Why is modulus operator not working for double in c#?

Question

Consider this:

double x,y;
x =120.0;
y = 0.05;

double z= x % y;

I tried this and expected the result to be 0, but it came out 0.04933333.

However,

x =120.0;
y = 0.5;
double z= x % y;

did indeed gave the correct result of 0.

What is happening here?

I tried Math.IEEERemainder(double, double) but it's not returning 0 either. What is going on here?

Also, as an aside, what is the most appropriate way to find remainder in C#?

Answer 1

doubles are never exact values. 120 is probably stored lossless, but 0.05 not. 0.5 is 1/2, so it could be stored lossless and you don't get a rounding error.

To have exact (decimal) numbers, use decimal instead.

decimal x,y;
x =120.0M;
y = 0.05M;

decimal z = x % y;  // z is 0

Answer 2

http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems can help you understand why you get these "strange" results. There's a particular precision that floating point numbers can have. Just try these queries and have a look at the results:

0.5 in base 2

0.05 in base

Answer 3

Modulus is defined in msdn as:

x - (x / y) * y

If you do it this way manually it seems to get the correct result.

Answer 4

You could do something like:

double a, b, r;

a = 120;
b = .05;

r = a - Math.floor(a / b) * b;

This should help ;)

Answer 5

Modulus should only be used with integer. The remainder come from an euclidean division. With double, you can have unexpected results.

See this article

Answer 6

I believe if you tried the same with decimal it would work properly.