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Code Inspection: Compare of float numbers with equality operator

Using the == operator to compare floating-point numbers is, generally, a bad idea. The problem arises from the fact that, typically, results of calculations have to be ‘fitted’ into floating-point representation, which does not always match the perceived reality of what result should be produced.

For example, suppose you take a root of -4 using var root = Complex.Sqrt(new Complex(-4, 0)). Then, you decide to ensure that the real part of the result is 0 (zero), by asserting that root.Real == 0.0. Unfortunately, the actual result of the calculation is (1.22460635382238E-16, 2), so the comparison will, predictably, fail.

In order to do these comparisons correctly, you need to specify a tolerance, i.e., a value to indicate by how much the result can diverge from the intended value. Then, the comparison above can be rewritten, e.g., as Math.Abs(root.Real)< 1.E-10. Similarly, comparisons between two values can take the form of Math.Abs(x - y)< tolerance. The tolerance value really depends on the precision of the calculations being performed but, in most cases, should be at least the Epsilon value ( float.Epsilon, double.Epsilon, and so on). The Epsilon is a constant that denotes the smallest non-zero value that can be represented with the type of floating number you are using. Please note that, in the majority of cases, the Epsilon value is too low to be used as a reliable tolerance value.

See Also

Last modified: 19 August 2016