C - Logical Operators

Logical operators in C are used to combine or invert boolean expressions. They are essential for decision making in control flow statements.

Learning Objectives

• Use &&, ||, and ! correctly.
• Explain short-circuit behavior and truthiness.

Prerequisites

• C - Operators
• C - Datatypes

List of Logical Operators

• && Logical AND
• || Logical OR
• ! Logical NOT

Short-circuit Evaluation

a && b evaluates b only if a is true (nonzero). a || b evaluates b only if a is false (zero).

#include <stdio.h>

int side(char *name, int v) { printf("%s\n", name); return v; }

int main(void) {
    int x = 1, y = 0;
    if (side("left", x) && side("right", y)) {
        printf("both\n");
    }
    if (side("left", x) || side("right", y)) {
        printf("one or both\n");
    }
}

Output shows when the right side is skipped.

Truthiness

Zero is false; nonzero is true. Logical operators return 0 or 1 in C.

Common Pitfalls

• Using & or | (bitwise) when you meant && or || (logical).
• Expecting logical operators to preserve non-boolean operand values—they normalize to 0 or 1.

Checks for Understanding

1. When is a || b’s right side evaluated?
2. What is !42?

Summary

Logical operators are fundamental for writing conditions and controlling program flow in C. Understand short-circuiting to avoid unintended side effects.