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

Dmitry Demeshchuk <>
Fri Nov 18 10:41:35 CET 2011

```That's not the correct way. You were measuring the processes memory in
the middle of calculations.

1> A = spawn(fun () -> test:a(), receive _ -> ok end end).
<0.46.0>
2> B = spawn(fun () -> test:b(), receive _ -> ok end end).
<0.48.0>
3> process_info(A, memory).
{memory,391814792}
4> process_info(B, memory).
{memory,401610112}

Note that I waited for several seconds before entering commands 3 and 4.

On Fri, Nov 18, 2011 at 1:32 PM, Barco You <> wrote:
> Very strange! By using another measuring method as below I got that your
> function is far better than mine in memory consumption.
>
> -module(test).
> -export([a/0, b/0]).
> a() ->
>         L = lists:seq(1, 10000000),
>         map2(fun (I, J) -> I + J end, L, L),
>                 stop ->
>                         ok
>         end.
> b() ->
>         L = lists:seq(1, 10000000),
>         map3(fun (I, J) -> I + J end, L, L),
>                 stop ->
>                         ok
>         end.
>
> map2(Fun, [H1 | T1], [H2 | T2]) ->
>         [Fun(H1, H2) | map2(Fun, T1, T2)];
> map2(_, [], []) ->
>         [].
> map3(_Fun, [], []) ->
>         [];
> map3(Fun, L1, L2) ->
>         map3(Fun, L1, L2, []).
> map3(_Fun, [], [], L) ->
>         lists:reverse(L);
> map3(Fun, [H1 | T1], [H2 | T2], L) ->
>         map3(Fun, T1, T2, [Fun(H1, H2) | L]).
>
> Eshell V5.8.4  (abort with ^G)
> 1> A = spawn(test,a,[]).
> <0.32.0>
> 2> B = spawn(test,b,[]).
> <0.34.0>
> 3> erlang:process_info(A,memory).
> {memory,176316700}
> 4> erlang:process_info(B,memory).
> {memory,306105040}
>
>
>
> On Fri, Nov 18, 2011 at 4:53 PM, Dmitry Demeshchuk <>
> wrote:
>>
>> I'm not sure it is so. Try running this function several times:
>>
>> memtest() ->
>>    erlang:garbage_collect(),
>>    M1 = proplists:get_value(total, erlang:memory()),
>>    a(),
>>    M2 = proplists:get_value(total, erlang:memory()),
>>    b(),
>>    M3 = proplists:get_value(total, erlang:memory()),
>>    {M2 - M1, M3 - M2}.
>>
>> The results are pretty the same.
>>
>> On Fri, Nov 18, 2011 at 12:36 PM, Barco You <> wrote:
>> > Umm! I don't think there would be some difference between times consumed
>> > by
>> > these two functions, but I will assume there are difference in memory
>> > consumption.
>> >
>> >
>> > On Fri, Nov 18, 2011 at 3:49 PM, Dmitry Demeshchuk
>> > <>
>> > wrote:
>> >>
>> >> I think it's meant that lists:reverse/1 is called at the end of the
>> >> _optimized_ code.
>> >>
>> >> Here's the module code:
>> >>
>> >> -module(test).
>> >> -export([a/0, b/0]).
>> >>
>> >> a() ->
>> >>    L = lists:seq(1, 10000000),    map2(fun (I, J) -> I + J end, L, L).
>> >>
>> >> b() ->
>> >>    L = lists:seq(1, 10000000),
>> >>    map3(fun (I, J) -> I + J end, L, L).
>> >>
>> >>
>> >> map2(Fun, [H1 | T1], [H2 | T2]) ->
>> >>    [Fun(H1, H2) | map2(Fun, T1, T2)];
>> >> map2(_, [], []) ->
>> >>    [].
>> >>
>> >> map3(_Fun, [], []) ->
>> >>   [];
>> >> map3(Fun, L1, L2) ->
>> >>   map3(Fun, L1, L2, []).
>> >>
>> >> map3(_Fun, [], [], L) ->
>> >>   lists:reverse(L);
>> >> map3(Fun, [H1 | T1], [H2 | T2], L) ->
>> >>   map3(Fun, T1, T2, [Fun(H1, H2) | L])
>> >>
>> >> Try to call timer:tc/3 yourself.
>> >>
>> >> On Fri, Nov 18, 2011 at 11:48 AM, Barco You <> wrote:
>> >> > According to the instruction attached by Ulf, the body-recursive and
>> >> > tail-recursive list function will be the same in consuming memory
>> >> > only
>> >> > when
>> >> > they call lists:reverse/1 at the end.
>> >> > So, I don't know how did you do the benchmarks. Did you compare these
>> >> > two
>> >> > methods with big enough lists?
>> >> > Or, I misunderstand the optimization instructions?
>> >> >
>> >> > BR,
>> >> > Barco
>> >> >
>> >> > On Fri, Nov 18, 2011 at 3:37 PM, Dmitry Demeshchuk
>> >> > <>
>> >> > wrote:
>> >> >>
>> >> >> Okay, I admit, this isn't an "honest" tail-recursed function, since
>> >> >> a
>> >> >> list concatenation operator is going to be called at the end.
>> >> >> However,
>> >> >> Erlang compiler optimizes such cases and converts them to
>> >> >> tail-recursive:
>> >> >> http://www.erlang.org/doc/efficiency_guide/myths.html#tail_recursive
>> >> >>
>> >> >> Also, I've ran benchmarks with both implementations: mine and yours.
>> >> >> And they result in the same performance.
>> >> >>
>> >> >> On Fri, Nov 18, 2011 at 11:06 AM, Barco You <>
>> >> >> wrote:
>> >> >> > Yes, Ryan's suggestion is a good generic solution for n lists and
>> >> >> > it's
>> >> >> > tail-recursed.
>> >> >> > Hi Dmitry,
>> >> >> > Your version is just recursed but not tail-recursed, because your
>> >> >> > function
>> >> >> > needs a piece of memory to stack the intermediate result for every
>> >> >> > round
>> >> >> > of
>> >> >> > recursive calls. To be tail-recursed, the recursive calls should
>> >> >> > eliminate
>> >> >> > the linearly-increased memory consumption by adding an extra
>> >> >> > variable
>> >> >> > (accumulator) and let the recursive function call it alone for
>> >> >> > every
>> >> >> > round.
>> >> >> >
>> >> >> > On Fri, Nov 18, 2011 at 2:53 PM, Dmitry Demeshchuk
>> >> >> > <>
>> >> >> > wrote:
>> >> >> >>
>> >> >> >> 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
>> >> >> >
>> >> >> >
>> >> >>
>> >> >>
>> >> >>
>> >> >> --
>> >> >> Best regards,
>> >> >> Dmitry Demeshchuk
>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Best regards,
>> >> Dmitry Demeshchuk
>> >
>> >
>>
>>
>>
>> --
>> Best regards,
>> Dmitry Demeshchuk
>
>

--
Best regards,
Dmitry Demeshchuk

```