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Below are some elegant set-processing functions that were created many years ago by IDL's founder, David Stern, but were never permanently built into our language. The one relevant to your question is clearly 'SetIntersection'.
;A somewhat belated reply to the numerous postings on finding the
;common elements of vectors:
;
;> Given vectors of the type...
;>
;> a = [1,2,3,4,5]
;> b = [3,4,5,6,7]
;>
;> What is the most efficient way to determine which values that occur in
;> a also occur in b (i.e., the values [3,4,5] occur in both a and b).
;>
;
;Below appear three IDL functions that operate on sets represented by
;arrays of positive integers. The SetIntersection(a,b) function
;returns the common elements, SetUnion(a,b) returns all unique elements
;in both arguments, and SetDifference(a,b) returns the elements
;(members) in a but not in b.
;
;It is faster than previously published functions, e.g. contain() and
;find_elements().
;
;Hope this helps,
;
;Research Systems, Inc.
;_________________________________________________________________
; Set operators. Union, Intersection, and Difference (i.e. return
; members of A that are not in B.)
;
; These functions operate on arrays of positive integers, which need
; not be sorted. Duplicate elements are ignored, as they have no
; effect on the result.
;
; The empty set is denoted by an array with the first element equal to
; -1.
;
; These functions will not be efficient on sparse sets with wide
; ranges, as they trade memory for efficiency. The HISTOGRAM function
; is used, which creates arrays of size equal to the range of the
; resulting set.
; For example:
; a = [2,4,6,8]
; b = [6,1,3,2]
; SetIntersection(a,b) = [ 2, 6] ; Common elements
; SetUnion(a,b) = [ 1, 2, 3, 4, 6, 8] ; Elements in either set
; SetDifference(a,b) = [ 4, 8] ; Elements in A but not in B
; SetIntersection(a,[3,5,7]) = -1 = Null Set
FUNCTION SetUnion, a, b
if a[0] lt 0 then return, b ;A union NULL = a
if b[0] lt 0 then return, a ;B union NULL = b
return, where(histogram([a,b], OMIN = omin)) + omin ;Return combined set
end
FUNCTION SetIntersection, a, b
minab = min(a, MAX=maxa) > min(b, MAX=maxb) ;Only need intersection of ranges
maxab = maxa James Jones
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