Closest divisible integer

Question

Example.

int a = 254;
int b = 25;

int c = (closest integer to `a` that is divisible by `b`)

How can I find the integer c? The result of that example is c = 250.

Answer 1

There are two cases to consider:

1. The closest integer that is less than or equal to a:

   int c1 = a - (a % b);
   

2. The closest integer that is greater than a:

   int c2 = (a + b) - (a % b);
   

Then we need to check which is closer to a and return that:

int c;
if (a - c1 > c2 - a) {
    c = c2;
} else {
    c = c1;
}

So we could create a closestInteger() method like this:

static int closestInteger(int a, int b) {
    int c1 = a - (a % b);
    int c2 = (a + b) - (a % b);
    if (a - c1 > c2 - a) {
        return c2;
    } else {
        return c1;
    }
}

Example:

System.out.println(closestInteger(254, 25));
System.out.println(closestInteger(9, 5));

Output:

250
10

Answer 2

You have to check on both sides of a. So we set 'c1' to the closest integer below (or equal to) a, and c2 to be the closest integer above it. Then compare the differences. If c1 is closer to a, set c to c1, otherwise c2.

int c1 = a - (a % b);
int c2 = c1 + b;
int c = a - c1 < c2 - a ? c1 : c2;