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Re: basic array-structure understanding question [message #63813 is a reply to message #63674]

Wed, 19 November 2008 08:47

Jean H.
Messages: 472
Registered: July 2006

Senior Member

> thanks Marc! the knot is coming undone..
> it's a pity that IDL does not associate the two arrays. But I guess
> nobody's perfect :) (or is there a way to TELL IDL to associate them?)
> cheers for the explanation. I'll go hunt around for some more of
> David's articles on this topic.
> juls

As Marc pointed it out, the link is very easy to conceptualize...
select the index on array A and select the corresponding data on array B!

A = [0,1,1,0,1] ;a mask
B = [1,2,3,4,5] ;some data

goodIndex = where(A eq 1)
print, B[goodIndex]
==> 2,3,5

Now, be careful when you do a subscription using where. Your code will
miserably fail in case there is no valid subscript!
ex:
A = [0,1,1,0,1] ;a mask
B = [1,2,3,4,5] ;some data

badIndex = where(A eq 3)
print, badIndex
==> -1
print, B[badIndex]
==> ERROR!

so, unless you are 100% sure that there is ALWAYS a valid index, you
should first search for the index, make sure there is at least 1 valid
one, and then do the subscription!

Jean

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[Message index]

		Re: basic array-structure understanding question By: julia.walterspiel on Wed, 19 November 2008 07:49
		Re: basic array-structure understanding question By: m_schellens on Wed, 19 November 2008 07:41
		Re: basic array-structure understanding question By: R.Bauer on Thu, 20 November 2008 00:42
		Re: basic array-structure understanding question By: Jean H. on Wed, 19 November 2008 08:47

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