;------------------------------------------------------------
;------------------------------------------------------------
;------------------------------------------------------------
;+
; NAME:different
;
; PURPOSE:calcule les elements differents de 2 matrices D'ENTIERS POSITIFS
;
; CATEGORY:calcule sur les matrices
;
; CALLING SEQUENCE:res=different(a,b)
;
; INPUTS:a et b:arrays of positive integers, which need
;               not be sorted. Duplicate elements are ignored, as they have no
;               effect on the result
;
; KEYWORD PARAMETERS:
;
; OUTPUTS:tableau
;
; COMMON BLOCKS:
;
; SIDE EFFECTS:
;
; The empty set is denoted by an array with the first element equal to
; -1.
;
; RESTRICTIONS:
;
; 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.
;
; EXAMPLE:
;
;   a = [2,4,6,8]
;   b = [6,1,3,2]
;   different(a,b) = [ 4, 8]         ; Elements in A but not in B
;
; MODIFICATION HISTORY: 
;
; http://www.dfanning.com/tips/set_operations.html
;-
;------------------------------------------------------------
;------------------------------------------------------------
;------------------------------------------------------------
FUNCTION different, 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) $
          *(1-Histogram(b, Min=mina, Max=maxa)), count)
IF count eq 0 THEN RETURN, -1 ELSE RETURN, r + mina
END