ObjectPascal
NaN
Constant
Not a real number Math unit
  const NaN = 0.0 / 0.0;
Description
The NaN constant provides a standard undetrmined number value for assigning to floating point variables.
 
It allows the floating point number to be treated as unassigned.
Related commands
Infinity Floating point value of infinite size
IsInfinite Checks whether a floating point number is infinite
IsNaN Checks to see if a floating point number holds a real number
 
Example code : Assign NAN to a number and then test using IsNaN
var
  float : Double;

begin
  // Set the number to an invalid number
  float := NaN;     // Equivalent to 0.0/0.0

  // Although an invalid number, we can still display it
  WriteLn('float = '+FloatToStr(float));

  // And we can test to see if it is a valid number
  if IsNaN(float)
  then WriteLn('float is not a number')
  else WriteLn('float = '+FloatToStr(float));
end;
Show full unit code
   float = NAN
   float is not a number