C if-else Statement

What You’ll Learn

The if-else statement handles two outcomes: when a condition is true, one block runs; when false, the else block runs instead. Use it whenever your program must always choose between two paths—pass or fail, even or odd, yes or no.

Definition and Usage

An if-else statement extends plain if with an alternative. C evaluates the condition once. If it is true (non-zero), the if body runs and the else body is skipped. If false (zero), the if body is skipped and the else body runs.

Use if-else when both outcomes need explicit code. If you only care about the true case, a plain if is enough. For three or more mutually exclusive cases, use an else if chain in the next tutorial.

📝 Syntax

Standard two-way branch in C:

if (condition) {
    // runs when condition is true
} else {
    // runs when condition is false
}

Flow

• Evaluate condition once.
• True → execute if block, skip else.
• False → skip if block, execute else.
• Continue with the next statement after the whole construct.

Preview: else if (next tutorial)

if (score >= 90) {
    printf("A\n");
} else if (score >= 80) {
    printf("B\n");
} else {
    printf("Below B\n");
}
/* See /c/statements/else-if for full coverage */

Compile

gcc program.c -std=c11 -o program

Examples Gallery

Compile with gcc example.c -std=c11 -o example. Each program shows a classic if-else pattern.

#include <stdio.h>

int main(void) {
    int number = 10;

    if (number > 0) {
        printf("The number is positive.\n");
    } else {
        printf("The number is non-positive.\n");
    }

    return 0;
}

The number is positive.

#include <stdio.h>

int main(void) {
    int n = 7;

    if (n % 2 == 0) {
        printf("%d is even.\n", n);
    } else {
        printf("%d is odd.\n", n);
    }

    return 0;
}

7 is odd.

#include <stdio.h>

int main(void) {
    int score = 42;

    if (score >= 50) {
        printf("Result: PASS (%d)\n", score);
    } else {
        printf("Result: FAIL (%d)\n", score);
    }

    return 0;
}

Result: FAIL (42)

#include <stdio.h>

int main(void) {
    int age = 25;
    char gender = 'F';

    if (age > 18) {
        if (gender == 'M') {
            printf("Adult male.\n");
        } else {
            printf("Adult female.\n");
        }
    } else {
        printf("Not an adult.\n");
    }

    return 0;
}

Adult female.

#include <stdio.h>

int main(void) {
    int a = 30;
    int b = 12;
    int min;

    if (a < b) {
        min = a;
    } else {
        min = b;
    }

    printf("min(%d, %d) = %d\n", a, b, min);
    return 0;
}

min(30, 12) = 12

🚀 Common Use Cases

• Pass/fail — threshold comparisons.
• Sign tests — positive vs zero/negative.
• Parity — even vs odd.
• Null safety — use pointer vs report error.
• Min/max — pick the smaller or larger of two values.
• Toggle flags — flip state between two choices.

📝 Notes

• else must follow an if or else if—never alone.
• Only one of if or else runs per evaluation.
• Without braces, else binds to the nearest preceding if.
• For three or more cases, use else if (see next tutorial).
• else needs no condition—it is the default when all prior tests fail.
• You can nest if-else inside either branch.

⚡ Optimization

Modern CPUs predict branches; keep hot paths in the if branch when one outcome is far more common. For simple min/max, the ternary operator min = (a < b) ? a : b; compiles to similar code but use if-else when beginners need clarity or when branches contain multiple statements.

💡 Best Practices