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Re: Testing for NODATA presence in a dataset [message #33374 is a reply to message #33372]

Thu, 26 December 2002 08:25

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Tom McGlynn (tam@lheapop.gsfc.nasa.gov) writes and
Bill Thompson confirms:

> That doesn't distinguish NaN from the infinities.
> The standard trick in any language for looking for NaN's is
>
> if x ne x then begin
> print,'This is a NaN'
> endif else ...

Humm, well, consider this little test in IDL 5.5 or 5.6
for Windows:

IDL> a = [ 1.0, 2.0, !Values.F_NAN, 4.0, !Values.F_NAN ]
IDL> print, a
1.00000 2.00000 NaN 4.00000 NaN
IDL> print, a(1)
2.00000

All well and good so far. Test the algorithm.

IDL> if a(1) ne a(1) THEN print, 'NAN' ELSE print, 'Number'
Number

Perfect. Working fine. Now text NAN.

IDL> print, a(2)
NaN
IDL> if a(2) ne a(2) THEN print, 'NAN' ELSE print, 'Number'
Number
% Program caused arithmetic error: Floating illegal operand

Oh, oh. What's up with that? And a floating illegal operand to
boot. :-(

How about the array in general?

IDL> print, array ne array
0 0 0 0 0
% Program caused arithmetic error: Floating illegal operand

Humm. I presume you guys have a reason for thinking
like you do. Any insights?

Cheers,

David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155

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[Message index]

		Re: Testing for NODATA presence in a dataset By: thompson on Thu, 26 December 2002 12:58
		Re: Testing for NODATA presence in a dataset By: David Fanning on Thu, 26 December 2002 08:25
		Re: Testing for NODATA presence in a dataset By: thompson on Tue, 24 December 2002 07:11
		Re: Testing for NODATA presence in a dataset By: Kenneth P. Bowman on Mon, 23 December 2002 06:42
		Re: Testing for NODATA presence in a dataset By: tam on Sun, 22 December 2002 18:33
		Re: Testing for NODATA presence in a dataset By: David Fanning on Sat, 21 December 2002 14:20
		Re: Testing for NODATA presence in a dataset By: tam on Mon, 30 December 2002 06:52

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