🧮 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

These two terms are related but not identical. An expression produces a value or performs an operation; a statement is a complete instruction that controls program execution or performs an action.

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 stores a value in an object. The assignment expression itself also has the value that was assigned.

int a;
int b;

b = (a = 10);

Assignment can therefore be chained, but separate assignments are often easier to read for beginners.

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 determines which operator binds more strongly when an expression contains different operator levels.

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);

Since both operands are integers, integer division is performed. The result is an integer; the fractional part is not retained.

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;

The integer operand is converted to a compatible floating-point type 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' participates in integer expressions because character types have integer representations.

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 evaluates its left operand, then its right operand. The value of the complete comma expression is the value of the rightmost 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

Avoid modifying the same scalar object multiple times in one expression when the operations are not safely sequenced.

i++ + ++i

Prefer clear steps:

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

This is easier to read, easier to debug, and avoids depending on unsafe evaluation behavior.

7.26 Common Expression Mistakes

7.27 Quick Revision

7.28 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
", a / b);
    printf("Floating Division = %.2f", (double)a / b);

    return 0;
}

7.30 Key Takeaway