JBS@IBM.COM (01/11/91)
> if(dnum*0d0 .ne. 0d0) write(*,1) >1 format(' The variable dnum is equal to NAN!') Is this correct? Suppose dnum is Inf. James B. Shearer
khb@chiba.Eng.Sun.COM (Keith Bierman fpgroup) (01/11/91)
In article <9101110259.AA28451@ucbvax.Berkeley.EDU> JBS@IBM.COM writes: Newsgroups: comp.lang.fortran Date: 11 Jan 91 02:56:52 GMT Sender: daemon@ucbvax.BERKELEY.EDU Lines: 5 > if(dnum*0d0 .ne. 0d0) write(*,1) >1 format(' The variable dnum is equal to NAN!') Is this correct? Suppose dnum is Inf. No, it isn't. Aside from reading the standard, let's try code! real dnum/1.0/,x/0.0/ dnum=dnum/x print*,dnum if(dnum*0d0 .ne. 0d0) print*," is nan?",dnum end f77 bork.f && a.out bork.f: MAIN: Inf is nan? Inf On some systems, vendors will have been kind and provided library functions. For example, on a Sun one might code something like real dnum/1.0/,x/0.0/ integer ir_isnan,ir_isinf ! libm function dnum=dnum/x print*,dnum if (ir_isnan(dnum)) print*,"dnum IS NaN" if (ir_isinf(dnum)) print*,"dnum IS Inf" end f77 bork.f && a.out bork.f: MAIN: Inf dnum IS Inf It would be more in keeping with the notion of standard compliance, to not treat ir_* as both a logical and an integer, so one might quite reasonably prefer to either declare ir* as logicals (despite the libm definition) or to test the value if (ir() .eq. 1) On systems which are ieee hw compliant, but have not provided the sw hooks, one can easily code these functions in whatever language seems most appropriate. To test for NaN it is probably best to use the result of the appropriate mask ANDed with the value under test. note that on a Sun, unless user overridden there will also be a message from ieee_retrospective, ala Warning: the following IEEE floating-point arithmetic exceptions occurred in this program and were never cleared: Division by Zero; Final note, if memory serves, the right mask is 0x7ff00000 cheers -- ---------------------------------------------------------------- Keith H. Bierman kbierman@Eng.Sun.COM | khb@chiba.Eng.Sun.COM SMI 2550 Garcia 12-33 | (415 336 2648) Mountain View, CA 94043
mcqueen@acf4.NYU.EDU (David M. McQueen) (01/12/91)
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.mcqueen@acf4.NYU.EDU (David M. McQueen) (01/12/91)
From mcqueen@acf4.NYU.EDU Fri Jan 11 14:00:26 1991
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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
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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.