FUNCTION rank_array, index
;+
; NAME:
;	rank_array
; PURPOSE:
;	Calculates rank array for a given index array
; CATEGORY:
;	gen/idl/toolbox/math
; CALLING SEQUENCE:
;	rank = rank_array(index)
; INPUTS:
;	index		array; type: integer
;					index array, i.e. array of indices returned
;					by the IDL sort function
; OUTPUTS:
;	rank		array; type: integer
;					rank array
; INCLUDE:
	@compile_opt.pro		; On error, return to caller
; CALLS:
;	IsType
; PROCEDURE:
;	The rank array inverts the index array. Applying rank to the sorted
;	array returns the array back into its original order: A[rank[index]] = A
; MODIFICATION HISTORY:
;	JAN-2002, Paul Hick (UCSD/CASS; pphick@ucsd.edu)
;-

n = n_elements(index)
r = make_array(n, type=IsType(index))
r[index] = lindgen(n)

RETURN, r  &  END