# [erlang-questions] [erlang-question] How to comprehend two lists synchronously?

Dmitry Demeshchuk <>
Fri Nov 18 07:53:25 CET 2011

```Hi, Barco.

Why do you think my version isn't tail-recursed? :) Take a look at
lists:map/2 implementation, for example. It's just the same.

List comprehensions just serve different purpose: for combinations
from multiple list sources. My guess is that people need this
operation more often than mapping over multiple list. Another problem
is that you should be sure that all those lists have the same length.

On Fri, Nov 18, 2011 at 10:38 AM, Barco You <> wrote:
> Hi Dmitry,
> What your suggested can really solve my problem, but it's not
> Tail-Recursion. The tail-recursed solution should look like this;
> map2(_Fun, [], []) ->
>    [];
> map2(Fun, L1, L2) ->
>    map2(Fun, L1, L2, []).
> map2(_Fun, [], [], L) ->
>    lists:reverse(L);
> map2(Fun, [H1 | T1], [H2 | T2], L) ->
>    map2(Fun, T1, T2, [Fun(H1, H2) | L]).
>
> However, I'm still disappointed with the list comprehension which is
> different from what I intuitively imagine about it.
>
> Regards,
> Barco
> On Fri, Nov 18, 2011 at 1:49 PM, Dmitry Demeshchuk <>
> wrote:
>>
>> My guess is you have to zip them together, or just write a
>> tail-recursed function:
>>
>> map2(Fun, [H1 | T1], [H2 | T2]) ->
>>    [Fun(H1, H2) | map2(Fun, T1, T2)];
>> map2(Fun, [], []) ->
>>    [].
>>
>> The second option definitely isn't a list comprehension, but it
>> requires less memory and has lesser complexity.
>>
>> On Fri, Nov 18, 2011 at 9:45 AM, Barco You <> wrote:
>> > Dear Erlangers,
>> >
>> > I hope to get a list from two lists like this:
>> > [{a1,b1}, {a2,b2}, {a3,b3}]      <-     [a1, a2 a3],  [b1, b2, b3].
>> > But if I use list comprehension, I got:
>> > 10>  [{D1,D2} ||  D1 <- [a1,a2,a3], D2 <- [b1,b2,b3]].
>> > [{a1,b1},
>> >  {a1,b2},
>> >  {a1,b3},
>> >  {a2,b1},
>> >  {a2,b2},
>> >  {a2,b3},
>> >  {a3,b1},
>> >  {a3,b2},
>> >  {a3,b3}]
>> >
>> > So, my questions is how to comprehend list in synchronous way in order
>> > to
>> > get what I want, rather than to compose the elements from two lists in
>> > all
>> > possible situations.
>> >
>> > Thank you,
>> > Barco
>> > _______________________________________________
>> > erlang-questions mailing list
>> >
>> > http://erlang.org/mailman/listinfo/erlang-questions
>> >
>> >
>>
>>
>>
>> --
>> Best regards,
>> Dmitry Demeshchuk
>
>

--
Best regards,
Dmitry Demeshchuk

```