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/GNU Fortran 5

9.151 ISNAN — Test for a NaN

Description:
ISNAN tests whether a floating-point value is an IEEE Not-a-Number (NaN).
Standard:
GNU extension
Class:
Elemental function
Syntax:
ISNAN(X)
Arguments:
X Variable of the type REAL.
Return value:
Returns a default-kind LOGICAL. The returned value is TRUE if X is a NaN and FALSE otherwise.
Example:
program test_nan
  implicit none
  real :: x
  x = -1.0
  x = sqrt(x)
  if (isnan(x)) stop '"x" is a NaN'
end program test_nan

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Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gfortran/ISNAN.html