What is the rationale for all comparisons returning false for IEEE754 NaN values?

Question

Why do comparisons of NaN values behave differently from all other values? That is, all comparisons with the operators ==, <=, >=, <, > where one or both values is NaN

Answer 1

It only looks peculiar because most programming environments that allow NaNs do not also allow 3-valued logic. If you throw 3-valued logic into the mix, it becomes consistent:

• (2.7 == 2.7) = true
• (2.7 == 2.6) = false
• (2.7 == NaN) = unknown
• (NaN == NaN) = unknown

Even .NET does not provide a bool? operator==(double v1, double v2) operator, so you are still stuck with the silly (NaN == NaN) = false result.