Double precision constant for Not a Number. NAN_DBL is considered unordered, which means all comparisons involving it are false except for not equal (operator!=). As a consequence, NAN_DBL == NAN_DBL is false but NAN_DBL!= NAN_DBL is true. Additionally, there are multiple bit representations for Not a Number, so if you must test if your value is NAN_DBL, use isnan().

public const double NAN_DBL = NaN