#	LMAP
#
#	Map list
#
#	Ralph E. Griswold
#
#	Last modified 12/15/85
#

# This procedure is modeled after the built-in string mapping function.
# The main difference is that the first argument is modified, rather
# than constructing a new list. This is in the interests of efficiency
# and likely uses, but is easily changed.
#
# Warning:  a2 and a3 are cached so that the table of correspondences
# is not rebuilt if lmap is called successively with the same mapping
# lists. Just because a2 and a3 are the same does not mean that they
# have the same values on successive calls of lmap. Thus, the
# equivalence of maps is taken from the value of a2 and a3, not their
# respective elements. This is deliberate, but could result in
# unexpected results.

procedure lmap(a1,a2,a3)
   static lmem2, lmem3, lmaptbl, tdefault
   local i
   initial tdefault := []
   if not(type(a1) == type(a2) == type(a3) == "list")
      then stop("illegal type in lmap")
   if not(*a2 = *a3)
      then stop("Run-time error:\nSecond and third arguments of lmap differ in length")
   if not(lmem2 === a2 & lmem3 === a3) then {	# if an argument is new, rebuild
      lmem2 := a2				# save for future reference
      lmem3 := a3
      lmaptbl := table(tdefault)		# new mapping table
      every i := 1 to *a2 do			# build the map
         lmaptbl[a2[i]] := a3[i]
      }
   every i := 1 to *a1 do			# map the values
      a1[i] := (tdefault ~=== lmaptbl[a1[i]])
   return a1
end