Division to the nearest 1 decimal place without floating point math?

Question

I am having some speed issues with my C# program and identified that this percentage calculation is causing a slow down. The calculation is simply n/d * 100. Both the numerator and denominator can be any integer number. The numerator can never be greater than the denominator and is never negative. Therefore, the result is always from 0-100. Right now, this is done by simply using floating point math and is somewhat slow, since it's being calculated tens of millions of times. I really don't need anything more accurate than to the nearest 0.1 percent. And, I just use this calculated value to see if it's bigger than a fixed constant value. I am thinking that everything should be kept as an integer, so the range with 0.1 accuracy would be 0-1000. Is there some way to calculate this percentage without floating point math?

Here is the loop that I am using with calculation:

for (int i = 0; i < simulationList.Count; i++)
{
    for (int j = i + 1; j < simulationList.Count; j++)
    {
        int matches = GetMatchCount(simulationList[i], simulationList[j]);
        if ((float)matches / (float)simulationList[j].Catchments.Count > thresPercent)
        {
            simulationList[j].IsOverThreshold = true;
        }
    }
}

Answer 1

Instead of n/d > c, you can use n > d * c (supposing that d > 0).
(c is the constant value you are comparing to.)

This way you don't need division at all.

However, watch out for the overflows.

Answer 2

If your units are in tenths instead of ones, then you can get your 0.1 accuracy using integer arithmetic:

Instead of:

for (...)
{
    float n = ...;
    float d = ...;

    if (n / d > 1.4) // greater than 140% ?

...do something like:

for (...)
{
    int n = 10 * ...;
    int d = ...;
    if (n / d > 14) // greater than 140% ?