Not-a-Number

(NaN) A special {IEEE floating point} value

representing the result of a numerical operation which cannot

return a valid number value. A NaN can be the result of an

invalid operation (e.g. division by zero?); the result

returned by a library function when it would be incorrect to

return a numeric value (e.g. ?) or an undetermined value

(e.g. uninitialised variable?).

NaN is encoded as a special {bit pattern} which typically

(always?) would otherwise represent a {floating-point} number

value. It is used to signal error returns where other

mechanisms are not convenient, e.g. a hardware {floating-point

unit} and to allow errors to propagate through a calculation.

Similar bit patterns are defined to represent positive and

negative {overflow} and {underflow}.

[ANSI/IEEE Std 754-1985].

[Correct examples?]

(1997-12-03)