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Re: Array element deletion [message #36386 is a reply to message #36385]

Fri, 12 September 2003 06:31

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

Edd Edmondson writes:

> Supposing I have a big array and I also have an array containing indices
> of unwanted elements. Is there a neat way of removing those elements from
> the array?
>
> Attempts using WHERE() have failed for me* and currently I'm looping
> through the array of indices and deleting them one at a time - which is
> fine when I've only got 20 or 30 of them but I doubt it'll scale well.

Here is a function from my web page:

FUNCTION SetDifference, a, b

; = a and (not b) = elements in A but not in B

mina = Min(a, Max=maxa)
minb = Min(b, Max=maxb)
IF (minb GT maxa) OR (maxb LT mina) THEN RETURN, a ;No intersection...
r = Where((Histogram(a, Min=mina, Max=maxa) NE 0) AND $
(Histogram(b, Min=mina, Max=maxa) EQ 0), count)
IF count eq 0 THEN RETURN, -1 ELSE RETURN, r + mina
END

Here is how it works. Suppose you have an array:

array = [3.5, 4.8, 9.3, 2.1, 7.6, 4.6]

And an array of indices you don't want:

bad = [2, 4]

You would do this:

possible = Indgen(N_Elements(array))
good = SetDifference(possible, bad)
Print, possible
Print, good
newArray = array[good]
Print, newArray

You can learn more about these set methods here:

http://www.dfanning.com/tips/set_operations.html

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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[Message index]

		Re: Array element deletion By: R.Bauer on Sat, 13 September 2003 10:05
		Re: Array element deletion By: Edd Edmondson on Fri, 12 September 2003 06:44
		Re: Array element deletion By: David Fanning on Fri, 12 September 2003 06:31
		Re: Array element deletion By: jjbezair on Fri, 12 September 2003 06:27
		Re: Array element deletion By: Edd Edmondson on Fri, 12 September 2003 06:19

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