list handling functions

Richard Carlsson <>
Fri Sep 19 00:41:23 CEST 2003


On Thu, 18 Sep 2003, Vlad Dumitrescu wrote:

> Are there any ways to simply express a 'map' (or similar) function
> that iterates over several lists at once? I use something like
>
> map2(Fun, A, B) ->
>     map2(Fun, A, B, []).
>
> map2(Fun, [], _, Res) ->
>     lists:reverse(Res);
> map2(Fun, _, [], Res) ->
>     lists:reverse(Res);
> map2(Fun, [Ha|Ta],[Hb|Tb],Res)->
>     map2(Fun, Ta, Tb, [Fun(Ha, Hb)|Res]).
>
> but this seems to be general enough to be found in a library. Or maybe
> list comprehensions could be used in a clever way?

I think the "standard" (but less efficient) solution is to first use
a "zip" function on the two lists, creating a single list of 2-tuples
(in most list libraries you'll also find a zip3 function for zipping 3
lists, and in some cases there are zip4...zip9 as well).

Then you just do a normal "map" on the tuples. Your map2 can be
viewed as the result of a deforestation-style optimization of zip+map.

	/Richard


Richard Carlsson ()   (This space intentionally left blank.)
E-mail: 	WWW: http://user.it.uu.se/~richardc/
 "Having users is like optimization: the wise course is to delay it."
   -- Paul Graham



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