.

int age;

age = 20;

age = 25;

After the second assignment, the value of age is 25.

3.7 Rules for Naming Variables

Variable names are identifiers and must follow the rules of the C language.

• A name may contain letters, digits and underscores.
• A name must not begin with a digit.
• Spaces are not allowed.
• C is case-sensitive.
• Keywords cannot be used as variable names.
• Choose meaningful names whenever possible.

3.8 Valid and Invalid Identifiers

3.9 C is Case-Sensitive

C treats uppercase and lowercase letters as different characters.

int age = 20;

int Age = 30;

int AGE = 40;

These are three different identifiers.

3.10 What is a Constant?

A constant is a value that is intended not to change during the relevant part of a program.

3.11 const Keyword

The const qualifier can be used to make an object read-only through that particular identifier.

const int MAX = 100;

After initialization, you should not assign a new value to MAX through that identifier.

const int MAX = 100;

/* MAX = 200;  invalid modification */

3.12 #define Constants

The preprocessor directive #define can be used to create a macro.

#define PI 3.14159

int main()
{
    printf("%f", PI);

    return 0;
}

The preprocessor replaces occurrences of the macro name according to the macro definition before compilation.

3.13 Variables and Memory

A variable is associated with a storage location in memory. The exact address and size depend on the type and the implementation.

int age = 25;

Later, when we learn pointers, we will see how a program can work with the address of such an object.

3.14 Multiple Variables

Multiple variables of the same type can be declared in one declaration.

int a, b, c;

They can also be initialized together.

int a = 10, b = 20, c = 30;

3.15 Complete Example

#include <stdio.h>

int main()
{
    int age = 20;
    float marks = 85.5;
    char grade = 'A';

    printf("Age = %d\n", age);

    printf("Marks = %.2f\n", marks);

    printf("Grade = %c\n", grade);

    return 0;
}

Output

3.16 Common Mistakes

• Using a variable before defining it when a definition is required.
• Using a keyword as a variable name.
• Starting a variable name with a digit.
• Using spaces in variable names.
• Confusing = with ==.
• Trying to modify an object declared with const.
• Using unclear variable names in larger programs.

3.17 Quick Revision

3.18 Quick MCQs

1. Which symbol is used for assignment?
   
   A) ==
   B) =
   C) !=
   D) >
   
   Answer: B


2. Which is a valid identifier?
   
   A) 2value
   B) student name
   C) student_name
   D) float
   
   Answer: C


3. Which keyword can be used to qualify an object as read-only?
   
   A) fixed
   B) constant
   C) const
   D) readonly
   
   Answer: C


4. Which of the following is case-sensitive in C?
   
   A) Identifiers
   B) Spaces
   C) Comments
   D) Output
   
   Answer: A


5. Which is a valid declaration?
   
   A) int 2age;
   B) int age;
   C) integer age;
   D) number age;
   
   Answer: B

3.19 Programming Problems

3.20 Key Takeaway

📦 4. Data Types

4.1 What is a Data Type?

A data type tells the compiler what kind of value an object can represent and helps determine how that object is stored and interpreted.

Choosing the correct data type is important because different types are designed for different kinds of data.

4.2 Main Categories of C Data Types

4.3 Basic Data Types

4.4 char Data Type

The char type is used to store a character value.

char grade = 'A';

printf("%c", grade);

Character constants are written using single quotes.

'A'

'B'

'7'

'@'

4.5 int Data Type

The int type is commonly used to represent integer values.

int age = 25;

int marks = 90;

printf("%d", age);

Integer values do not contain a fractional part.

4.6 float Data Type

The float type represents single-precision floating-point values.

float temperature = 36.5f;

printf("%.2f", temperature);

The suffix f can be used to indicate a floating constant of type float.

4.7 double Data Type

The double type provides double-precision floating-point values and is commonly preferred when more precision than float is useful.

double pi = 3.141592653589793;

printf("%.15f", pi);

4.8 void Data Type

The void type represents the absence of a value in certain contexts.

Example: Function with no return value

void display()
{
    printf("Hello");
}

A function declared with void return type does not return a value to its caller.

4.9 Type Modifiers

C provides type specifiers and modifiers that can be combined with integer types to change their range and representation.

4.10 signed and unsigned

Integer types can be signed or unsigned. A signed type can represent negative and non-negative values, while an unsigned type represents only non-negative values.

signed int temperature = -10;

unsigned int count = 100;

4.11 short and long

The keywords short and long can be used to request different integer ranges.

short int a;

long int b;

long long int c;

The exact size of integer types is implementation-dependent, so portable programs should not assume a particular byte size unless the implementation guarantees it.

4.12 sizeof() Operator

The sizeof operator gives the size in bytes of a type or object.

#include <stdio.h>

int main()
{
    printf("%zu\n", sizeof(char));

    printf("%zu\n", sizeof(int));

    printf("%zu\n", sizeof(float));

    printf("%zu\n", sizeof(double));

    return 0;
}

The result can vary between systems, especially for some integer types. Use sizeof() when your program needs the actual size on the current implementation.

4.13 Common Format Specifiers

4.14 Character vs Integer

A character constant such as 'A' and an integer constant such as 65 are different expressions, even though a character can be represented by an integer value in appropriate contexts.

char ch = 'A';

int number = 65;

printf("%c\n", ch);

printf("%d\n", number);

The numerical value associated with a character depends on the execution character set used by the implementation.

4.15 float vs double

4.16 Type Conversion

Type conversion happens when a value of one type is converted to another type.

Implicit Conversion

The compiler may automatically convert one type to another according to the rules of the C language.

int a = 10;

double b = a;

printf("%f", b);

Explicit Conversion

A programmer can explicitly request a conversion using a cast.

int a = 5;

int b = 2;

double result = (double)a / b;

printf("%f", result);

4.17 Important: Integer Division

When both operands of division are integers, integer division is performed.

int a = 5;

int b = 2;

printf("%d", a / b);

If a fractional result is required, convert at least one operand to a floating-point type.

printf("%f", (double)a / b);

4.18 Complete Example

#include <stdio.h>

int main()
{
    int age = 20;

    float marks = 85.5f;

    double pi = 3.141592653589793;

    char grade = 'A';

    printf("Age = %d\n", age);

    printf("Marks = %.2f\n", marks);

    printf("Pi = %.15f\n", pi);

    printf("Grade = %c\n", grade);

    printf("Size of int = %zu bytes\n",
           sizeof(int));

    return 0;
}

4.19 Common Mistakes

• Assuming every int is exactly 4 bytes on every C implementation.
• Confusing 'A' with "A".
• Forgetting the f suffix when you specifically want a float literal such as 3.14f.
• Expecting integer division to produce a fractional result.
• Using the wrong format specifier.
• Assuming float and double have the same precision.
• Assuming data type sizes are identical on every compiler and platform.

4.20 Quick Revision

4.21 Quick MCQs

1. Which data type is commonly used to store an integer?
   
   A) float
   B) int
   C) char
   D) void
   
   Answer: B


2. Which operator gives the size of a type or object?
   
   A) size
   B) length
   C) sizeof
   D) bytes
   
   Answer: C


3. Which is a character constant?
   
   A) "A"
   B) 'A'
   C) A
   D) `A`
   
   Answer: B


4. What is the result of 5 / 2 when both operands are int?
   
   A) 2.5
   B) 3
   C) 2
   D) 0
   
   Answer: C


5. Which type generally provides more precision than float?
   
   A) char
   B) int
   C) double
   D) void
   
   Answer: C

4.22 Programming Problems

4.23 Key Takeaway

⌨️ 5. Input & Output

5.1 What is Input and Output?

A program often needs to receive data from the user, process that data and display the result.

5.2 printf() Function

The printf() function is commonly used to display formatted output on the standard output stream.

#include <stdio.h>

int main()
{
    printf("Hello World");

    return 0;
}

5.3 Printing Text

Text written inside double quotation marks is printed as a string.

printf("Welcome to C Programming");

5.4 Printing Variables

Format specifiers are used to display values stored in variables.

int age = 25;

printf("%d", age);

5.5 Common Format Specifiers

5.6 Printing Integers

int a = 100;

printf("%d", a);

5.7 Printing Characters

char grade = 'A';

printf("%c", grade);

5.8 Printing Floating-Point Values

float marks = 85.5f;

printf("%f", marks);

5.9 Controlling Decimal Places

A precision such as %.2f can be used to display two digits after the decimal point.

float marks = 85.5678f;

printf("%.2f", marks);

Similarly:

printf("%.1f", marks);

printf("%.3f", marks);

5.10 Escape Sequences

Escape sequences represent special characters inside strings and character constants.

Example

printf("Hello\nWorld");

5.11 scanf() Function

The scanf() function is commonly used to read formatted input from the standard input stream.

int age;

scanf("%d", &age);

5.12 Why Do We Use & in scanf()?

For most ordinary scalar variables, scanf() needs the address of the object where the input value should be stored.

int age;

scanf("%d", &age);

5.13 Reading Multiple Values

int a, b;

scanf("%d %d", &a, &b);

If the input is:

then:

5.14 Reading Different Data Types

int age;
float marks;
char grade;

scanf("%d", &age);

scanf("%f", &marks);

scanf(" %c", &grade);

5.15 Reading a double

For scanf(), the conversion specifier for a double is %lf.

double price;

scanf("%lf", &price);

5.16 Reading a Character

char ch;

scanf(" %c", &ch);

printf("%c", ch);

The leading space in the format string tells scanf() to skip leading whitespace before reading the character.

5.17 Reading a String

A character array can be used to store a string. When using scanf() with %s, the array name is passed without &.

char name[30];

scanf("%29s", name);

printf("%s", name);

The width limit helps prevent writing more characters than the array can hold.

5.18 printf() vs scanf()

5.19 Complete Example

#include <stdio.h>

int main()
{
    int age;
    float marks;

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your marks: ");
    scanf("%f", &marks);

    printf("\nAge = %d\n", age);

    printf("Marks = %.2f\n", marks);

    return 0;
}

Sample Input

Sample Output

5.20 Multiple Input Example

#include <stdio.h>

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    printf("Sum = %d", a + b);

    return 0;
}

Input

Output

5.21 Common Input/Output Mistakes

• Forgetting & for ordinary scalar variables in scanf().
• Using the wrong format specifier.
• Using %f instead of %lf for a double argument in scanf().
• Using %d for a float or double input.
• Forgetting the newline escape sequence when separate output lines are needed.
• Using %s when the input can contain spaces.
• Forgetting the width limit when reading a string with scanf().
• Forgetting the leading space in a character input format when whitespace needs to be skipped.

5.22 Quick Revision

5.23 Quick MCQs

1. Which function is commonly used to display formatted output?
   
   A) scanf()
   B) printf()
   C) input()
   D) read()
   
   Answer: B


2. Which function is commonly used to read formatted input?
   
   A) printf()
   B) display()
   C) scanf()
   D) output()
   
   Answer: C


3. Which format specifier is commonly used for an int?
   
   A) %f
   B) %c
   C) %d
   D) %s
   
   Answer: C


4. Which specifier is used to read a double using scanf()?
   
   A) %d
   B) %f
   C) %lf
   D) %c
   
   Answer: C


5. Which escape sequence moves the cursor to the next line?
   
   A) \t
   B) \n
   C) \\
   D) \a
   
   Answer: B

5.24 Programming Problems

5.25 Key Takeaway

➕ 6. Operators

6.1 What is an Operator?

An operator is a symbol that tells the compiler to perform an operation on one or more operands.

6.2 Types of Operators in C

6.3 Arithmetic Operators

Arithmetic operators are used to perform mathematical calculations.

6.4 Arithmetic Example

int a = 10;
int b = 3;

printf("%d\n", a + b);
printf("%d\n", a - b);
printf("%d\n", a * b);
printf("%d\n", a / b);
printf("%d\n", a % b);

6.5 Modulus Operator %

The modulus operator gives the remainder of an integer division.

10 % 3

Common Uses

• Checking whether a number is even or odd.
• Finding remainders.
• Working with repeating patterns.
• Extracting digits from integers.

int n = 25;

if (n % 2 == 0)
{
    printf("Even");
}
else
{
    printf("Odd");
}

6.6 Integer Division

When both operands are integers, division produces an integer result.

int a = 5;
int b = 2;

printf("%d", a / b);

If you need a fractional result, use a floating-point operand.

printf("%f", (double)a / b);

6.7 Relational Operators

Relational operators compare two values. The result is an integer value of 0 or 1 in the usual conditional-expression context: 0 means false and nonzero means true.

Example

int a = 10;
int b = 20;

printf("%d\n", a > b);
printf("%d\n", a < b);
printf("%d\n", a == b);

6.8 Assignment Operators

Assignment operators store a value in a modifiable object.

Example

int a = 10;

a += 5;

printf("%d", a);

6.9 Logical Operators

Logical operators are commonly used to combine or negate conditions.

6.10 Logical AND &&

AND is true only when both conditions evaluate to true.

int age = 20;

if (age >= 18 && age <= 60)
{
    printf("Eligible");
}

6.11 Logical OR ||

OR is true when at least one condition evaluates to true.

6.12 Logical NOT !

NOT reverses the logical truth value of its operand.

int x = 0;

if (!x)
{
    printf("Condition is true");
}

6.13 Short-Circuit Evaluation

C evaluates logical AND and OR from left to right and may skip the remaining operands when the result is already known.

AND Example

if (x != 0 && 10 / x > 2)
{
    printf("Valid");
}

If x is 0, the second condition is not evaluated because the left side is already false.

OR Example

if (x == 0 || y > 10)
{
    printf("Condition satisfied");
}

6.14 Increment Operator ++

The increment operator increases a modifiable integer object by one.

int a = 5;

a++;

printf("%d", a);

6.15 Decrement Operator --

The decrement operator decreases a modifiable integer object by one.

int a = 5;

a--;

printf("%d", a);

6.16 Pre-Increment vs Post-Increment

Pre-Increment

int a = 5;

int b = ++a;

Post-Increment

int a = 5;

int b = a++;

6.17 Conditional Operator ?:

The conditional operator is a compact expression for choosing between two values.

int a = 10;
int b = 20;

int max = (a > b) ? a : b;

printf("%d", max);

6.18 Bitwise Operators

Bitwise operators operate on the individual bits of integer operands.

6.19 Bitwise Example

Consider:

int a = 5;
int b = 3;

In binary representation:

printf("%d", a & b);

Bitwise AND compares corresponding bits.

6.20 Bitwise XOR

XOR produces 1 when the corresponding bits are different.

int a = 5;
int b = 3;

printf("%d", a ^ b);

6.21 Shift Operators

Left Shift

int x = 5;

printf("%d", x << 1);

Right Shift

int x = 8;

printf("%d", x >> 1);

The exact behavior of shifts can depend on the signedness and value of the operand, especially for negative values. The simple examples above use positive integers.

6.22 Operator Summary

6.23 Operator Precedence

When an expression contains multiple operators, precedence determines which operators are grouped first.

int result = 10 + 5 * 2;

Multiplication has higher precedence than addition.

Use Parentheses for Clarity

int result = (10 + 5) * 2;

6.24 Associativity

When operators of the same precedence occur together, associativity determines how they are grouped.

For example, subtraction is generally left-associative.

20 - 5 - 3

6.25 Common Mistakes

• Confusing = with ==.
• Forgetting that integer division discards the fractional part.
• Confusing % with percentage.
• Confusing && with bitwise &.
• Confusing || with bitwise |.
• Confusing ++a with a++.
• Writing complicated expressions without parentheses.
• Assuming bitwise operations are the same as logical operations.

6.26 Quick Revision

6.27 Quick MCQs

1. What is the result of 10 % 3?
   
   A) 0
   B) 1
   C) 3
   D) 10
   
   Answer: B


2. Which operator is used for equality comparison?
   
   A) =
   B) ==
   C) !=
   D) >=
   
   Answer: B


3. What is the result of 5 / 2 when both operands are integers?
   
   A) 2.5
   B) 3
   C) 2
   D) 1
   
   Answer: C


4. Which operator means logical AND?
   
   A) &
   B) &&
   C) |
   D) ||
   
   Answer: B


5. What is the value of a after:
   
   int a = 5;
   ++a;
   
   A) 4
   B) 5
   C) 6
   D) 7
   
   Answer: C


6. Which operator is used for bitwise XOR?
   
   A) &
   B) |
   C) ^
   D) ~
   
   Answer: C

6.28 Programming Problems

6.29 Key Takeaway

🧮 7. Expressions

7.1 What is an Expression?

An expression is a combination of constants, variables, operators and function calls that represents a value.

7.2 Simple Expressions

int a = 10;
int b = 20;

int result = a + b;

The expression a + b produces the value 30.

7.3 Types of Expressions

7.4 Arithmetic Expressions

int a = 10;
int b = 3;

int x = a + b;
int y = a - b;
int z = a * b;
int p = a / b;
int q = a % b;

7.5 Relational Expressions

A relational expression compares two values.

int a = 10;
int b = 20;

printf("%d", a < b);

The expression a < b is true, so the result is nonzero; for this simple comparison it is 1.

7.6 Logical Expressions

int age = 25;

int result = age >= 18 && age <= 60;

printf("%d", result);

Both conditions are true, so the logical AND expression evaluates to true.

7.7 Assignment Expressions

int a;

a = 10;

The expression a = 10 assigns the value 10 to a.

7.8 Mixed Expressions

An expression may contain several different operators.

int result = 10 + 5 * 2;

Multiplication has higher precedence than addition.

7.9 Operator Precedence

Operator precedence determines which operators are grouped first when an expression contains multiple operators.

Important Precedence Order

7.10 Precedence Example 1

int result = 10 + 5 * 2;

7.11 Parentheses Change the Result

int result = (10 + 5) * 2;

7.12 Associativity

Associativity determines how operators with the same precedence are grouped.

Left-to-Right Example

int result = 20 - 5 - 3;

Multiplication and Division

int result = 20 / 5 * 2;

7.13 Integer Expression

int a = 5;
int b = 2;

int result = a / b;

Both operands are integers, so integer division is performed.

7.14 Floating-Point Expression

int a = 5;
int b = 2;

float result = (float)a / b;

printf("%.2f", result);

7.15 Type Conversion

Type conversion occurs when a value is converted from one data type to another.

Implicit Conversion

The compiler performs the conversion automatically when appropriate.

int a = 10;
double b = 2.5;

double result = a + b;

Here, a is converted to a compatible floating-point type for the calculation.

Explicit Conversion

The programmer explicitly requests a conversion using a cast.

int a = 5;
int b = 2;

double result = (double)a / b;

7.16 Type Casting

A cast has the form:

Example

int marks = 85;
int total = 100;

float percentage =
    (float)marks / total * 100;

printf("%.2f", percentage);

7.17 Character Expressions

In C, a character constant such as 'A' has an integer value associated with its character code.

char ch = 'A';

printf("%d", ch);

The exact numeric value depends on the execution character set. On systems using ASCII, 'A' is 65.

7.18 Truth Values in C

In conditions, zero represents false and any nonzero value represents true.

int a = 10;

if (a)
{
    printf("True");
}

int a = 0;

if (a)
{
    printf("True");
}
else
{
    printf("False");
}

7.19 Logical Expression Evaluation

int a = 10;
int b = 20;

int result = (a < b) && (b > 15);

7.20 Conditional Expression

int a = 10;
int b = 20;

int max = (a > b) ? a : b;

7.21 Step-by-Step Expression Evaluation

int result = 10 + 20 / 5 * 2 - 3;

Evaluate according to precedence and associativity.

7.22 Tricky Expression Example

int a = 5;
int b = 10;

int result = a + b * 2;

7.23 Increment Expressions

Pre-Increment

int a = 5;

int b = ++a;

Post-Increment

int a = 5;

int b = a++;

7.24 ⚠️ Avoid Dangerous Expressions

Do not write complicated expressions that modify the same scalar object multiple times when the evaluation order is not clearly defined.

For example, avoid expressions such as:

i++ + ++i

7.25 Common Mistakes

• Forgetting operator precedence.
• Assuming all operators are evaluated from left to right.
• Confusing integer division with floating-point division.
• Forgetting to cast before division.
• Confusing = with ==.
• Confusing & with &&.
• Confusing | with ||.
• Writing overly complicated expressions.

7.26 Quick Revision

7.27 Quick MCQs

1. What is the result of:
   
   10 + 5 * 2
   
   A) 30
   B) 20
   C) 25
   D) 15
   
   Answer: B


2. What is the result of:
   
   5 / 2
   
   when both operands are int?
   
   A) 2.5
   B) 3
   C) 2
   D) 1
   
   Answer: C


3. Which operator has higher precedence?
   
   A) +
   B) *
   C) =
   D) ||
   
   Answer: B


4. What is the result of:
   
   (10 + 5) * 2
   
   A) 20
   B) 25
   C) 30
   D) 15
   
   Answer: C


5. What is the value of b?
   
   int a = 5;
   int b = a++;
   
   A) 4
   B) 5
   C) 6
   D) Undefined
   
   Answer: B


6. What is the result of:
   
   (double)5 / 2
   
   A) 2
   B) 2.0
   C) 2.5
   D) 3
   
   Answer: C

7.28 Practice Problems

7.29 Key Takeaway

🔀 8. Decision Making

8.1 What is Decision Making?

Decision making allows a program to choose different actions depending on whether a condition is true or false.

8.2 Decision-Making Statements

8.3 if Statement

The if statement executes a block of code only when its condition evaluates to true.

Example

int age = 20;

if (age >= 18)
{
    printf("Eligible");
}

How it works

8.4 if-else Statement

The if-else statement provides two possible paths.

Example: Pass or Fail

int marks = 35;

if (marks >= 40)
{
    printf("Pass");
}
else
{
    printf("Fail");
}

8.5 Example: Even or Odd

int n;

scanf("%d", &n);

if (n % 2 == 0)
{
    printf("Even");
}
else
{
    printf("Odd");
}

8.6 Example: Positive, Negative or Zero

int n;

scanf("%d", &n);

if (n > 0)
{
    printf("Positive");
}
else if (n < 0)
{
    printf("Negative");
}
else
{
    printf("Zero");
}

8.7 else-if Ladder

An else-if ladder is used when there are multiple possible conditions.

Example: Grade Calculation

int marks;

scanf("%d", &marks);

if (marks >= 90)
{
    printf("Grade A");
}
else if (marks >= 75)
{
    printf("Grade B");
}
else if (marks >= 60)
{
    printf("Grade C");
}
else if (marks >= 40)
{
    printf("Grade D");
}
else
{
    printf("Fail");
}

8.8 Importance of Condition Order

In an else-if ladder, conditions are checked from top to bottom. Once a condition is true, its block executes and the remaining conditions are skipped.

Correct approach

if (marks >= 90)
{
    printf("A");
}
else if (marks >= 75)
{
    printf("B");
}
else if (marks >= 60)
{
    printf("C");
}

8.9 Nested if

A nested if is an if statement placed inside another if or else block.

int age = 25;
int hasID = 1;

if (age >= 18)
{
    if (hasID)
    {
        printf("Entry allowed");
    }
}

8.10 Multiple Conditions

Logical operators can be combined with decision-making statements.

int age = 25;

if (age >= 18 && age <= 60)
{
    printf("Eligible");
}

Using OR

int day = 1;

if (day == 1 || day == 7)
{
    printf("Weekend");
}

Using NOT

int available = 0;

if (!available)
{
    printf("Not available");
}

8.11 Problem: Largest of Two Numbers

int a, b;

scanf("%d %d", &a, &b);

if (a > b)
{
    printf("%d", a);
}
else
{
    printf("%d", b);
}

8.12 Problem: Largest of Three Numbers

int a, b, c;

scanf("%d %d %d", &a, &b, &c);

if (a >= b && a >= c)
{
    printf("%d", a);
}
else if (b >= a && b >= c)
{
    printf("%d", b);
}
else
{
    printf("%d", c);
}

8.13 Problem: Leap Year

A year is a leap year if it is divisible by 400, or if it is divisible by 4 but not by 100.

int year;

scanf("%d", &year);

if (year % 400 == 0 ||
    (year % 4 == 0 && year % 100 != 0))
{
    printf("Leap Year");
}
else
{
    printf("Not a Leap Year");
}

8.14 switch Statement

The switch statement selects one block from multiple cases based on the value of an integer-compatible expression.

8.15 switch Example

int day;

scanf("%d", &day);

switch(day)
{
    case 1:
        printf("Monday");
        break;

    case 2:
        printf("Tuesday");
        break;

    case 3:
        printf("Wednesday");
        break;

    default:
        printf("Invalid day");
}

8.16 Why break is Used

The break statement terminates the switch statement and transfers control to the statement after the switch.

int n = 1;

switch(n)
{
    case 1:
        printf("One");
        break;

    case 2:
        printf("Two");
        break;
}

8.17 default in switch

The default label executes when none of the case values match.

int choice = 5;

switch(choice)
{
    case 1:
        printf("Add");
        break;

    case 2:
        printf("Subtract");
        break;

    default:
        printf("Invalid choice");
}

8.18 Calculator Using switch

int a, b;
char op;

scanf("%d %c %d", &a, &op, &b);

switch(op)
{
    case '+':
        printf("%d", a + b);
        break;

    case '-':
        printf("%d", a - b);
        break;

    case '*':
        printf("%d", a * b);
        break;

    case '/':
        if (b != 0)
        {
            printf("%d", a / b);
        }
        else
        {
            printf("Division by zero is not allowed");
        }
        break;

    default:
        printf("Invalid operator");
}

8.19 if-else vs switch

8.20 Menu-Driven Program

switch is commonly used to implement menu-driven programs.

int choice;

printf("1. Add\n");
printf("2. Subtract\n");
printf("3. Multiply\n");
printf("4. Exit\n");

scanf("%d", &choice);

switch(choice)
{
    case 1:
        printf("Addition selected");
        break;

    case 2:
        printf("Subtraction selected");
        break;

    case 3:
        printf("Multiplication selected");
        break;

    case 4:
        printf("Exit");
        break;

    default:
        printf("Invalid choice");
}

8.21 Nested Decision Example

Nested decisions are useful when one condition depends on another.

int marks;

scanf("%d", &marks);

if (marks >= 40)
{
    if (marks >= 75)
    {
        printf("Distinction");
    }
    else
    {
        printf("Pass");
    }
}
else
{
    printf("Fail");
}

8.22 Common Mistakes

• Using = instead of == in a comparison.
• Forgetting braces when multiple statements belong to a condition.
• Writing incorrect condition order in an else-if ladder.
• Forgetting break in a switch case when fall-through is not intended.
• Forgetting the default case when invalid input should be handled.
• Using switch when range-based conditions are required.
• Forgetting to handle division by zero.
• Writing overly complicated nested if statements.

8.23 Problem-Solving Method

Before writing an if-else program, identify the condition first.

Example

Problem: Check whether a number is positive, negative or zero.

8.24 Boundary Cases

Boundary testing is very important in programming problems.

8.25 Quick Revision

8.26 Quick MCQs

1. Which statement is used to make a decision based on a condition?
   
   A) for
   B) if
   C) printf
   D) scanf
   
   Answer: B


2. Which statement provides two alternatives?
   
   A) if
   B) if-else
   C) switch only
   D) for
   
   Answer: B


3. Which keyword terminates a switch case when fall-through is not intended?
   
   A) stop
   B) exit
   C) break
   D) continue
   
   Answer: C


4. Which keyword handles unmatched switch cases?
   
   A) else
   B) default
   C) otherwise
   D) none
   
   Answer: B


5. What is the output?
   
   int n = 10;
   if (n > 5)
   {
     printf("Yes");
   }
   
   A) No
   B) Yes
   C) 10
   D) Error
   
   Answer: B


6. Which is best suited for checking multiple fixed menu choices?
   
   A) switch
   B) while
   C) do-while
   D) continue
   
   Answer: A

8.27 Practice Problems

8.28 Key Takeaway

🔁 9. Loops

9.1 What is a Loop?

A loop is a control structure that repeatedly executes a block of statements while a specified condition is satisfied.

9.2 Why Do We Need Loops?

Loops reduce repetitive code and make programs shorter, easier to maintain and easier to understand.

printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");
printf("Hello\n");

The same task can be performed using:

for(int i = 1; i <= 5; i++)
{
    printf("Hello\n");
}

9.3 Types of Loops in C

9.4 for Loop

The for loop is commonly used when the number of iterations is known or can be expressed conveniently.

Example

for(int i = 1; i <= 5; i++)
{
    printf("%d\n", i);
}

9.5 How for Loop Works

Example: i = 1

9.6 while Loop

The while loop repeatedly executes a block while its condition remains true.

Example

int i = 1;

while(i <= 5)
{
    printf("%d\n", i);
    i++;
}

9.7 How while Loop Works

9.8 do-while Loop

The do-while loop executes its body first and checks the condition afterward.

Example

int i = 1;

do
{
    printf("%d\n", i);
    i++;
}
while(i <= 5);

9.9 Important Difference: while vs do-while

The key difference is when the condition is checked.

int i = 10;

while(i < 5)
{
    printf("Hello");
}

int i = 10;

do
{
    printf("Hello");
}
while(i < 5);

9.10 Counting with Loops

A common use of loops is counting from one number to another.

for(int i = 1; i <= 10; i++)
{
    printf("%d ", i);
}

9.11 Counting in Reverse

for(int i = 10; i >= 1; i--)
{
    printf("%d ", i);
}

9.12 Print Even Numbers

for(int i = 2; i <= 20; i += 2)
{
    printf("%d ", i);
}

9.13 Print Odd Numbers

for(int i = 1; i <= 20; i += 2)
{
    printf("%d ", i);
}

9.14 Sum of First N Numbers

We can use a loop to calculate the sum from 1 to N.

int n;
int sum = 0;

scanf("%d", &n);

for(int i = 1; i <= n; i++)
{
    sum = sum + i;
}

printf("%d", sum);

9.15 Factorial

The factorial of a non-negative integer n is the product of all positive integers from 1 to n.

int n;
long long fact = 1;

scanf("%d", &n);

for(int i = 1; i <= n; i++)
{
    fact = fact * i;
}

printf("%lld", fact);

9.16 Multiplication Table

int n;

scanf("%d", &n);

for(int i = 1; i <= 10; i++)
{
    printf("%d x %d = %d\n",
           n, i, n * i);
}

9.17 Number of Digits

We can repeatedly divide an integer by 10 to remove its last digit.

int n;
int count = 0;

scanf("%d", &n);

if(n == 0)
{
    count = 1;
}
else
{
    if(n < 0)
        n = -n;

    while(n != 0)
    {
        n = n / 10;
        count++;
    }
}

printf("%d", count);

9.18 Reverse a Number

To reverse a number, repeatedly extract the last digit using the modulus operator.

int n;
int reverse = 0;

scanf("%d", &n);

while(n != 0)
{
    int digit = n % 10;

    reverse = reverse * 10 + digit;

    n = n / 10;
}

printf("%d", reverse);

9.19 Palindrome Number

A number is a palindrome if it reads the same from left to right and right to left.

int n;
int original;
int reverse = 0;

scanf("%d", &n);

original = n;

while(n != 0)
{
    int digit = n % 10;

    reverse = reverse * 10 + digit;

    n = n / 10;
}

if(original == reverse)
{
    printf("Palindrome");
}
else
{
    printf("Not Palindrome");
}

9.20 Prime Number

A prime number is an integer greater than 1 that has exactly two positive divisors: 1 and itself.

int n;
int isPrime = 1;

scanf("%d", &n);

if(n < 2)
{
    isPrime = 0;
}
else
{
    for(int i = 2; i * i <= n; i++)
    {
        if(n % i == 0)
        {
            isPrime = 0;
            break;
        }
    }
}

if(isPrime)
{
    printf("Prime");
}
else
{
    printf("Not Prime");
}

9.21 break Statement

The break statement immediately terminates the nearest enclosing loop.

for(int i = 1; i <= 10; i++)
{
    if(i == 5)
    {
        break;
    }

    printf("%d ", i);
}

9.22 continue Statement

The continue statement skips the remaining statements in the current iteration and proceeds with the next iteration.

for(int i = 1; i <= 5; i++)
{
    if(i == 3)
    {
        continue;
    }

    printf("%d ", i);
}

9.23 break vs continue

9.24 Nested Loops

A loop inside another loop is called a nested loop.

for(int i = 1; i <= 3; i++)
{
    for(int j = 1; j <= 3; j++)
    {
        printf("* ");
    }

    printf("\n");
}

9.25 Pattern Printing

for(int i = 1; i <= 5; i++)
{
    for(int j = 1; j <= i; j++)
    {
        printf("* ");
    }

    printf("\n");
}

9.26 Multiple Variables in for Loop

A for loop can contain more than one initialization or update expression.

for(int i = 1, j = 5;
    i <= 5;
    i++, j--)
{
    printf("%d %d\n", i, j);
}

9.27 Infinite Loop

A loop that never becomes false is called an infinite loop.

while(1)
{
    printf("Hello\n");
}

9.28 Common Loop Mistakes

• Forgetting to initialize the loop variable.
• Forgetting to update the loop variable.
• Using the wrong loop condition.
• Creating an unintended infinite loop.
• Using i < n when i <= n is required, or vice versa.
• Incorrectly placing break or continue.
• Using the wrong variable inside nested loops.
• Forgetting that integer division removes the fractional part.

9.29 Which Loop Should I Use?

9.30 Problem-Solving Method for Loops

Example: Sum from 1 to N

9.31 Quick Revision

9.32 Quick MCQs

1. Which loop is commonly used when the number of iterations is known?
   
   A) if
   B) for
   C) switch
   D) goto
   
   Answer: B


2. Which loop executes its body at least once?
   
   A) for
   B) while
   C) do-while
   D) if
   
   Answer: C


3. Which keyword terminates a loop?
   
   A) continue
   B) break
   C) stop
   D) exitloop
   
   Answer: B


4. Which keyword skips the current iteration?
   
   A) break
   B) skip
   C) continue
   D) next
   
   Answer: C


5. What is the output?
   
   for(int i = 1; i <= 3; i++)
   {
     printf("%d ", i);
   }
   
   A) 0 1 2
   B) 1 2 3
   C) 1 2
   D) 2 3 4
   
   Answer: B


6. How many times does this loop execute?
   
   for(int i = 1; i <= 5; i++)
   {
     printf("*");
   }
   
   A) 4
   B) 5
   C) 6
   D) Infinite
   
   Answer: B

9.33 Practice Problems

9.34 Key Takeaway

📊 10. Arrays

10.1 What is an Array?

An array is a collection of elements of the same data type stored in contiguous memory locations.

10.2 Why Do We Need Arrays?

Arrays allow us to store and process multiple values using a single variable name.

int marks[5];

marks[0] = 80;
marks[1] = 75;
marks[2] = 90;
marks[3] = 65;
marks[4] = 88;

10.3 Array Index

C arrays use zero-based indexing. This means the first element has index 0.

10.4 Array Declaration

Examples

int numbers[10];

float marks[5];

char letters[26];

double prices[20];

10.5 Array Initialization

int numbers[5] = {10, 20, 30, 40, 50};

The compiler assigns the values to indexes starting from 0.

10.6 Initialization Without Specifying Size

int numbers[] = {10, 20, 30, 40, 50};

The compiler determines the size from the number of initializers.

10.7 Partial Initialization

int numbers[5] = {10, 20};

The remaining elements are initialized to zero for this initialization form.

10.8 Accessing Array Elements

int numbers[5] = {10, 20, 30, 40, 50};

printf("%d", numbers[2]);

10.9 Modifying an Array Element

int numbers[5] = {10, 20, 30, 40, 50};

numbers[2] = 100;

printf("%d", numbers[2]);

10.10 Reading Array Elements

A loop is commonly used to read values into an array.

int n;

scanf("%d", &n);

int arr[n];

for(int i = 0; i < n; i++)
{
    scanf("%d", &arr[i]);
}

10.11 Printing Array Elements

for(int i = 0; i < n; i++)
{
    printf("%d ", arr[i]);
}

10.12 Array Traversal

Traversing an array means visiting each element one by one.

int arr[] = {10, 20, 30, 40, 50};

int n = 5;

for(int i = 0; i < n; i++)
{
    printf("%d ", arr[i]);
}

10.13 Sum of Array Elements

int n;
int sum = 0;

scanf("%d", &n);

int arr[n];

for(int i = 0; i < n; i++)
{
    scanf("%d", &arr[i]);

    sum += arr[i];
}

printf("Sum = %d", sum);

10.14 Average of Array Elements

int n;
int sum = 0;

scanf("%d", &n);

int arr[n];

for(int i = 0; i < n; i++)
{
    scanf("%d", &arr[i]);

    sum += arr[i];
}

double average = (double)sum / n;

printf("Average = %.2f", average);

10.15 Maximum Element

int n;

scanf("%d", &n);

int arr[n];

for(int i = 0; i < n; i++)
{
    scanf("%d", &arr[i]);
}

int max = arr[0];

for(int i = 1; i < n; i++)
{
    if(arr[i] > max)
    {
        max = arr[i];
    }
}

printf("Maximum = %d", max);

10.16 Minimum Element

int min = arr[0];

for(int i = 1; i < n; i++)
{
    if(arr[i] < min)
    {
        min = arr[i];
    }
}

printf("Minimum = %d", min);

10.17 Count Even and Odd Elements

int even = 0;
int odd = 0;

for(int i = 0; i < n; i++)
{
    if(arr[i] % 2 == 0)
    {
        even++;
    }
    else
    {
        odd++;
    }
}

printf("Even = %d\n", even);
printf("Odd = %d", odd);

10.18 Searching in an Array

Searching means checking whether a particular value exists in the array.

10.19 Linear Search

Linear search checks each element one by one until the target is found or the array ends.

int key;
int found = 0;

scanf("%d", &key);

for(int i = 0; i < n; i++)
{
    if(arr[i] == key)
    {
        found = 1;
        break;
    }
}

if(found)
{
    printf("Element Found");
}
else
{
    printf("Element Not Found");
}

10.20 Find Position of an Element

int key;
int position = -1;

scanf("%d", &key);

for(int i = 0; i < n; i++)
{
    if(arr[i] == key)
    {
        position = i;
        break;
    }
}

if(position != -1)
{
    printf("Index = %d", position);
}
else
{
    printf("Not Found");
}

10.21 Reverse an Array

for(int i = n - 1; i >= 0; i--)
{
    printf("%d ", arr[i]);
}

10.22 Copy One Array to Another

int arr2[n];

for(int i = 0; i < n; i++)
{
    arr2[i] = arr[i];
}

10.23 Count Positive, Negative and Zero

int positive = 0;
int negative = 0;
int zero = 0;

for(int i = 0; i < n; i++)
{
    if(arr[i] > 0)
    {
        positive++;
    }
    else if(arr[i] < 0)
    {
        negative++;
    }
    else
    {
        zero++;
    }
}

printf("Positive = %d\n", positive);
printf("Negative = %d\n", negative);
printf("Zero = %d", zero);

10.24 Second Largest Element

Finding the second largest element is an important problem-solving exercise.

int largest = arr[0];
int second = arr[0];

for(int i = 1; i < n; i++)
{
    if(arr[i] > largest)
    {
        second = largest;
        largest = arr[i];
    }
    else if(arr[i] > second &&
            arr[i] != largest)
    {
        second = arr[i];
    }
}

printf("Largest = %d\n", largest);
printf("Second Largest = %d", second);

10.25 Find Duplicate Elements

Nested loops can be used to compare every element with the elements after it.

for(int i = 0; i < n; i++)
{
    for(int j = i + 1; j < n; j++)
    {
        if(arr[i] == arr[j])
        {
            printf("%d ", arr[i]);
            break;
        }
    }
}

10.26 Find Unique Elements

An element is unique if it appears only once in the array.

for(int i = 0; i < n; i++)
{
    int count = 0;

    for(int j = 0; j < n; j++)
    {
        if(arr[i] == arr[j])
        {
            count++;
        }
    }

    if(count == 1)
    {
        printf("%d ", arr[i]);
    }
}

10.27 Frequency of an Element

int key;
int count = 0;

scanf("%d", &key);

for(int i = 0; i < n; i++)
{
    if(arr[i] == key)
    {
        count++;
    }
}

printf("Frequency = %d", count);

10.28 Largest and Smallest Together

int max = arr[0];
int min = arr[0];

for(int i = 1; i < n; i++)
{
    if(arr[i] > max)
    {
        max = arr[i];
    }

    if(arr[i] < min)
    {
        min = arr[i];
    }
}

printf("Maximum = %d\n", max);
printf("Minimum = %d", min);

10.29 Swapping Two Array Elements

int temp;

temp = arr[i];

arr[i] = arr[j];

arr[j] = temp;

10.30 Reverse Array In-Place

int start = 0;
int end = n - 1;

while(start < end)
{
    int temp = arr[start];

    arr[start] = arr[end];

    arr[end] = temp;

    start++;
    end--;
}

10.31 Introduction to Sorting

Sorting means arranging elements in a particular order.

10.32 Bubble Sort

Bubble sort repeatedly compares adjacent elements and swaps them if they are in the wrong order.

for(int i = 0; i < n - 1; i++)
{
    for(int j = 0; j < n - 1 - i; j++)
    {
        if(arr[j] > arr[j + 1])
        {
            int temp = arr[j];

            arr[j] = arr[j + 1];

            arr[j + 1] = temp;
        }
    }
}

10.33 Two-Dimensional Array

A two-dimensional array is commonly used to represent rows and columns, such as a matrix.

int matrix[3][3];

10.34 Reading a Matrix

int rows, cols;

scanf("%d %d", &rows, &cols);

int matrix[rows][cols];

for(int i = 0; i < rows; i++)
{
    for(int j = 0; j < cols; j++)
    {
        scanf("%d", &matrix[i][j]);
    }
}

10.35 Printing a Matrix

for(int i = 0; i < rows; i++)
{
    for(int j = 0; j < cols; j++)
    {
        printf("%d ", matrix[i][j]);
    }

    printf("\n");
}

10.36 Matrix Addition

Two matrices can be added when they have the same dimensions.

for(int i = 0; i < rows; i++)
{
    for(int j = 0; j < cols; j++)
    {
        result[i][j] =
            matrix1[i][j] + matrix2[i][j];
    }
}

10.37 Main Diagonal Elements

In a square matrix, main diagonal elements have equal row and column indexes.

for(int i = 0; i < n; i++)
{
    printf("%d ", matrix[i][i]);
}

10.38 Row Sum

for(int i = 0; i < rows; i++)
{
    int sum = 0;

    for(int j = 0; j < cols; j++)
    {
        sum += matrix[i][j];
    }

    printf("Row %d Sum = %d\n",
           i + 1, sum);
}

10.39 Column Sum

for(int j = 0; j < cols; j++)
{
    int sum = 0;

    for(int i = 0; i < rows; i++)
    {
        sum += matrix[i][j];
    }

    printf("Column %d Sum = %d\n",
           j + 1, sum);
}

10.40 Array Problem-Solving Pattern

Example

Find the maximum element.

10.41 Common Array Mistakes

• Forgetting that array indexing starts from 0.
• Accessing an index outside the valid range.
• Using i <= n instead of i < n while traversing.
• Forgetting to initialize variables such as sum, count or maximum.
• Using an incorrect loop limit.
• Confusing array index with array value.
• Forgetting to check special cases such as an empty or too-small input where applicable.

10.42 Quick Revision

10.43 Quick MCQs

1. What is the index of the first element of a C array?
   
   A) 0
   B) 1
   C) -1
   D) 2
   
   Answer: A


2. What is the last index of an array containing n elements?
   
   A) n
   B) n + 1
   C) n - 1
   D) 0
   
   Answer: C


3. Which loop is commonly used to traverse an array?
   
   A) for
   B) switch
   C) if
   D) goto
   
   Answer: A


4. Which algorithm checks elements one by one to find a target?
   
   A) Binary search
   B) Linear search
   C) Merge sort
   D) Selection sort
   
   Answer: B


5. Which array represents rows and columns?
   
   A) 1D array
   B) 2D array
   C) Pointer
   D) Structure
   
   Answer: B

10.44 Practice Problems

10.45 Key Takeaway

🔤 11. Strings

11.1 What is a String?

A string in C is a sequence of characters terminated by a special character called the null character \0.

11.2 Character vs String

11.3 Character Array

A character array can be used to store a string.

char name[20];

This creates an array capable of storing characters.

11.4 String Initialization

char name[] = "Venu";

C automatically adds the null character \0 at the end.

11.5 String Size

The array must have enough space for all characters plus the null character.

char name[5] = "Venu";

11.6 Printing a String

char name[] = "Venu";

printf("%s", name);

The %s format specifier is used to print a string.

11.7 Accessing Individual Characters

char name[] = "Venu";

printf("%c\n", name[0]);
printf("%c\n", name[1]);
printf("%c\n", name[2]);
printf("%c\n", name[3]);

11.8 Traversing a String

char str[] = "HELLO";

for(int i = 0; str[i] != '\0'; i++)
{
    printf("%c ", str[i]);
}

11.9 Reading a Single Word using scanf()

char name[20];

scanf("%s", name);

printf("%s", name);

11.10 Reading a Line using fgets()

Use fgets() when you want to read a line that may contain spaces.

char name[50];

fgets(name, sizeof(name), stdin);

printf("%s", name);

11.11 String Library Functions

C provides several useful string functions through the <string.h> header file.

#include <string.h>

11.12 strlen()

The strlen() function returns the number of characters in a string, excluding the terminating \0.

#include <stdio.h>
#include <string.h>

int main()
{
    char str[] = "HELLO";

    printf("%zu", strlen(str));

    return 0;
}

11.13 strcpy()

The strcpy() function copies the contents of one string into another character array.

char source[] = "Hello";
char destination[20];

strcpy(destination, source);

printf("%s", destination);

11.14 strcat()

The strcat() function appends one string to another.

char first[30] = "Hello ";
char second[] = "World";

strcat(first, second);

printf("%s", first);

11.15 strcmp()

The strcmp() function compares two strings lexicographically.

char a[] = "apple";
char b[] = "apple";

if(strcmp(a, b) == 0)
{
    printf("Equal");
}
else
{
    printf("Not Equal");
}

11.16 Find String Length Without strlen()

char str[100];

scanf("%99s", str);

int length = 0;

while(str[length] != '\0')
{
    length++;
}

printf("Length = %d", length);

11.17 Count Vowels

char str[100];
int vowels = 0;

scanf("%99s", str);

for(int i = 0; str[i] != '\0'; i++)
{
    char ch = str[i];

    if(ch == 'a' || ch == 'e' ||
       ch == 'i' || ch == 'o' ||
       ch == 'u' ||
       ch == 'A' || ch == 'E' ||
       ch == 'I' || ch == 'O' ||
       ch == 'U')
    {
        vowels++;
    }
}

printf("Vowels = %d", vowels);

11.18 Count Consonants

A consonant is an alphabetic character that is not a vowel.

char str[100];
int consonants = 0;

scanf("%99s", str);

for(int i = 0; str[i] != '\0'; i++)
{
    char ch = str[i];

    if((ch >= 'A' && ch <= 'Z') ||
       (ch >= 'a' && ch <= 'z'))
    {
        if(!(ch == 'a' || ch == 'e' ||
             ch == 'i' || ch == 'o' ||
             ch == 'u' ||
             ch == 'A' || ch == 'E' ||
             ch == 'I' || ch == 'O' ||
             ch == 'U'))
        {
            consonants++;
        }
    }
}

printf("Consonants = %d", consonants);

11.19 Count Digits in a String

char str[100];
int digits = 0;

scanf("%99s", str);

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] >= '0' && str[i] <= '9')
    {
        digits++;
    }
}

printf("Digits = %d", digits);

11.20 Count Spaces

char str[100];
int spaces = 0;

fgets(str, sizeof(str), stdin);

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] == ' ')
    {
        spaces++;
    }
}

printf("Spaces = %d", spaces);

11.21 Convert Lowercase to Uppercase

You can convert lowercase English letters using ASCII arithmetic.

char str[100];

scanf("%99s", str);

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] >= 'a' && str[i] <= 'z')
    {
        str[i] = str[i] - 'a' + 'A';
    }
}

printf("%s", str);

11.22 Convert Uppercase to Lowercase

char str[100];

scanf("%99s", str);

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] >= 'A' && str[i] <= 'Z')
    {
        str[i] = str[i] - 'A' + 'a';
    }
}

printf("%s", str);

11.23 Reverse a String

#include <string.h>

char str[100];

scanf("%99s", str);

int length = strlen(str);

for(int i = length - 1; i >= 0; i--)
{
    printf("%c", str[i]);
}

11.24 Palindrome String

A string is a palindrome if it reads the same from both directions.

#include <string.h>

char str[100];

scanf("%99s", str);

int left = 0;
int right = strlen(str) - 1;

int palindrome = 1;

while(left < right)
{
    if(str[left] != str[right])
    {
        palindrome = 0;
        break;
    }

    left++;
    right--;
}

if(palindrome)
{
    printf("Palindrome");
}
else
{
    printf("Not Palindrome");
}

11.25 Compare Two Strings Without strcmp()

char a[100];
char b[100];

scanf("%99s", a);
scanf("%99s", b);

int equal = 1;

int i = 0;

while(a[i] != '\0' || b[i] != '\0')
{
    if(a[i] != b[i])
    {
        equal = 0;
        break;
    }

    i++;
}

if(equal)
{
    printf("Equal");
}
else
{
    printf("Not Equal");
}

11.26 Copy String Without strcpy()

char source[100];
char destination[100];

scanf("%99s", source);

int i = 0;

while(source[i] != '\0')
{
    destination[i] = source[i];
    i++;
}

destination[i] = '\0';

printf("%s", destination);

11.27 Concatenate Without strcat()

char a[100];
char b[50];

scanf("%99s", a);
scanf("%49s", b);

int i = 0;
int j = 0;

while(a[i] != '\0')
{
    i++;
}

while(b[j] != '\0')
{
    a[i] = b[j];

    i++;
    j++;
}

a[i] = '\0';

printf("%s", a);

11.28 Frequency of a Character

char str[100];
char key;

scanf("%99s", str);
scanf(" %c", &key);

int count = 0;

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] == key)
    {
        count++;
    }
}

printf("Frequency = %d", count);

11.29 Remove Spaces from a String

char str[200];

fgets(str, sizeof(str), stdin);

int j = 0;

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] != ' ')
    {
        str[j] = str[i];
        j++;
    }
}

str[j] = '\0';

printf("%s", str);

11.30 Count Words in a Sentence

A simple word-counting approach is to count transitions from whitespace to a non-whitespace character.

char str[200];

fgets(str, sizeof(str), stdin);

int words = 0;
int inWord = 0;

for(int i = 0; str[i] != '\0'; i++)
{
    if(str[i] != ' ' &&
       str[i] != '\n' &&
       str[i] != '\t')
    {
        if(!inWord)
        {
            words++;
            inWord = 1;
        }
    }
    else
    {
        inWord = 0;
    }
}

printf("Words = %d", words);

11.31 Check Whether a String Contains Only Alphabets

char str[100];

scanf("%99s", str);

int valid = 1;

for(int i = 0; str[i] != '\0'; i++)
{
    if(!((str[i] >= 'A' && str[i] <= 'Z') ||
         (str[i] >= 'a' && str[i] <= 'z')))
    {
        valid = 0;
        break;
    }
}

if(valid)
{
    printf("Only Alphabets");
}
else
{
    printf("Contains Non-Alphabet Characters");
}

11.32 First Non-Repeating Character

A nested-loop approach can be used to find the first character that appears only once.

char str[100];

scanf("%99s", str);

int found = 0;

for(int i = 0; str[i] != '\0'; i++)
{
    int count = 0;

    for(int j = 0; str[j] != '\0'; j++)
    {
        if(str[i] == str[j])
        {
            count++;
        }
    }

    if(count == 1)
    {
        printf("%c", str[i]);
        found = 1;
        break;
    }
}

if(!found)
{
    printf("No non-repeating character");
}

11.33 Array of Strings

Multiple strings can be stored using a two-dimensional character array.

char names[3][20] =
{
    "Venu",
    "Ravi",
    "Sita"
};

for(int i = 0; i < 3; i++)
{
    printf("%s\n", names[i]);
}

11.34 Important: scanf() vs fgets()

11.35 Why is \0 Important?

String functions and many string operations need to know where the string ends.

The \0 tells C that the string has ended.

11.36 Common String Mistakes

• Forgetting space for the null character.
• Using == to compare string contents.
• Using scanf("%s") when the input contains spaces.
• Reading more characters than the array can hold.
• Forgetting #include <string.h> when using standard string functions.
• Forgetting to add \0 when manually constructing a string.
• Confusing 'A' with "A".

11.37 Quick Revision

11.38 Quick MCQs

1. Which character terminates a C string?
   
   A) '\n'
   B) '\0'
   C) '0'
   D) '\t'
   
   Answer: B


2. Which format specifier is used to print a string?
   
   A) %c
   B) %d
   C) %s
   D) %f
   
   Answer: C


3. Which function finds the length of a string?
   
   A) strcpy()
   B) strlen()
   C) strcat()
   D) strcmp()
   
   Answer: B


4. Which function compares two strings?
   
   A) strcpy()
   B) strlen()
   C) strcmp()
   D) strcat()
   
   Answer: C


5. Which function concatenates strings?
   
   A) strcat()
   B) strlen()
   C) strcmp()
   D) strchr()
   
   Answer: A


6. Which function can read a line containing spaces?
   
   A) scanf("%s")
   B) fgets()
   C) strlen()
   D) strcpy()
   
   Answer: B

11.39 Practice Problems

11.40 Key Takeaway

🔧 12. Functions

12.1 What is a Function?

A function is a named block of code designed to perform a specific task.

12.2 Why Do We Need Functions?

• Functions make programs easier to understand.
• They reduce code repetition.
• They make debugging easier.
• They improve code organization.
• They allow code reuse.
• Large problems can be divided into smaller problems.

12.3 Basic Function Syntax

return_type function_name(parameters)
{
    // statements

    return value;
}

12.4 Simple Function

#include <stdio.h>

void greet()
{
    printf("Hello!");
}

int main()
{
    greet();

    return 0;
}

12.5 Calling a Function

A function executes when it is called.

greet();

12.6 Function Declaration / Prototype

A function prototype tells the compiler about the function before it is used.

int add(int, int);

Complete example:

#include <stdio.h>

int add(int, int);

int main()
{
    int result = add(10, 20);

    printf("%d", result);

    return 0;
}

int add(int a, int b)
{
    return a + b;
}

12.7 Function Definition

int add(int a, int b)
{
    return a + b;
}

This contains the actual implementation of the function.

12.8 Function Call

int result = add(10, 20);

Here, 10 and 20 are arguments passed to the function.

12.9 Parameters and Arguments

int add(int a, int b)
{
    return a + b;
}

int result = add(10, 20);

12.10 Function with Return Value

int square(int n)
{
    return n * n;
}

int main()
{
    int result = square(5);

    printf("%d", result);

    return 0;
}

12.11 void Function

A void function does not return a value.

void display()
{
    printf("Welcome to C");
}

12.12 Function Without Parameters

void message()
{
    printf("Hello World");
}

int main()
{
    message();

    return 0;
}

12.13 Parameters Without Return Value

void printSum(int a, int b)
{
    printf("%d", a + b);
}

int main()
{
    printSum(10, 20);

    return 0;
}

12.14 No Parameters With Return Value

int getNumber()
{
    return 100;
}

int main()
{
    int x = getNumber();

    printf("%d", x);

    return 0;
}

12.15 Four Common Types of Functions

12.16 Add Two Numbers Using Function

int add(int a, int b)
{
    return a + b;
}

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    printf("%d", add(a, b));

    return 0;
}

12.17 Find Maximum Using Function

int maximum(int a, int b)
{
    if(a > b)
        return a;

    return b;
}

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    printf("Maximum = %d", maximum(a, b));

    return 0;
}

12.18 Check Even or Odd Using Function

int isEven(int n)
{
    return n % 2 == 0;
}

int main()
{
    int n;

    scanf("%d", &n);

    if(isEven(n))
        printf("Even");
    else
        printf("Odd");

    return 0;
}

12.19 Check Positive, Negative or Zero

void checkNumber(int n)
{
    if(n > 0)
        printf("Positive");
    else if(n < 0)
        printf("Negative");
    else
        printf("Zero");
}

int main()
{
    int n;

    scanf("%d", &n);

    checkNumber(n);

    return 0;
}

12.20 Check Prime Number Using Function

int isPrime(int n)
{
    if(n < 2)
        return 0;

    for(int i = 2; i * i <= n; i++)
    {
        if(n % i == 0)
            return 0;
    }

    return 1;
}

int main()
{
    int n;

    scanf("%d", &n);

    if(isPrime(n))
        printf("Prime");
    else
        printf("Not Prime");

    return 0;
}

12.21 Factorial Using Function

long long factorial(int n)
{
    long long fact = 1;

    for(int i = 1; i <= n; i++)
    {
        fact *= i;
    }

    return fact;
}

int main()
{
    int n;

    scanf("%d", &n);

    printf("%lld", factorial(n));

    return 0;
}

12.22 GCD Using Function

The greatest common divisor can be calculated efficiently using the Euclidean algorithm.

int gcd(int a, int b)
{
    while(b != 0)
    {
        int temp = b;

        b = a % b;

        a = temp;
    }

    return a;
}

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    printf("GCD = %d", gcd(a, b));

    return 0;
}

12.23 LCM Using Function

int gcd(int a, int b)
{
    while(b != 0)
    {
        int temp = b;

        b = a % b;

        a = temp;
    }

    return a;
}

long long lcm(int a, int b)
{
    if(a == 0 || b == 0)
        return 0;

    return (long long)a / gcd(a, b) * b;
}

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    printf("LCM = %lld", lcm(a, b));

    return 0;
}

12.24 What is Recursion?

Recursion occurs when a function calls itself.

12.25 Recursive Factorial

long long factorial(int n)
{
    if(n <= 1)
        return 1;

    return n * factorial(n - 1);
}

int main()
{
    int n;

    scanf("%d", &n);

    printf("%lld", factorial(n));

    return 0;
}

12.26 Recursive Fibonacci

int fibonacci(int n)
{
    if(n == 0)
        return 0;

    if(n == 1)
        return 1;

    return fibonacci(n - 1)
         + fibonacci(n - 2);
}

int main()
{
    int n;

    scanf("%d", &n);

    printf("%d", fibonacci(n));

    return 0;
}

12.27 Sum of First N Natural Numbers

int sumN(int n)
{
    if(n == 0)
        return 0;

    return n + sumN(n - 1);
}

int main()
{
    int n;

    scanf("%d", &n);

    printf("%d", sumN(n));

    return 0;
}

12.28 Passing an Array to a Function

An array can be passed to a function along with its size.

int sumArray(int arr[], int n)
{
    int sum = 0;

    for(int i = 0; i < n; i++)
    {
        sum += arr[i];
    }

    return sum;
}

int main()
{
    int arr[] = {10, 20, 30, 40, 50};

    int n = 5;

    printf("%d", sumArray(arr, n));

    return 0;
}

12.29 Maximum Element Using Function

int maximum(int arr[], int n)
{
    int max = arr[0];

    for(int i = 1; i < n; i++)
    {
        if(arr[i] > max)
        {
            max = arr[i];
        }
    }

    return max;
}

int main()
{
    int arr[] = {10, 50, 20, 80, 30};

    int n = 5;

    printf("Maximum = %d",
           maximum(arr, n));

    return 0;
}

12.30 Passing a String to a Function

#include <stdio.h>

void display(char str[])
{
    printf("%s", str);
}

int main()
{
    char name[] = "Venu";

    display(name);

    return 0;
}

12.31 Scope of Variables

Scope determines where a variable can be accessed.

12.32 Local Variable

void test()
{
    int x = 10;

    printf("%d", x);
}

The variable x is local to the function.

12.33 Global Variable

#include <stdio.h>

int x = 100;

void display()
{
    printf("%d", x);
}

int main()
{
    display();

    return 0;
}

12.34 static Variable

A local static variable retains its stored value between function calls.

void counter()
{
    static int count = 0;

    count++;

    printf("%d\n", count);
}

int main()
{
    counter();
    counter();
    counter();

    return 0;
}

12.35 Call by Value

In C, ordinary function arguments are passed by value. The function receives a copy of the argument.

void change(int x)
{
    x = 100;
}

int main()
{
    int a = 10;

    change(a);

    printf("%d", a);

    return 0;
}

12.36 Modifying a Variable Using a Pointer

To modify the caller's variable, we can pass its address using a pointer.

void change(int *x)
{
    *x = 100;
}

int main()
{
    int a = 10;

    change(&a);

    printf("%d", a);

    return 0;
}

12.37 Swap Two Numbers Using Function

void swap(int *a, int *b)
{
    int temp = *a;

    *a = *b;

    *b = temp;
}

int main()
{
    int a, b;

    scanf("%d %d", &a, &b);

    swap(&a, &b);

    printf("%d %d", a, b);

    return 0;
}

12.38 How Function Calling Works

12.39 How to Design a Good Function

1. Give the function one clear responsibility.
2. Use a meaningful function name.
3. Keep the function reasonably small.
4. Pass only the data the function needs.
5. Return a useful result when appropriate.
6. Avoid unnecessary global variables.

12.40 Function-Based Problem Solving

12.41 Common Function Mistakes

• Calling a function before declaring it when no suitable declaration is visible.
• Using the wrong return type.
• Forgetting to return a value from a non-void function.
• Passing the wrong number or type of arguments.
• Forgetting that ordinary C arguments are passed by value.
• Creating recursion without a proper base case.
• Using too many global variables.
• Giving one function too many unrelated responsibilities.

12.42 Quick Revision

12.43 Quick MCQs

1. What is a function?
   
   A) A variable
   B) A reusable block of code
   C) A data type
   D) An operator
   
   Answer: B


2. Which keyword indicates that a function returns no value?
   
   A) null
   B) empty
   C) void
   D) zero
   
   Answer: C


3. What is recursion?
   
   A) A loop
   B) A function calling itself
   C) A variable assignment
   D) An array
   
   Answer: B


4. What should recursive functions normally have to stop recursion?
   
   A) Pointer
   B) Array
   C) Base case
   D) Structure
   
   Answer: C


5. Which keyword is used to send a value back from a function?
   
   A) break
   B) return
   C) continue
   D) goto
   
   Answer: B


6. Ordinary C function arguments are generally passed:
   
   A) By value
   B) By name
   C) By class
   D) By object
   
   Answer: A

12.44 Practice Problems

12.45 Key Takeaway

👉 13. Pointers

13.1 What is a Pointer?

A pointer is a variable that stores the memory address of another variable.

int x = 10;

int *p = &x;

13.2 Why Do We Need Pointers?

• To work directly with memory addresses.
• To modify variables inside functions.
• To efficiently work with arrays and strings.
• To dynamically allocate memory.
• To create data structures such as linked lists and trees.
• To work with structures and system-level programming.

13.3 Understanding Memory

Every variable is stored somewhere in computer memory. That location has an address.

int x = 10;

printf("Value = %d\n", x);
printf("Address = %p\n", (void*)&x);

13.4 Address Operator &

The & operator gives the memory address of a variable.

int x = 25;

printf("%p", (void*)&x);

13.5 Pointer Declaration

int *p;

This declares p as a pointer to an integer.

13.6 Pointer Initialization

int x = 10;

int *p = &x;

13.7 Dereference Operator *

The * operator is used to access the value stored at the address held by a pointer.

int x = 10;

int *p = &x;

printf("%d", *p);

13.8 Complete Pointer Example

#include <stdio.h>

int main()
{
    int x = 50;

    int *p = &x;

    printf("Value of x = %d\n", x);

    printf("Address of x = %p\n", (void*)&x);

    printf("Value stored in p = %p\n", (void*)p);

    printf("Value using *p = %d\n", *p);

    return 0;
}

13.9 Modifying a Variable Using a Pointer

int x = 10;

int *p = &x;

*p = 100;

printf("%d", x);

Changing *p changes the original variable because the pointer points to that variable.

13.10 Pointer Memory Diagram

13.11 Pointer Types

int x = 10;
char c = 'A';
float f = 3.14f;

int *p1 = &x;
char *p2 = &c;
float *p3 = &f;

The pointer type should correspond to the type of object it points to.

13.12 NULL Pointer

A null pointer is a pointer that intentionally does not point to a valid object.

int *p = NULL;

Before dereferencing a pointer, make sure it points to a valid object.

if(p != NULL)
{
    printf("%d", *p);
}

13.13 Pointer and Array

The name of an array can be used in many expressions as a pointer to its first element.

int arr[] = {10, 20, 30};

printf("%d", *arr);

arr refers to the first element in this expression.

13.14 Accessing Array Using Pointer

int arr[] = {10, 20, 30, 40};

int *p = arr;

for(int i = 0; i < 4; i++)
{
    printf("%d ", *(p + i));
}

13.15 Pointer Arithmetic

Pointers can be incremented and decremented. The movement is based on the size of the pointed-to type.

int arr[] = {10, 20, 30};

int *p = arr;

printf("%d\n", *p);

p++;

printf("%d\n", *p);

13.16 Pointer Expressions

*(p + 0)
*(p + 1)
*(p + 2)

13.17 Pointer Difference

Pointers to elements of the same array can be subtracted to determine the number of elements between them.

int arr[] = {10, 20, 30, 40};

int *p = &arr[0];
int *q = &arr[3];

printf("%td", q - p);

13.18 Passing Pointer to a Function

void change(int *p)
{
    *p = 100;
}

int main()
{
    int x = 10;

    change(&x);

    printf("%d", x);

    return 0;
}

13.19 Swap Two Numbers Using Pointers

void swap(int *a, int *b)
{
    int temp = *a;

    *a = *b;

    *b = temp;
}

int main()
{
    int x = 10;
    int y = 20;

    swap(&x, &y);

    printf("%d %d", x, y);

    return 0;
}

13.20 Pointer and String

char str[] = "HELLO";

char *p = str;

while(*p != '\0')
{
    printf("%c ", *p);

    p++;
}

13.21 Pointer to String Literal

const char *p = "Hello";

printf("%s", p);

Using const char * communicates that the characters of the string literal should not be modified through the pointer.

13.22 Pointer to Pointer

A pointer can itself have an address, so another pointer can store its address.

int x = 10;

int *p = &x;

int **q = &p;

printf("%d\n", x);

printf("%d\n", *p);

printf("%d\n", **q);

13.23 sizeof Pointer

int *p;

printf("%zu", sizeof(p));

The size of a pointer depends on the system and implementation. It is not necessarily the same as the size of the object it points to.

13.24 Dynamic Memory Allocation

Dynamic memory allows a program to request memory during runtime.

The main functions are:

• malloc()
• calloc()
• realloc()
• free()

These functions are declared in <stdlib.h>.

13.25 malloc()

malloc() allocates a requested number of bytes and returns a pointer to the allocated memory if successful.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *p = malloc(5 * sizeof(int));

    if(p == NULL)
    {
        return 1;
    }

    for(int i = 0; i < 5; i++)
    {
        p[i] = (i + 1) * 10;
    }

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", p[i]);
    }

    free(p);

    return 0;
}

13.26 calloc()

calloc() allocates space for multiple elements and initializes the allocated bytes to zero.

int *p = calloc(5, sizeof(int));

if(p == NULL)
{
    return 1;
}

for(int i = 0; i < 5; i++)
{
    printf("%d ", p[i]);
}

free(p);

13.27 realloc()

realloc() changes the size of a previously allocated memory block.

int *p = malloc(3 * sizeof(int));

if(p == NULL)
{
    return 1;
}

p[0] = 10;
p[1] = 20;
p[2] = 30;

int *temp = realloc(p, 5 * sizeof(int));

if(temp != NULL)
{
    p = temp;

    p[3] = 40;
    p[4] = 50;
}

free(p);

13.28 free()

free() releases dynamically allocated memory.

int *p = malloc(10 * sizeof(int));

if(p != NULL)
{
    free(p);

    p = NULL;
}

13.29 Dangling Pointer

A dangling pointer is a pointer that refers to an object or memory region that is no longer valid.

int *p = malloc(sizeof(int));

if(p != NULL)
{
    free(p);

    p = NULL;
}

Setting the pointer to NULL after freeing helps avoid accidentally using the old address.

13.30 Wild Pointer

A pointer that has not been initialized may contain an indeterminate value and must not be dereferenced.

int *p;

/* Do not do: *p = 10; */

Initialize pointers before using them.

int *p = NULL;

13.31 Pointer and const

const can be used in different ways with pointers.

const int *p;

The pointed-to integer should not be modified through p.

int *const p = &x;

The pointer itself cannot be redirected to another address after initialization.

13.32 Array of Pointers

int a = 10;
int b = 20;
int c = 30;

int *p[3] = {&a, &b, &c};

for(int i = 0; i < 3; i++)
{
    printf("%d ", *p[i]);
}

13.33 Pointer to an Array

int arr[3] = {10, 20, 30};

int (*p)[3] = &arr;

printf("%d", (*p)[1]);

13.34 Pointer with Structure

struct Student
{
    int age;
};

int main()
{
    struct Student s = {20};

    struct Student *p = &s;

    printf("%d", p->age);

    return 0;
}

The -> operator is commonly used to access structure members through a pointer.

13.35 Function Pointer Introduction

A function pointer stores the address of a function.

int add(int a, int b)
{
    return a + b;
}

int main()
{
    int (*fp)(int, int) = add;

    printf("%d", fp(10, 20));

    return 0;
}

13.36 Comparing Pointers

Pointers can be compared for equality or inequality. Relational comparisons are meaningful when pointers refer to elements of the same array.

int arr[3] = {10, 20, 30};

int *p = &arr[0];
int *q = &arr[0];

if(p == q)
{
    printf("Same address");
}

13.37 Pointer with 2D Array

int arr[2][3] =
{
    {10, 20, 30},
    {40, 50, 60}
};

for(int i = 0; i < 2; i++)
{
    for(int j = 0; j < 3; j++)
    {
        printf("%d ", arr[i][j]);
    }

    printf("\n");
}

13.38 Pointer Formula

13.39 Common Pointer Mistakes

• Dereferencing an uninitialized pointer.
• Dereferencing a NULL pointer.
• Accessing memory after it has been freed.
• Forgetting to free dynamically allocated memory.
• Going outside the bounds of an array using pointers.
• Using the wrong pointer type.
• Confusing & with *.
• Confusing p with *p.

13.40 Quick Revision

13.41 Quick MCQs

1. What does a pointer store?
   
   A) Only integers
   B) A memory address
   C) A function name only
   D) A keyword
   
   Answer: B


2. Which operator obtains the address of a variable?
   
   A) *
   B) &
   C) %
   D) #
   
   Answer: B


3. Which operator dereferences a pointer?
   
   A) *
   B) &
   C) #
   D) %
   
   Answer: A


4. What does NULL represent for a pointer?
   
   A) It points to integer zero as an object
   B) It intentionally points to no valid object
   C) It always points to address 1
   D) It is a string
   
   Answer: B


5. Which function allocates dynamic memory?
   
   A) printf()
   B) malloc()
   C) scanf()
   D) strlen()
   
   Answer: B


6. Which function releases dynamically allocated memory?
   
   A) delete()
   B) remove()
   C) free()
   D) clear()
   
   Answer: C

13.42 Practice Problems

13.43 Key Takeaway

🏗️ 14. Structures & Unions

14.1 What is a Structure?

A structure is a user-defined data type that allows us to group different types of data under one name.

For example, a student may have:

• Name → character array
• Age → integer
• Marks → float
• Roll number → integer

Instead of storing these as unrelated variables, we can group them using a structure.

14.2 Structure Syntax

struct Student
{
    int roll;
    char name[50];
    float marks;
};

Here Student is the structure tag.

14.3 Creating a Structure Variable

struct Student s1;

Now s1 is a variable of type struct Student.

14.4 Accessing Structure Members

The dot . operator is used to access members of a structure variable.

#include <stdio.h>

struct Student
{
    int roll;
    char name[50];
    float marks;
};

int main()
{
    struct Student s1;

    s1.roll = 101;

    s1.marks = 85.5;

    printf("Roll = %d\n", s1.roll);

    printf("Marks = %.2f\n", s1.marks);

    return 0;
}

14.5 Assigning a String to a Structure Member

A character array cannot normally be assigned using the = operator after declaration. Use strcpy() instead.

#include <stdio.h>
#include <string.h>

struct Student
{
    int roll;
    char name[50];
};

int main()
{
    struct Student s1;

    s1.roll = 101;

    strcpy(s1.name, "Venu");

    printf("%d\n", s1.roll);

    printf("%s\n", s1.name);

    return 0;
}

14.6 Structure Initialization

struct Student
{
    int roll;
    char name[50];
    float marks;
};

struct Student s1 =
{
    101,
    "Venu",
    85.5
};

14.7 Designated Initialization

C also allows members to be initialized by name.

struct Student s1 =
{
    .roll = 101,
    .name = "Venu",
    .marks = 85.5
};

14.8 Taking Input into a Structure

#include <stdio.h>

struct Student
{
    int roll;
    char name[50];
    float marks;
};

int main()
{
    struct Student s;

    scanf("%d", &s.roll);

    scanf("%49s", s.name);

    scanf("%f", &s.marks);

    printf("Roll = %d\n", s.roll);

    printf("Name = %s\n", s.name);

    printf("Marks = %.2f\n", s.marks);

    return 0;
}

14.9 Array of Structures

We can create an array containing multiple structure variables.

struct Student
{
    int roll;
    char name[50];
    float marks;
};

struct Student students[3];

14.10 Example: Multiple Students

#include <stdio.h>

struct Student
{
    int roll;
    char name[50];
    float marks;
};

int main()
{
    struct Student s[3];

    for(int i = 0; i < 3; i++)
    {
        scanf("%d", &s[i].roll);

        scanf("%49s", s[i].name);

        scanf("%f", &s[i].marks);
    }

    for(int i = 0; i < 3; i++)
    {
        printf("%d %s %.2f\n",
               s[i].roll,
               s[i].name,
               s[i].marks);
    }

    return 0;
}

14.11 Passing Structure to a Function

#include <stdio.h>

struct Student
{
    int roll;
    float marks;
};

void display(struct Student s)
{
    printf("Roll = %d\n", s.roll);

    printf("Marks = %.2f\n", s.marks);
}

int main()
{
    struct Student s = {101, 90.5};

    display(s);

    return 0;
}

14.12 Returning a Structure from a Function

#include <stdio.h>

struct Point
{
    int x;
    int y;
};

struct Point createPoint()
{
    struct Point p = {10, 20};

    return p;
}

int main()
{
    struct Point p = createPoint();

    printf("%d %d", p.x, p.y);

    return 0;
}

14.13 Pointer to Structure

struct Student
{
    int roll;
    float marks;
};

int main()
{
    struct Student s = {101, 85.5};

    struct Student *p = &s;

    printf("%d\n", p->roll);

    printf("%.2f\n", p->marks);

    return 0;
}

14.14 Arrow Operator ->

When we have a pointer to a structure, we can use -> to access its members.

struct Student *p = &s;

p->roll;

The following two expressions are equivalent:

p->roll

(*p).roll

14.15 Nested Structure

A structure can contain another structure as a member.

struct Date
{
    int day;
    int month;
    int year;
};

struct Student
{
    int roll;
    char name[50];

    struct Date dob;
};

14.16 Accessing Nested Structure

struct Student s =
{
    101,
    "Venu",
    {5, 8, 2000}
};

printf("%d", s.dob.year);

14.17 Structure Containing an Array

struct Student
{
    int roll;
    char name[50];
    int marks[5];
};

A structure member can itself be an array.

14.18 Real-World Example: Employee

struct Employee
{
    int id;
    char name[50];
    float salary;
};

int main()
{
    struct Employee e =
    {
        101,
        "Ravi",
        45000
    };

    printf("ID = %d\n", e.id);

    printf("Name = %s\n", e.name);

    printf("Salary = %.2f\n", e.salary);

    return 0;
}

14.19 Structure vs Array

14.20 What is a Union?

A union is a user-defined data type similar to a structure, but all members share the same memory location.

union Data
{
    int i;
    float f;
    char c;
};

14.21 Union Example

#include <stdio.h>

union Data
{
    int i;
    float f;
    char c;
};

int main()
{
    union Data d;

    d.i = 100;

    printf("%d\n", d.i);

    d.f = 3.14f;

    printf("%.2f\n", d.f);

    return 0;
}

A union should generally be read through the member that was most recently written, subject to the rules of the C standard.

14.22 Structure Memory

In a structure, each member has its own storage.

struct Data
{
    int a;
    float b;
    char c;
};

14.23 Union Memory

In a union, all members share the same memory location. Only one member's value is normally stored at a time.

14.24 Structure vs Union

14.25 typedef with Structure

typedef can create a convenient alias for a type.

typedef struct
{
    int roll;
    char name[50];
} Student;

Now we can write:

Student s1;

instead of:

struct Student s1;

14.26 typedef Example

#include <stdio.h>

typedef struct
{
    int id;
    float salary;
} Employee;

int main()
{
    Employee e = {101, 45000};

    printf("%d\n", e.id);

    printf("%.2f\n", e.salary);

    return 0;
}

14.27 Self-Referential Structure

A structure can contain a pointer to another object of the same structure type.

struct Node
{
    int data;

    struct Node *next;
};

14.28 Structure Padding

The compiler may insert unused bytes between structure members to satisfy alignment requirements.

struct Example
{
    char c;
    int x;
};

Therefore, you should not assume that the structure size is always exactly the sum of the individual member sizes.

printf("%zu", sizeof(struct Example));

14.29 Copying Structures

Structures of the same type can be assigned directly.

struct Student s1 = {101, "Venu", 90};

struct Student s2;

s2 = s1;

The members are copied as part of the structure assignment.

14.30 Comparing Structures

C does not provide a general == operator for comparing two structures.

Compare the required members individually.

if(s1.roll == s2.roll &&
   s1.marks == s2.marks)
{
    printf("Equal");
}

14.31 Common Structure Mistakes

• Forgetting the semicolon after the structure definition.
• Using . incorrectly with a structure pointer.
• Using -> with an ordinary structure variable.
• Trying to assign a character array using = after declaration.
• Assuming structure size is always the sum of member sizes.
• Reading a union member different from the one most recently written without understanding the relevant C rules.

14.32 Quick Revision

14.33 Quick MCQs

1. Which keyword is used to define a structure?
   
   A) record
   B) struct
   C) structure
   D) object
   
   Answer: B


2. Which operator accesses a structure member?
   
   A) .
   B) ->
   C) *
   D) &
   
   Answer: A


3. Which operator is used with a pointer to a structure?
   
   A) .
   B) ->
   C) ::
   D) %
   
   Answer: B


4. What is shared by members of a union?
   
   A) Different arrays
   B) Same memory location
   C) Different functions
   D) Different files
   
   Answer: B


5. Which keyword creates a type alias?
   
   A) alias
   B) typedef
   C) rename
   D) type
   
   Answer: B


6. Which structure is important for linked lists?
   
   A) Nested structure only
   B) Self-referential structure
   C) Empty structure
   D) Union
   
   Answer: B

14.34 Practice Problems

14.35 Key Takeaway

⚙️ 15. Preprocessor & Header Files

15.1 What is the C Preprocessor?

The C preprocessor is a program that processes source code before the actual compilation begins.

Preprocessor directives begin with the # symbol.

#include <stdio.h>
#define PI 3.14159

15.2 Common Preprocessor Directives

15.3 #include

The #include directive tells the preprocessor to include the contents of another file.

#include <stdio.h>

This allows us to use functions such as printf() and scanf().

15.4 System Header Files

Standard C libraries provide commonly used functions through header files.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

15.5 <> vs " " in #include

There are two common forms of the include directive.

#include <stdio.h>

#include "myheader.h"

15.6 #define

The #define directive creates a macro.

#define PI 3.14159

Wherever the macro name appears later in the source, the preprocessor substitutes its replacement text.

#include <stdio.h>

#define PI 3.14159

int main()
{
    printf("%f", PI);

    return 0;
}

15.7 Object-like Macro

#define MAX 100

#define MIN 0

#define COLLEGE "ABC College"

These are called object-like macros because they do not take arguments.

15.8 Macro with Expression

#define PI 3.14159

#define LIMIT 50

#define SIZE 10

15.9 Function-like Macro

A macro can accept arguments.

#define SQUARE(x) ((x) * (x))

printf("%d", SQUARE(5));

15.10 Why Parentheses Matter in Macros

Always use parentheses carefully when writing expression macros.

Prefer:

#define SQUARE(x) ((x) * (x))

instead of:

#define SQUARE(x) x * x

The parenthesized version avoids many operator precedence problems when the macro is used inside larger expressions.

15.11 Macro with Multiple Arguments

#define MAX(a, b) ((a) > (b) ? (a) : (b))

printf("%d", MAX(10, 20));

15.12 Macro vs Function

15.13 Macro Side Effects

Be careful when passing expressions with side effects to function-like macros.

#define SQUARE(x) ((x) * (x))

int i = 5;

int result = SQUARE(i++);

This is dangerous because the argument may be expanded more than once.

15.14 #undef

The #undef directive removes a previously defined macro.

#define SIZE 100

#undef SIZE

15.15 Conditional Compilation

Conditional compilation allows parts of the source code to be included or excluded depending on preprocessor conditions.

#if condition

    /* code */

#endif

15.16 #if

#define VERSION 2

#if VERSION == 2

printf("Version 2");

#endif

The code between #if and #endif is included only when the condition evaluates as true.

15.17 #if and #else

#define DEBUG 0

#if DEBUG

printf("Debug mode");

#else

printf("Normal mode");

#endif

15.18 #elif

#define VERSION 2

#if VERSION == 1

printf("Version 1");

#elif VERSION == 2

printf("Version 2");

#else

printf("Other version");

#endif

15.19 #ifdef

#ifdef checks whether a macro is defined.

#define DEBUG

#ifdef DEBUG

printf("Debug mode enabled");

#endif

15.20 #ifndef

#ifndef checks whether a macro is not defined.

#ifndef DEBUG

printf("DEBUG is not defined");

#endif

15.21 Header Guards

Header guards prevent the same header file from being included multiple times in a single compilation unit.

#ifndef STUDENT_H
#define STUDENT_H

struct Student
{
    int roll;
    char name[50];
};

#endif

15.22 Creating Your Own Header File

We can create our own header file containing declarations and reusable code.

Example file:

mathutils.h

Content:

#ifndef MATHUTILS_H
#define MATHUTILS_H

int add(int a, int b);
int subtract(int a, int b);

#endif

15.23 Using a User-defined Header

Create a source file containing the function definitions.

#include "mathutils.h"

int add(int a, int b)
{
    return a + b;
}

int subtract(int a, int b)
{
    return a - b;
}

15.24 Including Your Header in main()

#include <stdio.h>
#include "mathutils.h"

int main()
{
    printf("%d\n", add(10, 20));

    printf("%d\n", subtract(20, 10));

    return 0;
}

15.25 Header File Structure

This separation makes larger programs easier to organize and maintain.

15.26 Standard vs User-defined Header

15.27 #pragma

#pragma provides implementation-specific instructions to the compiler.

Its exact behavior depends on the compiler and platform.

#pragma once

Many compilers support #pragma once as a way to prevent multiple inclusion of a header.

15.28 Predefined Macros

C implementations provide several predefined macros.

#include <stdio.h>

int main()
{
    printf("File: %s\n", __FILE__);

    printf("Line: %d\n", __LINE__);

    printf("Date: %s\n", __DATE__);

    printf("Time: %s\n", __TIME__);

    return 0;
}

Common predefined macros include:

• __FILE__ → current source file name
• __LINE__ → current line number
• __DATE__ → compilation date
• __TIME__ → compilation time

15.29 assert.h

The assert.h header provides the assert() macro for debugging checks.

#include <assert.h>

int x = 10;

assert(x > 0);

If the expression is false, the assertion normally reports a diagnostic and terminates the program.

15.30 Common Preprocessor Mistakes

• Forgetting the # before a directive.
• Writing a function-like macro without careful parentheses.
• Passing expressions with side effects to macros.
• Including the wrong header file.
• Forgetting header guards.
• Assuming #pragma behavior is portable.
• Using macros when a normal function would be clearer and safer.

15.31 Quick Revision

15.32 Quick MCQs

1. Which symbol begins a preprocessor directive?
   
   A) $
   B) #
   C) @
   D) %
   
   Answer: B


2. Which directive includes a header file?
   
   A) #define
   B) #include
   C) #ifdef
   D) #undef
   
   Answer: B


3. Which directive defines a macro?
   
   A) #define
   B) #include
   C) #if
   D) #endif
   
   Answer: A


4. Which directive removes a macro definition?
   
   A) #remove
   B) #delete
   C) #undef
   D) #clear
   
   Answer: C


5. Which directive checks whether a macro is defined?
   
   A) #ifdef
   B) #ifndef
   C) #check
   D) #defined
   
   Answer: A


6. Which directive checks whether a macro is not defined?
   
   A) #ifdef
   B) #ifndef
   C) #notdefined
   D) #ifnot
   
   Answer: B


7. Which header is commonly used for printf()?
   
   A) string.h
   B) stdio.h
   C) math.h
   D) stdlib.h
   
   Answer: B


8. Which directive ends a conditional compilation block?
   
   A) #end
   B) #endif
   C) #stop
   D) #finish
   
   Answer: B

15.33 Practice Problems

15.34 Key Takeaway

📁 16. File Handling in C

16.1 What is File Handling?

File handling allows a C program to store data in a file and retrieve that data later.

Normally, variables store data temporarily in memory. When the program ends, that data is lost. Files provide a way to store data persistently.

16.2 File Pointer

C uses a special pointer called a file pointer to work with files.

FILE *fp;

The type FILE is defined in stdio.h.

#include <stdio.h>

int main()
{
    FILE *fp;

    return 0;
}

16.3 Opening a File - fopen()

The fopen() function is used to open a file.

FILE *fp;

fp = fopen("data.txt", "r");

The first argument is the file name and the second argument is the opening mode.

16.4 Checking Whether a File Opened Successfully

Always check whether fopen() returned NULL.

#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("data.txt", "r");

    if(fp == NULL)
    {
        printf("File could not be opened");
        return 1;
    }

    printf("File opened successfully");

    fclose(fp);

    return 0;
}

16.5 Closing a File - fclose()

After finishing file operations, close the file using fclose().

fclose(fp);

16.6 File Opening Modes

16.7 Writing to a File - fprintf()

The fprintf() function writes formatted data to a file.

#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("student.txt", "w");

    if(fp == NULL)
    {
        printf("Unable to open file");
        return 1;
    }

    fprintf(fp, "Name: Venu\n");
    fprintf(fp, "Marks: 85\n");

    fclose(fp);

    return 0;
}

The file student.txt will contain:

16.8 Writing a Character - fputc()

#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("data.txt", "w");

    if(fp == NULL)
        return 1;

    fputc('A', fp);
    fputc('B', fp);
    fputc('C', fp);

    fclose(fp);

    return 0;
}

The file will contain:

16.9 Writing a String - fputs()

#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("data.txt", "w");

    if(fp == NULL)
        return 1;

    fputs("Welcome to C Programming\n", fp);

    fclose(fp);

    return 0;
}

16.10 Reading from a File - fscanf()

The fscanf() function reads formatted data from a file.

#include <stdio.h>

int main()
{
    FILE *fp;

    char name[50];
    int marks;

    fp = fopen("student.txt", "r");

    if(fp == NULL)
        return 1;

    fscanf(fp, "%49s %d", name, &marks);

    printf("Name: %s\n", name);
    printf("Marks: %d\n", marks);

    fclose(fp);

    return 0;
}

16.11 Reading a Character - fgetc()

#include <stdio.h>

int main()
{
    FILE *fp;

    int ch;

    fp = fopen("data.txt", "r");

    if(fp == NULL)
        return 1;

    ch = fgetc(fp);

    if(ch != EOF)
        printf("%c", ch);

    fclose(fp);

    return 0;
}

16.12 EOF - End of File

EOF represents the end-of-file condition.

Functions such as fgetc() return an int so that they can represent every possible unsigned character value as well as EOF.

int ch;

while((ch = fgetc(fp)) != EOF)
{
    putchar(ch);
}

16.13 Reading a Complete File

#include <stdio.h>

int main()
{
    FILE *fp;

    int ch;

    fp = fopen("data.txt", "r");

    if(fp == NULL)
    {
        printf("File not found");
        return 1;
    }

    while((ch = fgetc(fp)) != EOF)
    {
        putchar(ch);
    }

    fclose(fp);

    return 0;
}

16.14 Reading a String - fgets()

fgets() reads a line or part of a line from a file into a character array.

#include <stdio.h>

int main()
{
    FILE *fp;

    char line[100];

    fp = fopen("data.txt", "r");

    if(fp == NULL)
        return 1;

    while(fgets(line, sizeof line, fp) != NULL)
    {
        printf("%s", line);
    }

    fclose(fp);

    return 0;
}

16.15 Append Data - "a"

Append mode adds new data to the end of an existing file.

#include <stdio.h>

int main()
{
    FILE *fp;

    fp = fopen("data.txt", "a");

    if(fp == NULL)
        return 1;

    fprintf(fp, "New record added\n");

    fclose(fp);

    return 0;
}

16.16 Write Mode - "w"

Write mode creates a file if it does not exist. If the file already exists, opening it in "w" mode truncates its previous contents.

fp = fopen("data.txt", "w");

16.17 Read Mode - "r"

Read mode opens an existing file for reading.

fp = fopen("data.txt", "r");

If the file does not exist or cannot be opened, fopen() returns NULL.

16.18 File Position Indicator

C maintains a position indicator associated with an open stream. Reading and writing normally move this position forward.

16.19 fseek()

The fseek() function changes the file position indicator.

fseek(fp, 0, SEEK_SET);

Common origins are:

• SEEK_SET → beginning of file
• SEEK_CUR → current position
• SEEK_END → end of file

16.20 ftell()

ftell() returns the current file position indicator as a value of type long on typical implementations.

long position;

position = ftell(fp);

printf("Position = %ld", position);

16.21 rewind()

The rewind() function moves the file position indicator back to the beginning.

rewind(fp);

It also clears the stream's error and end-of-file indicators.

16.22 Binary Files

Binary files store data as bytes rather than as human-readable text.

Binary mode is commonly used for structures and other data where preserving a byte representation is useful.

fp = fopen("data.dat", "wb");

16.23 fwrite()

fwrite() writes blocks of bytes to a file.

#include <stdio.h>

struct Student
{
    int roll;
    float marks;
};

int main()
{
    FILE *fp;

    struct Student s = {101, 85.5};

    fp = fopen("student.dat", "wb");

    if(fp == NULL)
        return 1;

    fwrite(&s, sizeof s, 1, fp);

    fclose(fp);

    return 0;
}

16.24 fread()

fread() reads blocks of bytes from a file into memory.

#include <stdio.h>

struct Student
{
    int roll;
    float marks;
};

int main()
{
    FILE *fp;

    struct Student s;

    fp = fopen("student.dat", "rb");

    if(fp == NULL)
        return 1;

    if(fread(&s, sizeof s, 1, fp) == 1)
    {
        printf("%d %.2f", s.roll, s.marks);
    }

    fclose(fp);

    return 0;
}

16.25 Text File vs Binary File

16.26 Copy One File to Another

#include <stdio.h>

int main()
{
    FILE *source;
    FILE *destination;

    int ch;

    source = fopen("source.txt", "rb");

    destination = fopen("copy.txt", "wb");

    if(source == NULL || destination == NULL)
    {
        printf("Unable to open file");
        return 1;
    }

    while((ch = fgetc(source)) != EOF)
    {
        fputc(ch, destination);
    }

    fclose(source);
    fclose(destination);

    return 0;
}

16.27 Count Characters in a File

#include <stdio.h>

int main()
{
    FILE *fp;

    int ch;
    long count = 0;

    fp = fopen("data.txt", "r");

    if(fp == NULL)
        return 1;

    while((ch = fgetc(fp)) != EOF)
    {
        count++;
    }

    printf("Characters = %ld", count);

    fclose(fp);

    return 0;
}

16.28 Count Lines in a File

#include <stdio.h>

int main()
{
    FILE *fp;

    int ch;
    int lines = 0;

    fp = fopen("data.txt", "r");

    if(fp == NULL)
        return 1;

    while((ch = fgetc(fp)) != EOF)
    {
        if(ch == '\n')
        {
            lines++;
        }
    }

    printf("Lines = %d", lines);

    fclose(fp);

    return 0;
}

16.29 File Error Handling

Always check the return value of file operations.

FILE *fp = fopen("data.txt", "r");

if(fp == NULL)
{
    perror("data.txt");
    return 1;
}

perror() prints a message describing the most recent error associated with certain library or system operations.

16.30 Common File Handling Mistakes

• Forgetting to check whether fopen() returned NULL.
• Forgetting to close an opened file.
• Using "w" when you actually want to preserve existing contents.
• Using the wrong file mode.
• Using a character type instead of int when checking the result of fgetc() against EOF.
• Ignoring the return value of fread() or other important file operations.
• Assuming binary files are portable across all machines when writing raw C structures.

16.31 Quick Revision

16.32 Quick MCQs

1. Which type is used for a file pointer?
   
   A) FILE *
   B) file *
   C) FilePointer
   D) pointer_file
   
   Answer: A


2. Which function opens a file?
   
   A) open()
   B) fopen()
   C) fileopen()
   D) create()
   
   Answer: B


3. Which function closes a file?
   
   A) close()
   B) fclose()
   C) fileclose()
   D) endfile()
   
   Answer: B


4. Which mode opens a file for reading?
   
   A) "r"
   B) "w"
   C) "a"
   D) "x"
   
   Answer: A


5. Which mode can erase existing contents when opening a file?
   
   A) "r"
   B) "w"
   C) "a"
   D) "rb"
   
   Answer: B


6. Which mode is used to append data?
   
   A) "r"
   B) "w"
   C) "a"
   D) "x"
   
   Answer: C


7. Which function writes formatted data to a file?
   
   A) printf()
   B) fprintf()
   C) fwrite()
   D) fput()
   
   Answer: B


8. Which function reads one character from a file?
   
   A) fgets()
   B) fscanf()
   C) fgetc()
   D) fread()
   
   Answer: C


9. Which function writes binary blocks?
   
   A) fwrite()
   B) fprintf()
   C) fputs()
   D) fputc()
   
   Answer: A


10. Which function changes the file position?
    
    A) ftell()
    B) fseek()
    C) rewindfile()
    D) move()
    
    Answer: B

16.33 Practice Problems

16.34 Key Takeaway

🧠 17. Dynamic Memory Allocation

17.1 What is Dynamic Memory Allocation?

Dynamic Memory Allocation allows a C program to request memory during program execution.

Unlike fixed-size arrays, dynamically allocated memory can be requested according to the actual requirements of the program.

17.2 Static vs Dynamic Memory

17.3 Stack and Heap Memory

Dynamic memory is generally allocated from the heap.

The exact memory layout is implementation-dependent, but the stack and heap are useful concepts for understanding C memory management.

17.4 malloc()

malloc() allocates a block of uninitialized memory of the requested size.

#include <stdlib.h>

int *ptr;

ptr = malloc(5 * sizeof *ptr);

Here, memory is requested for five integers.

17.5 Example Using malloc()

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *ptr;

    ptr = malloc(5 * sizeof *ptr);

    if(ptr == NULL)
    {
        printf("Memory allocation failed");
        return 1;
    }

    for(int i = 0; i < 5; i++)
    {
        ptr[i] = (i + 1) * 10;
    }

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", ptr[i]);
    }

    free(ptr);

    return 0;
}

17.6 Does malloc() Initialize Memory?

No. The bytes returned by malloc() have indeterminate values until your program writes appropriate values into them.

int *ptr = malloc(5 * sizeof *ptr);

Do not assume the allocated integers are initialized to zero.

17.7 calloc()

calloc() allocates memory for multiple elements and initializes the allocated bytes to zero.

int *ptr;

ptr = calloc(5, sizeof *ptr);

The first argument specifies the number of elements. The second specifies the size of each element.

17.8 Example Using calloc()

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *ptr;

    ptr = calloc(5, sizeof *ptr);

    if(ptr == NULL)
    {
        printf("Memory allocation failed");
        return 1;
    }

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", ptr[i]);
    }

    free(ptr);

    return 0;
}

17.9 malloc() vs calloc()

17.10 realloc()

realloc() changes the size of a previously allocated memory block.

ptr = realloc(ptr, 10 * sizeof *ptr);

The block may be moved to a different memory location. Therefore, use the pointer returned by realloc().

17.11 Example Using realloc()

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *ptr;

    ptr = malloc(3 * sizeof *ptr);

    if(ptr == NULL)
        return 1;

    for(int i = 0; i < 3; i++)
    {
        ptr[i] = (i + 1) * 10;
    }

    int *temp = realloc(ptr, 5 * sizeof *ptr);

    if(temp == NULL)
    {
        free(ptr);
        return 1;
    }

    ptr = temp;

    ptr[3] = 40;
    ptr[4] = 50;

    for(int i = 0; i < 5; i++)
    {
        printf("%d ", ptr[i]);
    }

    free(ptr);

    return 0;
}

17.12 Safe Use of realloc()

Do not directly overwrite your only pointer with realloc() if you need to handle allocation failure safely.

Prefer a temporary pointer:

int *temp;

temp = realloc(ptr, new_size);

if(temp != NULL)
{
    ptr = temp;
}
else
{
    /* Original ptr is still valid */
}

17.13 free()

free() releases dynamically allocated memory back to the implementation.

free(ptr);

After freeing memory, the pointer value becomes indeterminate. If you keep the pointer variable, assigning NULL to it can help prevent accidental reuse.

free(ptr);
ptr = NULL;

17.14 Dynamic Memory Diagram

17.15 Checking for NULL

Dynamic allocation functions return a null pointer if the allocation fails.

int *ptr = malloc(100 * sizeof *ptr);

if(ptr == NULL)
{
    printf("Memory allocation failed");
    return 1;
}

17.16 Dynamic Array

Dynamic allocation is useful when the number of elements is known only during program execution.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int n;

    printf("Enter number of elements: ");
    scanf("%d", &n);

    int *arr = malloc(n * sizeof *arr);

    if(arr == NULL)
        return 1;

    for(int i = 0; i < n; i++)
    {
        scanf("%d", &arr[i]);
    }

    for(int i = 0; i < n; i++)
    {
        printf("%d ", arr[i]);
    }

    free(arr);

    return 0;
}

17.17 Dynamic Array Representation

17.18 Dynamically Allocated String

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    const char *text = "Hello";

    char *str = malloc(strlen(text) + 1);

    if(str == NULL)
        return 1;

    strcpy(str, text);

    printf("%s", str);

    free(str);

    return 0;
}

17.19 Dynamic Structure

Structures can also be allocated dynamically.

#include <stdio.h>
#include <stdlib.h>

struct Student
{
    int roll;
    float marks;
};

int main()
{
    struct Student *s;

    s = malloc(sizeof *s);

    if(s == NULL)
        return 1;

    s->roll = 101;
    s->marks = 85.5;

    printf("Roll = %d\n", s->roll);
    printf("Marks = %.2f\n", s->marks);

    free(s);

    return 0;
}

17.20 Accessing Dynamic Structures

When a pointer points to a structure, the -> operator is used to access its members.

s->roll = 101;
s->marks = 85.5;

This is equivalent to:

(*s).roll = 101;
(*s).marks = 85.5;

17.21 Dynamic Array of Structures

#include <stdio.h>
#include <stdlib.h>

struct Student
{
    int roll;
    float marks;
};

int main()
{
    int n;

    scanf("%d", &n);

    struct Student *students =
        malloc(n * sizeof *students);

    if(students == NULL)
        return 1;

    for(int i = 0; i < n; i++)
    {
        scanf("%d %f",
              &students[i].roll,
              &students[i].marks);
    }

    for(int i = 0; i < n; i++)
    {
        printf("%d %.2f\n",
               students[i].roll,
               students[i].marks);
    }

    free(students);

    return 0;
}

17.22 Dynamic 2D Array

A two-dimensional array can also be allocated dynamically. One simple method is to allocate one contiguous block.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int rows = 3;
    int cols = 4;

    int *matrix =
        malloc(rows * cols * sizeof *matrix);

    if(matrix == NULL)
        return 1;

    for(int i = 0; i < rows; i++)
    {
        for(int j = 0; j < cols; j++)
        {
            matrix[i * cols + j] = i + j;
        }
    }

    for(int i = 0; i < rows; i++)
    {
        for(int j = 0; j < cols; j++)
        {
            printf("%d ", matrix[i * cols + j]);
        }

        printf("\n");
    }

    free(matrix);

    return 0;
}

17.23 Memory Leak

A memory leak occurs when dynamically allocated memory is no longer reachable by the program and has not been released.

int *ptr = malloc(100 * sizeof *ptr);

/* ptr is lost without calling free() */

ptr = NULL;

17.24 Dangling Pointer

A dangling pointer is a pointer that refers to memory whose lifetime has ended or that has already been freed.

int *ptr = malloc(sizeof *ptr);

*ptr = 10;

free(ptr);

/* ptr should not be dereferenced here */

A common defensive practice is:

free(ptr);
ptr = NULL;

17.25 Double Free

Calling free() more than once on the same allocation is invalid.

free(ptr);

/* Do not do this again: */
free(ptr);

Setting the pointer to NULL after freeing can help avoid accidental repeated freeing, because free(NULL) has no effect.

17.26 Use-After-Free

Accessing memory after it has been released is invalid.

free(ptr);

/* Invalid */
printf("%d", *ptr);

17.27 Using sizeof() Correctly

Prefer using the pointed-to type when allocating memory.

int *ptr;

ptr = malloc(10 * sizeof *ptr);

This style is easier to maintain if the pointer type changes later.

17.28 Dynamic Memory Workflow

17.29 Important Functions

17.30 Common Dynamic Memory Mistakes

• Forgetting to check whether allocation returned NULL.
• Forgetting to call free().
• Accessing memory after free().
• Freeing the same allocation twice.
• Writing beyond the allocated block.
• Losing the only pointer to allocated memory.
• Using the wrong size in malloc() or calloc().
• Overwriting the original pointer with realloc() without considering allocation failure.

17.31 Quick Revision

17.32 Quick MCQs

1. Which function allocates dynamic memory?
   
   A) malloc()
   B) allocate()
   C) memory()
   D) new()
   
   Answer: A


2. Which function releases dynamically allocated memory?
   
   A) delete()
   B) remove()
   C) free()
   D) release()
   
   Answer: C


3. Which function allocates and zero-initializes the allocated bytes?
   
   A) malloc()
   B) calloc()
   C) realloc()
   D) free()
   
   Answer: B


4. Which function changes the size of an allocation?
   
   A) malloc()
   B) calloc()
   C) realloc()
   D) resize()
   
   Answer: C


5. Where is dynamically allocated storage commonly obtained from?
   
   A) Heap
   B) Register
   C) Code segment
   D) Preprocessor
   
   Answer: A


6. What should be checked after malloc()?
   
   A) EOF
   B) NULL
   C) void
   D) zero
   
   Answer: B


7. What is a memory leak?
   
   A) Using too little memory
   B) Losing access to allocated memory without releasing it
   C) Reading a file
   D) Using a static variable
   
   Answer: B


8. What should you do after free(ptr) if you want to make ptr a safe null pointer?
   
   A) ptr = NULL
   B) ptr = 1
   C) ptr++
   D) realloc(ptr)
   
   Answer: A


9. Which function is declared in stdlib.h?
   
   A) malloc()
   B) printf()
   C) strlen()
   D) strcpy()
   
   Answer: A


10. Which is a dangerous operation?
    
    A) free(ptr)
    B) ptr = NULL
    C) Dereferencing ptr after free(ptr)
    D) malloc()
    
    Answer: C

17.33 Practice Problems

17.34 Key Takeaway

💻 18. Command Line Arguments

18.1 What are Command Line Arguments?

Command line arguments are values supplied to a C program when the program is started from a command line or terminal.

They allow a program to receive input without using scanf() during execution.

Example:

program.exe 10 20 30

Here 10, 20 and 30 are command line arguments.

18.2 main() with Command Line Arguments

A common form of the main function is:

int main(int argc, char *argv[])
{
    return 0;
}

The two parameters are:

• argc → argument count
• argv → argument vector, an array of pointers to the argument strings

18.3 argc - Argument Count

argc contains the number of command-line arguments, including the program name.

Example:

program.exe 10 20

The value of argc is:

Why 3?

18.4 argv - Argument Vector

argv is an array of strings containing the command-line arguments.

program.exe 10 20

18.5 Display Command Line Arguments

#include <stdio.h>

int main(int argc, char *argv[])
{
    printf("Argument count = %d\n\n", argc);

    for(int i = 0; i < argc; i++)
    {
        printf("argv[%d] = %s\n", i, argv[i]);
    }

    return 0;
}

Suppose the program is executed as:

program.exe Hello 123 C

18.6 Important: Command Line Arguments are Strings

All command-line arguments are received as strings.

program.exe 10 20

Therefore:

argv[1]

contains the string:

"10"

It is not automatically an integer.

18.7 Converting Strings to Integers - atoi()

The atoi() function converts a string representing an integer into an int.

#include <stdlib.h>

int num;

num = atoi(argv[1]);

Example:

program.exe 25

int num = atoi(argv[1]);

Now:

18.8 Add Two Command Line Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        printf("Usage: program number1 number2");
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[2]);

    printf("Sum = %d", a + b);

    return 0;
}

Example:

program.exe 10 20

18.9 Subtract Two Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        printf("Usage: program number1 number2");
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[2]);

    printf("Difference = %d", a - b);

    return 0;
}

18.10 Multiply Two Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        printf("Usage: program number1 number2");
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[2]);

    printf("Product = %d", a * b);

    return 0;
}

18.11 Divide Two Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc != 3)
    {
        printf("Usage: program number1 number2");
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[2]);

    if(b == 0)
    {
        printf("Division by zero is not allowed");
        return 1;
    }

    printf("Quotient = %d", a / b);

    return 0;
}

18.12 Sum of Multiple Command Line Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    int sum = 0;

    for(int i = 1; i < argc; i++)
    {
        sum += atoi(argv[i]);
    }

    printf("Sum = %d", sum);

    return 0;
}

Example:

program.exe 10 20 30 40

18.13 Find the Largest Command Line Number

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc < 2)
    {
        printf("Provide at least one number");
        return 1;
    }

    int largest = atoi(argv[1]);

    for(int i = 2; i < argc; i++)
    {
        int value = atoi(argv[i]);

        if(value > largest)
        {
            largest = value;
        }
    }

    printf("Largest = %d", largest);

    return 0;
}

18.14 Find the Smallest Command Line Number

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc < 2)
    {
        printf("Provide at least one number");
        return 1;
    }

    int smallest = atoi(argv[1]);

    for(int i = 2; i < argc; i++)
    {
        int value = atoi(argv[i]);

        if(value < smallest)
        {
            smallest = value;
        }
    }

    printf("Smallest = %d", smallest);

    return 0;
}

18.15 Average of Command Line Numbers

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc < 2)
    {
        printf("Provide at least one number");
        return 1;
    }

    double sum = 0;

    for(int i = 1; i < argc; i++)
    {
        sum += atoi(argv[i]);
    }

    double average = sum / (argc - 1);

    printf("Average = %.2f", average);

    return 0;
}

18.16 strtol() - Safer Integer Conversion

atoi() is simple, but it provides limited error reporting. For more robust programs, strtol() can be used.

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>

int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage: program number");
        return 1;
    }

    char *end;
    long value;

    errno = 0;

    value = strtol(argv[1], &end, 10);

    if(end == argv[1] || *end != '\0')
    {
        printf("Invalid integer");
        return 1;
    }

    if(errno == ERANGE ||
       value < INT_MIN ||
       value > INT_MAX)
    {
        printf("Integer out of range");
        return 1;
    }

    printf("Number = %ld", value);

    return 0;
}

18.17 Passing a Character or Word

#include <stdio.h>

int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage: program name");
        return 1;
    }

    printf("Hello %s!", argv[1]);

    return 0;
}

Example:

program.exe Venu

18.18 Arguments Containing Spaces

If an argument contains spaces, the shell normally separates it into multiple arguments unless it is quoted.

program.exe "Venu Gopal"

Here:

argv[1]

18.19 Command Line Argument Diagram

18.20 Command Line Arguments vs scanf()

18.21 Checking the Number of Arguments

A program should check whether the required arguments have been provided before accessing them.

if(argc != 3)
{
    printf("Usage: program number1 number2");
    return 1;
}

18.22 Command Line Calculator

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    if(argc != 4)
    {
        printf("Usage: program number operator number");
        return 1;
    }

    int a = atoi(argv[1]);
    int b = atoi(argv[3]);

    char op = argv[2][0];

    switch(op)
    {
        case '+':
            printf("Result = %d", a + b);
            break;

        case '-':
            printf("Result = %d", a - b);
            break;

        case '*':
            printf("Result = %d", a * b);
            break;

        case '/':
            if(b == 0)
            {
                printf("Division by zero is not allowed");
                return 1;
            }

            printf("Result = %d", a / b);
            break;

        default:
            printf("Invalid operator");
    }

    return 0;
}

Example:

program.exe 10 + 20

18.23 Passing a File Name

Command-line arguments are often useful for supplying file names.

#include <stdio.h>

int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage: program filename");
        return 1;
    }

    FILE *fp = fopen(argv[1], "r");

    if(fp == NULL)
    {
        printf("Unable to open file");
        return 1;
    }

    printf("File opened successfully");

    fclose(fp);

    return 0;
}

Example:

program.exe data.txt

18.24 Common Mistakes

• Forgetting that argv contains strings.
• Using argv[1] without checking argc.
• Performing arithmetic directly on strings.
• Forgetting to convert numeric arguments.
• Assuming the program name is not counted in argc.
• Using atoi() when detailed input validation is required.
• Forgetting that arguments containing spaces may need quoting in the command shell.

18.25 Quick Revision

18.26 Quick MCQs

1. What does argc represent?
   
   A) Argument count
   B) Argument character
   C) Array count
   D) Character count
   
   Answer: A


2. What does argv represent?
   
   A) Argument vector
   B) Argument value
   C) Array variable
   D) Automatic variable
   
   Answer: A


3. Which argument normally contains the program name?
   
   A) argv[0]
   B) argv[1]
   C) argv[2]
   D) argc
   
   Answer: A


4. If a program is run as:
   
   program.exe 10 20
   
   what is argc?
   
   A) 2
   B) 3
   C) 10
   D) 20
   
   Answer: B


5. What type of data does argv contain?
   
   A) Integer values
   B) Character strings
   C) Floating-point values only
   D) Structures
   
   Answer: B


6. Which function can convert a string to int?
   
   A) atoi()
   B) itoa()
   C) stringtoint()
   D) convert()
   
   Answer: A


7. Which function provides more robust integer conversion and error handling?
   
   A) atoi()
   B) strtol()
   C) printf()
   D) scanf()
   
   Answer: B


8. Which header declares atoi() and strtol()?
   
   A) stdio.h
   B) stdlib.h
   C) string.h
   D) math.h
   
   Answer: B


9. Which should be checked before using argv[2]?
   
   A) EOF
   B) argc
   C) NULL
   D) sizeof
   
   Answer: B


10. Command line arguments are most directly useful for:
    
    A) Supplying startup input to a program
    B) Declaring variables
    C) Creating loops
    D) Defining structures
    
    Answer: A

18.27 Practice Problems

18.28 Key Takeaway

🔢 19. Bitwise Programming

19.1 What is Bitwise Programming?

Bitwise programming operates directly on the individual bits of an integer.

A bit can have only two values: 0 or 1.

Bitwise operations are commonly used in low-level programming, embedded systems, networking, operating systems and programming interviews.

19.2 Binary Representation

Computers internally represent integer values using binary digits.

8-bit binary representation

For example:

10 = 00001010

19.3 Bitwise Operators

19.4 Bitwise AND (&)

The AND operation produces 1 only when both corresponding bits are 1.

int result = 5 & 3;

printf("%d", result);

19.5 AND Truth Table

19.6 Bitwise OR (|)

OR produces 1 when at least one corresponding bit is 1.

int result = 5 | 3;

printf("%d", result);

19.7 Bitwise XOR (^)

XOR produces 1 when the corresponding bits are different.

int result = 5 ^ 3;

printf("%d", result);

19.8 XOR Truth Table

19.9 Bitwise NOT (~)

The NOT operator flips every bit: 0 becomes 1 and 1 becomes 0.

int x = 5;

printf("%d", ~x);

For signed integers, the exact decimal result of ~x depends on the integer representation. On modern two's-complement systems, ~5 is commonly -6.

19.10 Left Shift (<<)

The left-shift operator moves bits toward the left.

int x = 5;

printf("%d", x << 1);

For a nonnegative value, shifting left by one position corresponds to multiplying by 2 when the result is representable.

19.11 Right Shift (>>)

The right-shift operator moves bits toward the right.

int x = 20;

printf("%d", x >> 2);

19.12 Important Shift Rules

• Shifting by a negative count is undefined behavior.
• Shifting by a count greater than or equal to the width of the promoted left operand is undefined behavior.
• Left-shifting signed values requires care because overflow can result in undefined behavior.
• Right-shifting a negative signed integer is implementation-defined.
• For predictable bit manipulation, unsigned integers are often preferable.

19.13 Check Odd or Even Using Bitwise AND

The least significant bit determines whether a nonnegative integer is odd or even.

if(n & 1)
{
    printf("Odd");
}
else
{
    printf("Even");
}

Example:

7 = 00000111
1 = 00000001

7 & 1 = 1

19.14 Check Whether a Bit is Set

To check bit position k, use:

if(n & (1u << k))
{
    printf("Bit is set");
}
else
{
    printf("Bit is not set");
}

Example: check bit 2 of 5.

5 = 00000101

1u << 2 = 00000100

5 & 4 = 4

19.15 Set a Bit

Setting a bit means changing it to 1.

n = n | (1u << k);

Short form:

n |= (1u << k);

Example:

n = 8;       /* 1000 */
k = 1;

n |= (1u << k);

/* Result = 1010 = 10 */

19.16 Clear a Bit

Clearing a bit means changing it to 0.

n = n & ~(1u << k);

Short form:

n &= ~(1u << k);

19.17 Toggle a Bit

Toggling changes:

n ^= (1u << k);

19.18 Bit Mask

A bit mask is a value used to select or modify specific bits.

1u << k

creates a mask with bit k set.

19.19 Count Set Bits

A simple method is to examine every bit.

#include <stdio.h>

int main()
{
    unsigned int n;
    int count = 0;

    scanf("%u", &n);

    while(n != 0)
    {
        count += n & 1u;
        n >>= 1;
    }

    printf("Set bits = %d", count);

    return 0;
}

For example:

13 = 1101

19.20 Efficient Set Bit Counting

Brian Kernighan's method repeatedly removes the lowest set bit.

while(n != 0)
{
    n = n & (n - 1);
    count++;
}

This performs one iteration for each set bit.

19.21 Check Whether a Number is a Power of 2

A positive power of two has exactly one set bit.

if(n > 0 && (n & (n - 1)) == 0)
{
    printf("Power of 2");
}
else
{
    printf("Not a power of 2");
}

Examples:

19.22 Clear the Lowest Set Bit

n = n & (n - 1);

This removes the lowest set bit from n.

n = 12;

/*
12 = 1100
11 = 1011
*/

n & (n - 1)

/*
1100
1011
----
1000
*/

Result = 8

19.23 Isolate the Lowest Set Bit

unsigned int lowest = n & (~n + 1);

An equivalent common expression is:

unsigned int lowest = n & -n;

For unsigned arithmetic, a useful portable formulation is to work with unsigned values when doing bit manipulation.

19.24 Swap Two Values Using XOR

XOR has the property:

a = a ^ b;
b = a ^ b;
a = a ^ b;

However, in normal C programs a temporary variable is usually clearer and safer.

19.25 Find the Unique Element Using XOR

If every number occurs exactly twice except one number, XOR can find the unique value.

int arr[] = {4, 1, 2, 1, 2};

int result = 0;

for(int i = 0; i < 5; i++)
{
    result ^= arr[i];
}

printf("%d", result);

Because:

19.26 Using Bits as Flags

Individual bits can represent multiple yes/no settings inside one integer.

#define READ_PERMISSION   (1u << 0)
#define WRITE_PERMISSION  (1u << 1)
#define EXEC_PERMISSION   (1u << 2)

unsigned int permissions = 0;

permissions |= READ_PERMISSION;
permissions |= WRITE_PERMISSION;

Now multiple independent flags are stored in one value.

19.27 Set, Clear and Toggle a Bit

#include <stdio.h>

int main()
{
    unsigned int n;
    int k;

    scanf("%u %d", &n, &k);

    /* Set */
    n |= (1u << k);

    printf("After set: %u\n", n);

    /* Clear */
    n &= ~(1u << k);

    printf("After clear: %u\n", n);

    /* Toggle */
    n ^= (1u << k);

    printf("After toggle: %u\n", n);

    return 0;
}

19.28 Print Binary Representation

#include <stdio.h>

void printBinary(unsigned int n)
{
    unsigned int mask = 1u <<
                         (sizeof(unsigned int) * 8 - 1);

    while(mask != 0)
    {
        printf("%d", (n & mask) != 0);

        mask >>= 1;
    }
}

int main()
{
    unsigned int n;

    scanf("%u", &n);

    printBinary(n);

    return 0;
}

19.29 Bitwise vs Logical Operators