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Re: Strange WHERE issue? [message #76290 is a reply to message #76289]

Thu, 26 May 2011 19:01

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

polystethylene writes:

>
> Wonder if anybody can help me with this...
>
> Essentially I'm just varying a parameter (1st column) and testing chi
> squared (2nd column), and I want to find chi squared minimum + 1 in
> order find my 1 sigma error bars...
>
> I have the following code:
>
> c = WHERE(chisqarr_ph[1,*] EQ MIN(chisqarr_ph[1,*]))
>
>
> d = WHERE(chisqarr_ph[1,*] LE (chisqarr_ph[1,c]+1))
>
>
>
> I have a problem with the line d = WHERE(chisqarr_ph[1,*] LE
> (chisqarr_ph[1,c]+1)), which returns -1.

This is one of the classic Where gotchas:

http://www.idlcoyote.com/misc_tips/noidea.html

You are only comparing the *first* value in your
array, because the value on the right of the Boolean
expression is a vector of one element.

And, you should be VERY careful using these WHERE
expressions with floating values. You will not get
the results you expect at least some of the time.
See, for example, this article:

http://www.idlcoyote.com/math_tips/razoredge.html

You might want to have a look at the program Floats_Equal
in the Coyote Library:

http://www.idlcoyote.com/programs/floats_equal.pro

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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[Message index]

		Re: Strange WHERE issue? By: Jeremy Bailin on Fri, 27 May 2011 12:58
		Re: Strange WHERE issue? By: Fabzou on Thu, 26 May 2011 19:40
		Re: Strange WHERE issue? By: polystethylene on Thu, 26 May 2011 19:34
		Re: Strange WHERE issue? By: David Fanning on Thu, 26 May 2011 19:24
		Re: Strange WHERE issue? By: ben.bighair on Thu, 26 May 2011 19:05
		Re: Strange WHERE issue? By: David Fanning on Thu, 26 May 2011 19:01

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