Question

I’m trying to think of a function in C that would satisfy the following conditions:

• It accepts an integer greater than 0 as an argument;
• It rounds that integer up to the nearest value so that only the first
  digit is not a zero

For example:

53 comes out as 60..

197 comes out as 200..

4937 comes out as 5000..

Is there a way to do this so that the requirement is satisfied regardless of the number of trailing zeroes?

For example, I understand how I could do it in any individual case. divide 53 by 10 then ceil(), multiply by 10, but I would like one that can handle any value.

Opinions? Ideas?

Answer 1

It’s unnecessary to convert the number to a string and back. You can do this using basic modulo arithmetic and multiplication and division.

Here’s a pure numeric solution, hopefully somewhat more efficient in terms of running time:

int round_up_to_max_pow10(int n)
{
    int tmp = n;
    int i = 0;
    while ((tmp /= 10) >= 10) {
        i++;
    }

    if (n % (int)(pow(10, i + 1) + 0.5)) {
        tmp++;
    }

    for (; i >= 0; i--) {
        tmp *= 10;
    }

    return tmp;
}

printf("Original: %d; rounded: %d\n", 4937, round_up_to_max_pow10(4937));