How do I check if a double is a multiple of another double in c#?

Question

I am currently attempting:

if (4.750 % 0.125 == 0 )

but it looks like c# doesn't want decimal for mod operation. How do I check if a number is a multiple of 0.125 then?

thanks!

That might be tricky to check, see https://stackoverflow.com/questions/588004/is-floating-point-math-broken — Progman, Feb 19 '20 at 21:19
*"it looks like c# doesn't want decimal for mod operation"* what do you mean by this? Are you getting an error? If so, what is it? Also, those are `double`, not `decimal` types - add an `m` after the numbers to force them to `decimal`). — Rufus L, Feb 19 '20 at 21:26
Possibly use plain division and see whether the result is (nearly) an integer? The trick would be to define that "nearly" — Hans Keﬆing, Feb 19 '20 at 21:27
I would find a way to make them integers, so in this case multiple each side by 1000 so it would be 4750 % 125 — Yoosh, Feb 19 '20 at 21:28
Code in the question compiles and runs fine producing expected "true" as result. Please review your question and make sure to show [MCVE] as well write exact errors/unexpected behavior you observe. Make sure to check MSDN and https://stackoverflow.com/questions/20671518/how-does-modulus-operation-works-with-float-data-type and [edit] your post with all necessary details as well as how it does not work for you. — Alexei Levenkov, Feb 19 '20 at 21:31
If you're worried about floating-point rounding errors (i.e. you need precise values for comparison or other mathematic operations), use `decimal` instead of `double`. — Rufus L, Feb 19 '20 at 21:35

score 1 · Answer 1 · answered Feb 19 '20 at 21:25

1

Comparing floats/doubles with zero is pretty dangerous, because of the floating-point precision. You can do two things:

multiply both numbers by 1000 (on example), then cast to int and do your comparison

or

if (Math.Abs(a % b) < 0.001)

answered Feb 19 '20 at 21:25

Tearth

Why not just use `decimal` instead of `double`? – Rufus L Feb 19 '20 at 21:32
Yeah, then everything should be ok with `a % b == 0`. But OP is saying about doubles so I gave the solution for this type (maybe he had to use them instead of decimals). – Tearth Feb 19 '20 at 21:37

score 1 · Answer 2 · answered Feb 19 '20 at 21:30

What you could do is create a custom function to find the mod of any decimals for you:

static double DecimalMod(double a, double b) {
        if (a < b) {
            return a;
        }

        return DecimalMod(a - b, b);
}

And since comparing doubles will be annoying with floating-point precision, you'll want to use the bounds checking of:

if (Mathf.Abs(DecimalMod(4.750, 0.125)) < 0.01) {
     // Do Stuff
}```

2 Answers2