CODEWITHSUNDEEP
c#floating-point
alex
alex

Reputation: 1278

C# float magically changing value


I have a float holding a very important value, which has to be VERY exact.
The problem I have is I'm changing the value of the float ALWAYS only + and - (No division here).
After changing the value about 20 times, the value isn't 0.05 as expected, its 0.0499989.
Why is this? Im not casting the float, I'm not dviding it, but it magically changes from 0.05 to 0.049989.
I tried Math.Round(value, 2); but it doesn't return 0.05 either. What should I do now??

Upvotes: 4

Views: 3891

Answers (4)

Stephen Canon
Stephen Canon

Reputation: 106147

Just like an int variable can only hold integers in a certain range, a float can only hold certain values. 0.05 is not one of them.

If you set an int variable to (say) 3.4, it won't actually hold the value 3.4; it will hold that value converted to a representable int value: 3.

Similarly, if you set a float variable to 0.05, it won't get that exact value; it will instead get that value converted to the closest value representable as a float. This is what you are seeing.

Upvotes: 2

SLaks
SLaks

Reputation: 887405

Float and double values are stored in binary (base 2).
Therefore, they cannot accurately represent numbers like .3 that have no finite-length representation in binary.

Similarly, a decimal, which is stored in base 10, cannot accurately represent numbers like 1/3 that have no finite-length representation in decimal.

You need an arbitrary-precision arithmetic library.

Upvotes: 4

BrokenGlass
BrokenGlass

Reputation: 160882

The problem is that with float some fractional numbers cannot be exactly represented. Consider using the decimal data type if you only use + and -, you shouldn't have that problem, since decimal uses a base 10 internally.

Upvotes: 2

Jeremy Fuller
Jeremy Fuller

Reputation: 3401

Floating point variables in many (if not most) languages only hold an imprecise approximation of the actual value. You can solve the issue in C# by using the decimal data type. See this SO question.

Upvotes: 11

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