NaN
In computing, NaN (Not a Number) is a value or symbol that is usually produced as the result of an operation on invalid input operands, especially in floating-point calculations. For example, most floating-point units are unable to calculate the square root of negative numbers, and will instead indicate that the operation was invalid and return a NaN result.
In floating-point calculations, NaN is not the same as infinity, although both are typically handled as special cases in floating-point representations of real numbers as well as in floating-point operations. An invalid operation is also not the same as an overflow (which might return an infinity) or an underflow (which would return the smallest normal number, a subnormal number, or zero).
A NaN does not compare equal to any floating-point number or NaN, even if the latter has an identical representation. One can therefore test whether a variable has a NaN value by comparing it to itself.
In the IEEE floating-point standard, arithmetic operations involving NaN always produce NaN, allowing the value to propagate through a calculation. In the current (proposed) revision of that standard, there are two exceptions to this rule: the maxnum and minnum functions return the maximum and minimum of two operands, and will favour numbers (if just one of the operands is a NaN then the other operand is returned). This behaviour is useful in the statistical processing of sparse data.
