⌨️ 5. Input & Output in C

5.1 What Is Input and Output?

User
  |
  | Input
  v
+----------------+
|    C Program   |
|                |
| Read -> Process|
|      -> Result |
+----------------+
  |
  | Output
  v
Screen

5.2 Standard Input/Output and <stdio.h>

C provides many standard input/output functions through the header file:

#include <stdio.h>

The name stdio comes from standard input/output

Basic console I/O functions

5.3 printf() — Displaying Output

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

Basic syntax

printf("format string", values);

Simple example

#include <stdio.h>

int main(void)
{
    printf("Hello, CodeBhavya!");
    return 0;
}

The text inside quotation marks is called a string literal

Printing multiple pieces of information

#include <stdio.h>

int main(void)
{
    int age = 20;

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

    return 0;
}

Here, %d tells printf() how the corresponding value should be formatted.

5.4 Format Strings and Format Specifiers

A format string contains ordinary text and special conversion specifications.

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

             |
             +---- %d = format specifier

5.5 Printing Integers

int

#include <stdio.h>

int main(void)
{
    int number = 125;

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

    return 0;
}

unsigned int

#include <stdio.h>

int main(void)
{
    unsigned int count = 500;

    printf("Count = %u\n", count);

    return 0;
}

long

long population = 500000L;

printf("Population = %ld\n", population);

long long

long long distance = 9000000000LL;

printf("Distance = %lld\n", distance);

5.6 Printing Floating-Point Values

Floating-point values are commonly represented using float, double, or long double.

double with printf()

#include <stdio.h>

int main(void)
{
    double price = 149.75;

    printf("Price = %f\n", price);

    return 0;
}

A float value is automatically promoted to double when passed as a variable argument to the printf() function, so %f is used for both ordinary float and double values in this context.

Printing with two decimal places

double price = 149.7568;

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

The output is rounded for the requested display precision:

149.76

5.7 Printing Characters

Use %c when displaying a character.

#include <stdio.h>

int main(void)
{
    char grade = 'A';

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

    return 0;
}

5.8 Printing Strings

Use %s to print a null-terminated character string.

#include <stdio.h>

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

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

    return 0;
}

char name[] = "Bhavya";

Memory:

+---+---+---+---+---+---+----+
| B | h | a | v | y | a | \0 |
+---+---+---+---+---+---+----+
                            ^
                            |
                     null terminator

The \0 character marks the end of a C string.

5.9 Escape Sequences

Escape sequences represent special characters inside character and string literals.

Newline

printf("Hello\nWorld");

Tab

printf("Name\tMarks\n");

Quotation mark

printf("He said \"Hello\"");

Backslash

printf("C:\\CodeBhavya\\C");

5.10 Width and Precision Basics

The format string can control how values are displayed.

Precision for floating-point output

double value = 12.34567;

printf("%.2f\n", value);
printf("%.3f\n", value);
printf("%.4f\n", value);

The precision specifies how many digits are displayed after the decimal point for the f conversion.

Field width

printf("%10d\n", 125);

The value is displayed using a minimum field width of 10 characters.

Combining width and precision

printf("%10.2f\n", 125.456);

5.11 scanf() — Reading Formatted Input

The scanf() function reads formatted input from the standard input stream.

Basic syntax

scanf("format", arguments);

Reading an integer

#include <stdio.h>

int main(void)
{
    int age;

    scanf("%d", &age);

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

    return 0;
}

5.12 Why Does scanf() Need &?

For most scalar variables, scanf() needs the address where it should store the input.

int age;

scanf("%d", &age);

Variable:

age
+-----------+
|     ?     |
+-----------+
     ^
     |
   &age
     |
     v
scanf() stores the input here

The & operator obtains the address of the variable. Pointers will explain this mechanism in much greater detail later.

char name[20];

scanf("%19s", name);

5.13 scanf() Format Specifiers

5.14 The Important %f vs %lf Difference

One of the most common C programming mistakes occurs when reading floating-point values.

For float

float price;

scanf("%f", &price);

For double

double price;

scanf("%lf", &price);

For printf()

float a = 10.5f;
double b = 20.5;

printf("%f\n", a);
printf("%f\n", b);

The difference is about the arguments each function receives. scanf() needs pointers to the exact destination types. Variable arguments passed to printf() follow the default argument promotions, which turn a float value into double. A float pointer does not become a double pointer.

5.15 Reading Multiple Values

Multiple values can be read using one scanf() statement.

#include <stdio.h>

int main(void)
{
    int a, b;

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

    printf("A = %d\n", a);
    printf("B = %d\n", b);

    return 0;
}

Input can be supplied as:

10 20

or, because ordinary whitespace separates these conversions:

10
20

5.16 getchar() — Reading One Character

The getchar() function reads one character from standard input.

#include <stdio.h>

int main(void)
{
    int ch;

    ch = getchar();

    printf("You entered: %c\n", ch);

    return 0;
}

5.17 putchar() — Writing One Character

The putchar() function writes one character to standard output.

#include <stdio.h>

int main(void)
{
    putchar('A');
    putchar('\n');

    return 0;
}

Using a variable

#include <stdio.h>

int main(void)
{
    char grade = 'A';

    putchar(grade);
    putchar('\n');

    return 0;
}

5.18 fgets() — Reading a Line of Text

The fgets() function is commonly used when a program needs to read a line of text into a character array.

Basic syntax

fgets(buffer, sizeof buffer, stdin);

Example

#include <stdio.h>

int main(void)
{
    char name[50];

    printf("Enter your name: ");

    fgets(name, sizeof name, stdin);

    printf("You entered: %s", name);

    return 0;
}

5.19 puts() — Writing a String

The puts() function writes a string followed by a newline.

#include <stdio.h>

int main(void)
{
    char message[] = "Welcome to CodeBhavya";

    puts(message);

    return 0;
}

Compared with printf("%s\n", message), puts() is a simple option when you only need to write a string followed by a newline.

5.20 scanf("%s") vs fgets()

scanf() with a width limit

char name[20];

scanf("%19s", name);

The width 19 prevents scanf() from storing more than 19 characters for this conversion, leaving space for the terminating \0.

5.21 Whitespace and scanf()

Different scanf() conversions treat whitespace differently.

Numeric conversions

Conversions such as %d, %f, and %lf skip leading whitespace automatically.

Character conversion

The %c conversion does not skip whitespace automatically.

char ch;

scanf("%c", &ch);

If a newline is already waiting in the input stream, this may read that newline.

Common solution

scanf(" %c", &ch);

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

5.22 Understanding Leftover Input

Consider:

int age;

scanf("%d", &age);

If the user enters:

20

the numeric conversion reads the 20. The newline entered after it can remain unread in the input stream.

User enters:

2 0 \n
|---|
  |
  +---- scanf("%d") consumes 20

The newline may remain:

\n
 ^
 |
unread input

If the next operation is:

char ch;

scanf("%c", &ch);

the character conversion may read that pending newline.

Common approach

scanf(" %c", &ch);

The leading whitespace in the format string skips preceding whitespace before reading the actual character.

Numeric input followed by fgets()

A similar issue can occur when a numeric scanf() is followed immediately by fgets(). The pending newline may be consumed by fgets(), making it appear as though the user entered an empty line.

5.23 Checking the Return Value of scanf()

scanf() returns information about how many input items were successfully converted and assigned.

Example

#include <stdio.h>

int main(void)
{
    int age;

    if (scanf("%d", &age) == 1)
    {
        printf("Age = %d\n", age);
    }
    else
    {
        printf("Invalid input\n");
    }

    return 0;
}

If the input successfully provides one integer, scanf() returns 1.

5.24 Standard Streams: stdin, stdout and stderr

C programs commonly interact with three predefined standard streams.

Example

#include <stdio.h>

int main(void)
{
    fprintf(stdout, "Normal output\n");
    fprintf(stderr, "Error message\n");

    return 0;
}

For beginner console programs, printf() is the usual choice for normal formatted output, while stderr is useful for diagnostic or error messages.

5.25 Output Buffering — Beginner View

Standard I/O may hold output temporarily in a buffer before sending it to its destination. A buffer is a temporary storage area.

For most beginner programs, you can simply think of printf() and related functions as sending output to stdout.

5.26 Complete Input and Output Program

The following program combines several ideas from this lesson.

#include <stdio.h>

int main(void)
{
    int age;
    double marks;
    char grade;
    char name[50];

    printf("Enter your name: ");
    fgets(name, sizeof name, stdin);

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

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

    printf("Enter your grade: ");
    scanf(" %c", &grade);

    printf("\n--- Student Details ---\n");
    printf("Name  : %s", name);
    printf("Age   : %d\n", age);
    printf("Marks : %.2f\n", marks);
    printf("Grade : %c\n", grade);

    return 0;
}

5.27 Common Input & Output Mistakes

int age;

scanf("%d", age);

scanf("%d", &age);

double value;

scanf("%f", &value);

scanf("%lf", &value);

scanf("%s", name);

printf("Hello");
printf("World");

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

5.28 Common Confusions

5.29 Input & Output Concept Map

C Input & Output
│
├── Standard I/O
│   └── <stdio.h>
│
├── Output
│   ├── printf()
│   ├── putchar()
│   └── puts()
│
├── Input
│   ├── scanf()
│   ├── getchar()
│   └── fgets()
│
├── Format Specifiers
│   ├── %d
│   ├── %u
│   ├── %f
│   ├── %lf
│   ├── %c
│   └── %s
│
├── Escape Sequences
│   ├── \n
│   ├── \t
│   ├── \\
│   └── \0
│
└── Standard Streams
    ├── stdin
    ├── stdout
    └── stderr