Question

Suppose you input A = 15, B = 6, The answer is 18. What algorithm do I need?

This is what I try, but it doesn’t work:

#include <stdio.h>

int main() {
int a, b;

scanf("%d %d", &a, &b);

for (  ; a % b != 0; a++ ) {
    if ( a % b == 0 ) {
        printf("%d\n", a);
        return a;
    }
}
return 0;
}

I get infinite loop.

Answer 1

The question (now) asks for:

The next multiple of b that is not less than a?

Using your notation of a and b, you can write it directly like this:

int NextMultipleUp(int a, int b)
{
    int r = a % b;
    if (r == 0)
        return a;
    else
        return a + b - r;
}

---

The question originally asked for

The next multiple of a that is not greater than b

And for that the answer is

int NextMultipleDown(int a, int b)
{
    return b - b % a;
}

This was the answer for which the original comments applied to.