| set difference, with duplicates [message #92702] |
Wed, 10 February 2016 12:03  |
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Russell[1]
Messages: 101
Registered: August 2011
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Senior Member |
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Hi everyone...
I have two long-integer arrays, one of which contains duplicate entries, and I'd like to find the elements in a but not in b. For example:
a=[2,3,4,5,2,3,5,2,4,3,10,100]
b=[2,10]
I'd like set operator that computes a dif b, but preserves the duplicate entries (and order if possible) for a. For example,
c = set_difference(a,b)
and I would want
c=[3,4,5,3,5,4,3,100]
I'm aware of the Coyote's cgsetdifference, but that does not preserve duplicates (or I didn't realize the right set of options).
Any ideas? If it helps, the a-array may be very long 10^5 elements and the b array will be 10^3 elements. I also expect the values to be very high, but I can compress them to the lowest possible integer arrays.
Thanks for any advice,
Russell
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| Re: set difference, with duplicates [message #92703 is a reply to message #92702] |
Wed, 10 February 2016 12:23  |
Burch
Messages: 28
Registered: December 2013
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Junior Member |
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On Wednesday, February 10, 2016 at 2:03:36 PM UTC-6, rrya...@gmail.com wrote:
> Hi everyone...
>
> I have two long-integer arrays, one of which contains duplicate entries, and I'd like to find the elements in a but not in b. For example:
>
> a=[2,3,4,5,2,3,5,2,4,3,10,100]
> b=[2,10]
>
>
> I'd like set operator that computes a dif b, but preserves the duplicate entries (and order if possible) for a. For example,
>
> c = set_difference(a,b)
>
> and I would want
>
> c=[3,4,5,3,5,4,3,100]
>
> I'm aware of the Coyote's cgsetdifference, but that does not preserve duplicates (or I didn't realize the right set of options).
>
> Any ideas? If it helps, the a-array may be very long 10^5 elements and the b array will be 10^3 elements. I also expect the values to be very high, but I can compress them to the lowest possible integer arrays.
>
> Thanks for any advice,
> Russell
One option is to use match2 from the IDL Astronomy Library:
http://idlastro.gsfc.nasa.gov/ftp/pro/misc/match2.pro
http://idlastro.gsfc.nasa.gov
For your example:
IDL> a = [2,3,4,5,2,3,5,2,4,3,10,100]
IDL> b = [2,10]
IDL> match2, a, b, a_in_b, b_in_a
Note that match 2 finds matching elements and returns -1 for elements with no match.
IDL> print, a_in_b
0 -1 -1 -1 0 -1 -1 0 -1 -1 1 -1
IDL> c = a[where(a_in_b eq -1)]
IDL> print, c
3 4 5 3 5 4 3 100
-Jeff
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