Arithmetic Operators

Big idea

Operators do math on values: +, -, *, /, and % (the remainder).

The idea

Two of these surprise everyone at first:

• / between two whole numbers throws away the fraction. 7 / 2 is 3, not 3.5. The next lesson shows how to keep the .5.
• % gives what's left over after dividing. 7 % 2 is 1 because 2 goes into 7 three times, with 1 left over.

Try it

Change the numbers and press Run.

#include <stdio.h>

int main(void) {
    int a = 17, b = 5;
    printf("%d + %d = %d\n", a, b, a + b);
    printf("%d - %d = %d\n", a, b, a - b);
    printf("%d * %d = %d\n", a, b, a * b);
    printf("%d / %d = %d\n", a, b, a / b);
    printf("%d %% %d = %d\n", a, b, a % b);
    return 0;
}

output
17 + 5 = 22
17 - 5 = 12
17 * 5 = 85
17 / 5 = 3
17 % 5 = 2

%% inside printf prints a single % — a lone % would start a format code.

What % is good for

The remainder answers everyday questions:

• Even or odd? n % 2 is 0 for even numbers, 1 for odd ones.
• Last digit? n % 10 — 1234 % 10 is 4.
• Minutes and seconds: 135 seconds is 135 / 60 = 2 minutes and 135 % 60 = 15 seconds.

#include <stdio.h>

int main(void) {
    int total = 200;
    printf("%d min %d sec\n", total / 60, total % 60);
    return 0;
}

3 min 20 sec

Watch it

Think of it like

Sharing 17 sweets among 5 friends. 17 / 5 is how many each friend gets (3), and 17 % 5 is how many are left in the bag (2).

🕷️ Spidey-sense: Web-shooters hold 7 cartridges and I've got 23. 23 / 7 = 3 full reloads, 23 % 7 = 2 spare. Division tells me the reloads, remainder tells me what's rattling around in my pocket.

Common mistake

Dividing by zero. x / 0 or x % 0 with whole numbers crashes the program — check the divisor first.

Key terms

• Operator — A symbol that does something to values, like + or %.
• Operand — A value an operator works on: in 7 + 2, the operands are 7 and 2.
• Remainder (%) — What's left after dividing whole numbers; also called modulo.