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Re: Efficient comparison of array location in two lists [message #75217 is a reply to message #75216]

Mon, 21 February 2011 18:45

Heinz Stege
Messages: 189
Registered: January 2003

Senior Member

On Mon, 21 Feb 2011 15:59:57 -0800 (PST), Matt Francis wrote:

> I have two sets of data, both comprised of a STRARR labelling the data
> and a FLTARR of the data itself.
>
> I want to compare the data in the two sets, but the order and number
> of labels are different, so I'd first need to to find which array
> elements in each FLTARR correspond to the same label.
>
> It's trivial to see how this can be accomplished by simply looping
> over either data set and using WHERE(STRCMP()), but is there a way to
> do this without the loop?
>
> To make it clear, say I had:
>
> foo1 = ['a','b','c']
> foo2 = ['a','c','b']
>
> I want a single line that produces:
>
> IDL_MAGIC(foo1,foo2,indx1,indx2)
> print,indx1,indx2
> -> [0,1,2] [0,2,1]
>
> So that could compare
>
> foo1_data[indx1] and foo2_data[indx2] in whatever way I wanted.
>
> Any thoughts?

You can manage it in a short and very fast way by use of the built-in
function value_locate:

indx1=sort(foo1)
if n_elements(foo1) ge 2 then $
indx2=value_locate(foo1[indx1],foo2) $
else $
; value locate does not work for vectors with 1 element.
; It would be nice, if ITT VIS could make it working in IDL 8.1.
indx2=lonarr(n_elements(foo2))
ii=where(foo1[indx1] eq foo2[indx2],count)
if count le 0 then message,'No matching labels found.'
indx1=indx1[ii]
indx2=indx2[ii]

The labels foo1 as well as foo2 are assumed to be unique.

I believe, value_locate is an often underestimated function.

Heinz

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[Message index]

		Re: Efficient comparison of array location in two lists By: Heinz Stege on Tue, 22 February 2011 02:18
		Re: Efficient comparison of array location in two lists By: Heinz Stege on Tue, 22 February 2011 02:16
		Re: Efficient comparison of array location in two lists By: Jeremy Bailin on Mon, 21 February 2011 21:21
		Re: Efficient comparison of array location in two lists By: wlandsman on Mon, 21 February 2011 20:02
		Re: Efficient comparison of array location in two lists By: David Fanning on Mon, 21 February 2011 19:17
		Re: Efficient comparison of array location in two lists By: Heinz Stege on Mon, 21 February 2011 18:45
		Re: Efficient comparison of array location in two lists By: wlandsman on Mon, 21 February 2011 16:43

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