| Re: Testing for NODATA presence in a dataset [message #33372] |
Thu, 26 December 2002 12:58  |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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Hmmm, you're right. This seems to be a Windows versus Unix thing. I just
tried it myself on my workstation and my laptop, both in IDL 5.4, and the
behavior under Windows was as you describe. I guess that means you have to use
the FINITE() function after all.
Looking up the documentation, I realize that the keywords /NAN and /INFINITY
were added in IDL version 5.2, so that FINITE(X,/NAN) is equivalent to X NE X,
except that it also works under Windows.
In older versions of IDL, I guess you'd have to check explicitly for infinities
to distinguish them from NaNs. Fortunately, there are only four different
infinity values, as opposed to 9E15 different NaNs.
Sorry for giving wrong information,
William Thompson
David Fanning <david@dfanning.com> writes:
> 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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| 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
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Senior Member |
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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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| Re: Testing for NODATA presence in a dataset [message #33378 is a reply to message #33374] |
Tue, 24 December 2002 07:11 |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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tam@lheapop.gsfc.nasa.gov (Tom McGlynn) writes:
> 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
I would like to echo Tom's response, and remind people that there is a whole
family of values which are interpreted as NaN. The values returned by
!VALUES.F_NAN and !VALUES.D_NAN are just the simplest forms. The "X NE X"
syntax will catch them all, and has the additional bonus of working no matter
whether the data is single precision, double precision, or even complex.
Bill Thompson
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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
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Member |
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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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| Re: Testing for NODATA presence in a dataset [message #33388 is a reply to message #33386] |
Sat, 21 December 2002 14:20 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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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
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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| Re: Testing for NODATA presence in a dataset [message #33464 is a reply to message #33374] |
Mon, 30 December 2002 06:52 |
tam
Messages: 48
Registered: February 2000
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Member |
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David Fanning wrote:
> 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
Just to follow up on Bill's message.... I did warn in my first message
that interpreters had been known to screw up this comparison, but I
believe the behaviour you see above is clearly non-compliant with
the IEEE 754 floating point standard. I almost never run IDL
under Windows, but I'd call this a bug -- though I daresay RSI
will call it a feature.
Using IDL 5.2 under Linux I have:
IDL> a=sqrt(-1)
%Program caused arithmetic error. Floating illegal operand.
IDL> print, a
-NaN
IDL> print a ne a
1
IDL> z=[0,0,a,a,0]
IDL> print, z ne z
0 0 1 1 0
I believe this to be 'correct' behavior but it appears that
it is not universally implemented this way within IDL. Of
course IDL has been implemented on non-IEEE machines (e.g., VAX)
and so completely consistent behavior may be impossible.
Let me add my apologies though for misleading anyone looking for
how to actually do something, rather than how they should be able
to do it.
Regards,
Tom McGlynn
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