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From: mcqueen@acf4.NYU.EDU (David M. McQueen)
Newsgroups: comp.lang.fortran
Subject: Re: detecting Nan's
Message-ID: <9101111859.AA27169@acf4.NYU.EDU>
Date: 11 Jan 91 18:59:59 GMT
Lines: 54

From mcqueen@acf4.NYU.EDU  Fri Jan 11 14:00:26 1991
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From: mcqueen@acf4.NYU.EDU (David M. McQueen)
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Relay-Version: version nyu B notes v1.6.1 1/11/90; site acf4.NYU.EDU
From: mcqueen@acf4.NYU.EDU (David M. McQueen)
Date: 11 Jan 91 13:59 EST
Date-Received: 11 Jan 91 13:59 EST
Subject: Re: detecting Nan's
Message-ID: <13160008@acf4.NYU.EDU>
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Newsgroups: comp.lang.fortran
Posting-Version: version nyu B notes v1.6.1 1/11/90; site acf4.NYU.EDU
Organization: New York University
References: <9101110259.AA28451@ucbvax.Berkeley.EDU>

Recently, Frank Elliott, one of the mathematicians here at the Courant
Institute, found himself in the position of trying to detect NaN in his
C code on a Stellar GS2000. Although the man pages suggested that there were
library functions for NaN-detection (e.g., "isnand") , in fact the installed
library did not have these functions. Frank came up with a clever kludge
which I have translated to (and used in) Fortran. We have taken to calling
this "Elliott's Device". The Fortran translation below is not guaranteed to
be portable, but I think it could be made portable with relatively little
work. I would not recommend this for numerically intensive applications :-).

      character*80 test
      x = 0.0
      y = 0.0
      z = x/y
      write(6,*) z
      write(test,*) z
c     Elliott's Device
      write(6,'(a)') test
      if (test(1:4) .eq. ' NaN') write(6,*) 'z is NaN'
      end

-------------------------------------------------------------------------------
PS:

I tried to post this earlier and it may or may not have actually gotten out. I
have received email (from Peter Shenkin; thank you, Peter) that the line:

      write(test,*) z

is an illegal unformatted internal write. I have done this successfully on
a Sun 3/50 running f77 under SunOS 4.1. No guarantees can be made about
other systems.