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/GNU Fortran 7

9.159 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-7.1.0/gfortran/ISNAN.html