Step 1 of 6

if and else

So far, every line of main ran every time. Real programs need to make decisions: do this if the player is alive, that if they're not. The if statement runs a block of code only when a condition is true.

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

Exactly one of the two blocks runs. The else part is optional; without it, nothing happens when the condition is false.

#include <stdio.h>

int main(void) {
    int temperature;
    scanf("%d", &temperature);
    if (temperature >= 30) {
        printf("hot\n");
    } else {
        printf("not hot\n");
    }
    printf("done\n");
    return 0;
}

34

hot
done

After the if/else, the program continues with the next line either way, which is why done always prints.

Comparison operators

A comparison produces 1 for true or 0 for false.

The = versus == trap

= assigns and == compares. if (x = 5) doesn't check whether x is 5: it stores 5 in x, and since 5 is non-zero the condition is always true. The compiler warns about this (using the result of an assignment as a condition without parentheses); when you see that warning, you almost certainly meant ==.

Your turn: read a health value and print alive if it's greater than 0, otherwise dead.

Next: else if chains