function compare_lists,list1,list2,tf=tf

;+
;
; COMPARE_LISTS
; A function to determine which elements from one list are present in
; a second.
;
; Call Sequence:
; Result = COMPARE_LISTS(LIST1, LIST2 [, /TF])
;
; Inputs:
; LIST1, LIST2 - Two arrays of possibly differing lengths, but
;                containing the same type of variable (e.g., not
;                mixing strings and integers).
;
; Returns:
; An integer array of same length as LIST1 with -1 where that element
; does not exist in LIST2 and the index of the first occurence of that
; element if it does exist in LIST2.
;
; Keywords:
; TF - Set /TF to return a 0/1 true-false array instead an array of
;      indices.
;
; Katie Peek / Nov 2006
;
;-

; Create the integer array to be filled.
n = n_elements(list1)
l1 = intarr(n)

; Loop over elements in LIST1.
for i=0L,n-1 do begin
  w = where(list2 eq list1[i])
  if keyword_set(tf) then begin
    case 1 of
      (w[0] eq -1): l1[i] = 0
      (w[0] ne -1): l1[i] = 1
      else: print,"Don't look now, but you just broke Logic."
    endcase
  endif else l1[i] = w[0]
endfor

; Return the integer array.
return,l1
end