#415 – Be Careful When Checking Floating Point Numbers for Equality

When you check two floating point numbers for equality in C#, you might sometimes be surprised at the result.

Consider the following example.  f2 is equal to six sixths (1.0), minus 1.0–which should be equal to 0.0.  But when we compare the result to 0.0, we see that the values are not equal.

            float f1 = 1.0f / 6.0f;
            float f2 = (f1 * 6.0f) - 1.0f;   // Should be 0.0
            float f3 = 0.0f;

            bool check = (f2 == f3);    // Should be true, but is false !

This happens because floating point numbers can’t necessarily be represented exactly when stored in a floating point variable.  Instead, the value stored can be very close, but not equal to, the desired value.

Here’s the output from the example above.  f2 should be equal to 0.0, after our calculation.  It is instead very nearly equal to 0.0.  Because of this, the comparison to the constant 0.0 fails.