I've been trying to find a standards-compliant way to check for Infinite and NaN values in Fortran 90/95 but it proved harder than I thought.

  • I tried to manually create Inf and NaN variables using the binary representation described in IEEE 754, but I found no such functionality.
  • I am aware of the intrinsic ieee_arithmetic module in Fortran 2003 with the ieee_is_nan() and ieee_is_finite() intrinsic functions. However it's not supported by all the compilers (notably gfortran as of version 4.9).

Defining infinity and NaN at the beginning like pinf = 1. / 0 and nan = 0. / 0 seems hackish to me and IMHO can raise some building problems - for example if some compilers check this in compile time one would have to provide a special flag.

Is there a way I can implement in standard Fortran 90/95?

function isinf(x)
! Returns .true. if x is infinity, .false. otherwise
...
end function isinf

and isnan()?

fortran fortran95
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