>This works, but it's a plist and thus O[n] for lookup. I assumed the questioner wanted an O[1] data structure; thus a hash table is the better answer. (Granted

>This works, but it's a plist and thus O[n] for lookup. I assumed the questioner wanted an O[1] data structure; thus a hash table is the better answer. (Granted it makes little difference when n is small.) There was a test on some Common Lisp implementations, of hashtables versus plists on access speeds. On small (<30?) datasets, plists were much faster than hashtables. Bigger than that, the constant access speed of hashtables makes HTs a more sane choice. The other thing is memory consumption -- on CL, plists are very frugal.