Why is my number being rounded incorrectly?

Question

This feels like the kind of code that only fails in-situ, but I will attempt to adapt it into a code snippet that represents what I\'m seeing.

float f = myFl         


        

Answer 1

Floating point has limited accuracy, and is based on binary rather than decimal. The decimal number 13.45 cannot be precisely represented in binary floating point, so rounds down. The multiplication by 20 further exaggerates the loss of precision. At this point you have 268.999... - not 269 - therefore the conversion to integer truncates to 268.

To get rounding to the nearest integer, you could try adding 0.5 before converting back to integer.

For "perfect" arithmetic, you could try using a Decimal or Rational numeric type - I believe C# has libraries for both, but am not certain. These will be slower, however.

EDIT - I have found a "decimal" type so far, but not a rational - I may be wrong about that being available. Decimal floating point is inaccurate, just like binary, but it's the kind of inaccuracy we're used to, so it gives less surprising results.