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Re: Testing for NODATA presence in a dataset [message #33386 is a reply to message #33385]

Sun, 22 December 2002 18:33

tam
Messages: 48
Registered: February 2000

Member

David Fanning <david@dfanning.com> wrote in message news:<MPG.186e97123b2778b2989a81@news.frii.com>...
> Jonathan Greenberg (greenberg@ucdavis.edu) writes:
>
>> I'm having a problem testing for whether an entry in an array is NAN --
>> doing something like:
>>
>> If (value EQ !VALUES.F_NAN) then begin
>> print,'Not a number'
>> Endif else begin
>> print,'Is a number!'
>> Endelse
>>
>> Will always return 'Is a number', even if I set:
>> value = !VALUES.F_NAN
>>
>> What's going wrong with this?
>
> The problem is that NAN is ... well, not a number.
> Thus, you can't use it in expressions that
> require a number. (Think of it as a mathematical
> Catch-22, if you like.)
>
> The proper way to write this code is like this:
>
> If Finite(value) EQ 0 then begin
> print,'Not a number'
> Endif else begin
> print,'Is a number!'
> Endelse

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 ...

This can get optimized away if the compiler/interpreter
is poorly designed. Seemed to work for me in a quick
test though for IDL 5.2 on Linux. NaN's are not equal
to anything --- even themselves.

Regards,
Tom McGlynn

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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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