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Interesting property of sort [message #53994]

Wed, 16 May 2007 05:41

cmancone
Messages: 30
Registered: May 2007

Member

while attempting to build a faster star matching algorithm, I
"discovered" an interesting property of sort - or, an interesting use
for it. Of course, you guys have all probably seen this before
anyway, but I thought I would share it because it can be very useful.
Essentially, I found the easy way to make a sort lookup table. So,
let's say you have the array:

test = [5,4,1,3,2]

and you sort it:

sortindex = sort(test)
sorted = test[sortindex]

now, what if you want to know where the 5 (or any other number) ended
up in the sorted array? It turns out that you can make a lookup table
by simply sorting sortindex.

lookup = sort(sortindex)

so, if you wanted to know where the first element of test ended up in
the sorted list, you would say:

print,lookup[0]
; prints "4"
print,sorted[lookup[0]]
; prints "5"

In retrospect, I suppose this result isn't suprising, and probably
should have been immediately obvious. Still, I thought I'd share.

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[Message index]

		Interesting property of sort By: cmancone on Wed, 16 May 2007 05:41
		Re: Interesting property of sort By: cmancone on Thu, 17 May 2007 10:55
		Re: Interesting property of sort By: David Fanning on Wed, 16 May 2007 10:01
		Re: Interesting property of sort By: cmancone on Wed, 16 May 2007 09:52
		Re: Interesting property of sort By: wlandsman@jhu.edu on Wed, 16 May 2007 08:55
		Re: Interesting property of sort By: David Fanning on Wed, 16 May 2007 07:37
		Re: Interesting property of sort By: cmancone on Wed, 16 May 2007 07:23
		Re: Interesting property of sort By: David Fanning on Wed, 16 May 2007 06:50

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