C# float magically changing value

Question

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 divisi

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.