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Re: Filtering out NaNs [message #8633 is a reply to message #8528]

Sun, 23 March 1997 00:00

Mark Hadfield
Messages: 783
Registered: May 1995

Senior Member

Stein Vidar Hagfors Haugan <steinhh@rigil.uio.no> wrote in
article <5gp9oq$8hp$1@ratatosk.uio.no>...
>
> NaNs are characterized by the fact that they are *not* equal
> to any number (that's what it says, isn't it :-)
>
> In fact, it's not even equal to itself - and this is the distinguishing
> feature that must be used to pick them out. I.e.,
>
> a(where(a ne a)) = missing_flag

I've always used the finite function, eg:

a(where(1-finite(a))) = missing_flag

However FINITE returns 0 for infinities as well as NaNs and I suppose this
could be a problem in some instances.

I just tried 4 different comparison operators against pairs of NaN's and
found that on my system (IDL 5.0 prerelease on WinNT/Intel) NaN IS equal to
itself, but it's also greater than itself.

IDL> a = !values.f_nan
IDL> print, a eq a, a ne a, a gt a, a lt a
1 0 1 0
% Program caused arithmetic error: Floating illegal operand

Curious.

============================================================ ==
Mark Hadfield NIWA (Taihoro Nukurangi)
PO Box 14-901
m.hadfield@niwa.cri.nz Wellington, New Zealand

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[Message index]

		Filtering out NaNs By: iarla on Wed, 19 March 1997 00:00
		Re: Filtering out NaNs By: steinhh on Wed, 19 March 1997 00:00
		Re: Filtering out NaNs By: Mark Hadfield on Sun, 23 March 1997 00:00
		Re: Filtering out NaNs By: Christian Marquardt on Fri, 21 March 1997 00:00

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