Multiply and divide

Add multiplication, division and the divide-by-zero check.

The multiply and divide functions

Now we add multiply and divide functions. multiply multiplies two numbers; divide divides them. For division we must check b == 0 to prevent a divide-by-zero error:

int multiply(int a, int b) {
    return a * b;
}

int divide(int a, int b) {
    if (b == 0) return 0;
    return a / b;
}

Division by zero is undefined; the check prevents a crash.

Exercise Add multiply and divide

Add int multiply(int a, int b) and int divide(int a, int b) functions to the code below. Inside divide, include if (b == 0) return 0;. Call both in main: printf("%d\n", multiply(5, 3)); and printf("%d", divide(6, 3));.

Show example solution

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int multiply(int a, int b) {
    return a * b;
}

int divide(int a, int b) {
    if (b == 0) return 0;
    return a / b;
}

int main(void) {
    printf("Calculator\n");
    printf("%d\n", add(5, 3));
    printf("%d\n", subtract(5, 3));
    printf("%d\n", multiply(5, 3));
    printf("%d", divide(6, 3));
    return 0;
}

The divide-by-zero danger

Division by zero causes the program to crash immediately. So we must check the divisor in the divide function:

if (b == 0) return 0;  // safe return

You can return a different value instead of zero; what matters is that the program does not crash. Production code usually uses an error code or a special value.

Test yourself

multiply return statement

int multiply(int a, int b) { return a ____ b; }

Show answer

int multiply(int a, int b) { return a * b; }

What should be checked before division?
- a > 0
- b == 0
- b > 0
- a < 0
Show answer

b == 0 — The b == 0 check prevents a divide-by-zero error.
Match the functions

add

Addition
subtract

Subtraction
multiply

Multiplication
divide

Division

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