| Re: dynamic array workaround? [message #45098] |
Sun, 14 August 2005 17:55  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Jeff N. writes:
> I haven't seen anything in the help files that would lead me to believe
> that IDL has a way to do something like the following:
>
> Say I want to loop through a few lists of numbers, and create one big
> array of all numbers in the lists that meet some criteria. Since I
> don't know beforehand how many numbers will meet the criteria, I don't
> know how many elements that one big array will need.
>
> If I were writing code in a language like PERL maybe, I'd use a dynamic
> array, but I don't think that IDL supports those. Any idea as to how I
> could get around this? What I've been doing is to just initialize a
> scalar, then assign new elements to it in the loops, then just removing
> the 1st element when I'm done looping. I guess I could also loop
> twice, with the first round of loops serving to find out how many array
> elements I'd end up needing. Both approaches seem inefficient to me.
> What else could I try? I had a sneaky suspicion that the TEMPORARY
> function was what I needed, but somehow that doesn't quite look right
> either.
I'd probably use a pointer to do this:
IF Ptr_Valid(ptr) THEN ptr = Ptr_New([mylist]) ELSE $
*ptr = [Temporary(*ptr), mynewList]
finalList = Temporary(*ptr)
If your lists contain integers, you can probably use the
HISTOGRAM function to do the concatenation. See the Histogram
tutorial and the article on Set Operations on Arrays:
http://www.dfanning.com/tips/histogram_tutorial.html
http://www.dfanning.com/tips/set_operations.html
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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