if and else

Big idea

if runs a block of code only when a condition is true. else runs when it isn't.

The idea

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

The condition goes in brackets. In C, a condition is just a number: 0 means false, anything else means true. Comparisons like age >= 18 produce 1 (true) or 0 (false).

int age = 20;
if (age >= 18) {
    printf("You can vote\n");
} else {
    printf("Not yet\n");
}

The else part is optional — leave it out when there's nothing to do on the false side.

Try it

Change temperature and run it again — try 30, then 31.

#include <stdio.h>

int main(void) {
    int temperature = 34;
    if (temperature > 30) {
        printf("It's hot. Drink water.\n");
    } else {
        printf("Nice weather.\n");
    }
    printf("Done.\n");
    return 0;
}

output
It's hot. Drink water.
Done.

Done. prints either way — it's outside both blocks.

Reading input and deciding

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);
    if (n % 2 == 0) {
        printf("%d is even\n", n);
    } else {
        printf("%d is odd\n", n);
    }
    return 0;
}

input
7

output
7 is odd

Watch it

Think of it like

A fork in a road with a sign: "If it's raining, take the covered path; otherwise take the park." You only walk one of the two paths.

🕷️ Spidey-sense: My spidey-sense is basically one big if. If danger, dodge. Else, keep eating my sandwich.

Common mistake

A semicolon right after the condition:

if (age >= 18);          // ✗ this ; is an empty statement — the if ends here
{
    printf("Vote!\n");  //   so this block ALWAYS runs
}

The compiler accepts it, which makes it hard to spot. No ; after if (...).

Key terms

• Condition — An expression that's true (non-zero) or false (zero).
• Block — Code grouped in { }.
• Branch — One of the paths an if/else can take.