Step 3 of 6

Remainder with %

The % operator (called modulo or remainder) gives what's left over after an integer division.

#include <stdio.h>

int main(void) {
    printf("%d %d\n", 17 / 5, 17 % 5);
    printf("%d %d\n", 20 / 5, 20 % 5);
    printf("%d\n", 9 % 2);
    return 0;
}

3 2
4 0
1

17 divided by 5 is 3 with 2 left over (3 times 5 is 15, and 17 minus 15 is 2). / gives the 3 and % gives the 2. Together they always satisfy (a / b) * b + a % b == a.

What it's good for

• Even or odd: n % 2 is 0 for even numbers and 1 for odd ones.
• Every Nth time: i % 10 == 0 is true for 0, 10, 20 and so on.
• Wrapping around: (hour + 5) % 24 keeps a clock hour between 0 and 23.
• Splitting units: 135 cents is 135 / 100 dollars and 135 % 100 cents. The same trick splits seconds into hours, minutes and seconds.

% only works on integers. For doubles there's a library function called fmod, but you'll rarely need it.

Your turn: a match lasted 754 seconds. Print it as minutes and seconds: 12 min 34 sec.

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