🧮 7. Expressions

7.1 What Is an Expression?

An expression is a combination of values, variables, operators, function calls, and parentheses that C can evaluate to produce a value.

total = price + tax;

7.2 Expression vs Statement

An expression describes a calculation, comparison, assignment, or function call. A statement is an instruction in the program. Adding a semicolon to an expression makes an expression statement, as the examples below show.

7.3 Main Types of Expressions

7.4 Arithmetic Expressions

Arithmetic expressions use numeric operands and arithmetic operators.

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);

7.5 Relational Expressions

Relational operators compare values. The result used by C for a comparison is an int: 1 represents true and 0 represents false.

int a = 10;
int b = 20;

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

7.6 Logical Expressions

Logical operators combine or modify conditions.

int age = 25;

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

printf("%d", eligible);

7.7 Short-Circuit Evaluation

C uses short-circuit evaluation for && and ||. The right-hand operand may not be evaluated when its result is already determined by the left-hand operand.

int x = 0;

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

7.8 Assignment Expressions

Assignment changes the stored value of a variable. It is also an expression, so its assigned value can be used in a larger expression.

int a;
int b;

b = (a = 10);

A chained assignment uses the value of one assignment in another. For learning and debugging, separate assignments often make each change easier to follow.

7.9 Compound Assignment Expressions

Compound assignment combines an operation with assignment.

7.10 Mixed Expressions

A mixed expression contains multiple operators, so C must determine how the expression is grouped.

int result = 10 + 5 * 2;

7.11 Operator Precedence — The Grouping Rules

Precedence tells us how operators and their operands are grouped. For example, multiplication groups more tightly than addition.

7.12 Parentheses — Make the Intent Obvious

int a = 10 + 5 * 2;
int b = (10 + 5) * 2;

7.13 Associativity

When operators have the same precedence, their associativity determines how they are grouped.

int result = 20 - 5 - 3;

int result = 20 / 5 * 2;

7.14 Integer Division

int a = 5;
int b = 2;

int result = a / b;

printf("%d", result);

Both operands are integers, so the quotient is an integer. The fractional part is discarded toward zero: 5 / 2 gives 2, and -5 / 2 gives -2.

7.15 Floating-Point Division

int a = 5;
int b = 2;

double result = (double)a / b;

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

7.16 Type Conversion in Expressions

C may convert operands to compatible types during expression evaluation. Conversions can be implicit or explicit.

Implicit Conversion

int count = 10;
double price = 2.5;

double total = count * price;

Here, one operand is a floating-point value. C converts the integer operand to the floating-point type used for the multiplication.

Explicit Conversion — Cast

int marks = 85;
int total = 100;

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

7.17 Character Expressions

A character constant such as 'A' has type int in C. Its numeric value comes from the execution character set, so it can take part in integer calculations.

char ch = 'A';

printf("%d", ch);

The exact numeric value comes from the execution character set. On an ASCII-based system, 'A' has value 65.

char ch = 'A';

printf("%c\n", ch);
printf("%c\n", ch + 1);

7.18 Truth Values in Expressions

In a C condition, 0 is false and any nonzero value is true.

int x = -5;

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

The value does not need to be exactly 1 to be true.

7.19 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;

7.20 Pre-Increment and Post-Increment

Both ++a and a++ increase a by one. The important difference is the value produced by the expression.

int a = 5;
int b = ++a;

int a = 5;
int b = a++;

7.21 Pre-Decrement and Post-Decrement

int a = 5;
int b = --a;

int a = 5;
int b = a--;

7.22 The Comma Operator

The comma operator finishes its left operand, including any changes it makes, before evaluating its right operand. The right operand supplies the result of the comma expression.

int a, b;

int result = (a = 10, b = 20, a + b);

7.23 Expression Evaluation — A Worked Example

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

7.24 Evaluation Order vs Precedence — Critical C Concept

This is one of the most important ideas in C expressions: precedence is about grouping; evaluation order is a separate issue.

int x = f() + g();

7.25 Undefined Behavior from Unsafe Side Effects

A side effect is a change made while evaluating an expression, such as increasing a variable. Do not combine changes to the same variable unless C guarantees their order.

i++ + ++i

Prefer clear steps:

i++;
i++;
int result = i;

The separate statements make the order clear. You can check the value after each line instead of guessing an output from undefined behavior.

7.26 Common Expression Mistakes

#include <stdio.h>

int main(void)
{
    int index = -1;
    unsigned int count = 5U;
    if (index < 0)
        puts("Invalid negative index");
    else if ((unsigned int)index < count)
        puts("Index is within range");
    else
        puts("Index is out of range");
    return 0;
}

Invalid negative index

7.28 Quick Revision

7.29 Quick MCQs

Select an answer first, then click Check Answer. A correct choice becomes green. If the choice is wrong, it becomes red and the correct option is shown in green. The correct answer and explanation appear below after checking.

Run your program to see the output.

#include <stdio.h>

int main()
{
    int result;

    result = 10 + 5 * 2;

    printf("Result = %d", result);

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int result;

    result = (10 + 5) * 2;

    printf("Result = %d", result);

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int result;

    result = 20 / 5 * 2;

    printf("Result = %d", result);

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int a, b;
    double average;

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

    average = (a + b) / 2.0;

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

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int obtained, total;
    double percentage;

    scanf("%d %d", &obtained, &total);

    percentage = (double)obtained / total * 100.0;

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

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int a = 5;
    int b = ++a;

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

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int a = 5;
    int b = a++;

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

    return 0;
}

Run your program to see the output.

#include <stdio.h>

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

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

    larger = (a > b) ? a : b;

    printf("Larger = %d", larger);

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int result;

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

    printf("Result = %d", result);

    return 0;
}

Run your program to see the output.

#include <stdio.h>

int main()
{
    int a, b;

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

    printf("Integer Division = %d\n", a / b);
    printf("Floating Division = %.2f", (double)a / b);

    return 0;
}

7.31 Key Takeaway