Apparent F#/BCL floating point bug

Question

The following is in FSI:

> System.Math.Round(0.2916, 2);;
val it : float = 0.29
> it * 100.;;
val it : float = 29.0
> int it;;
val it : int = 28
         


        

Answer 1

float is binary floating point data type. Not all decimal floating point values can be represented exactly as binary floating point. What FSI shows as 29 is actually 28.999999999999996.

If you want to store precise decimal floating point values, use decimal data type (to make a number literal decimal instead of float add m to it - e.g. 0.2916m).

> System.Math.Round(0.2916m, 2);;
val it : decimal = 0.29M
> it * 100m;;
val it : decimal = 29.00M
> int it;;
val it : int = 29