[erlang-questions] Merging maps with a fun like dict:merge/3

[Ivan Uemlianin]

Dear Alexander

Thanks!  That looks excellent!

I notice that your function F is different from the dict:merge/3 
function: instead of taking a key and the two values, is takes two 
values, defaulting the first to 0.

I've put my case clause into your framework:

merge_ap(F, [Head | Tail]) ->
     lists:foldl(fun(Y, X) ->
                         maps:merge(X, maps:map(fun(Key, Value) ->
                                                        case 
maps:is_key(Key, X) of
                                                            false ->
Value;
                                                            true ->
F(Key, Value, maps:get(Key, X))
                                                        end
                                                end,
                                                Y))
                 end,
                 Head,
                 Tail).

This has desired behaviour, and will probably still perform well. Using 
lists:foldl is a brilliant idea.

Best wishes

Ivan


On 26/08/2015 09:16, Alexander Petrovsky wrote:
> Hi!
>
> I've make some little benchmarks, and noticed that the most effective 
> way is:
>
> merge(F, [Head | Tail]) ->
>     lists:foldl(fun(Y, X) -> maps:merge(X, maps:map(fun(Key, Value) -> 
> F(maps:get(Key, X, 0), Value) end, Y)) end, Head, Tail).
>
>
> 2015-08-23 20:05 GMT+03:00 Ivan Uemlianin <ivan@REDACTED 
> <mailto:ivan@REDACTED>>:
>
>     Dear All
>
>     The maps module does not have a merge/3 like dict:merge/3, which
>     calls a function fun(Key, Val1, Val2) when both dicts have the
>     same key.  From the documentation, "If a key occurs in both
>     dictionaries then Fun is called with the key and both values to
>     return a new value."
>
>     The dict documentation says that dict:merge/3 is faster than an
>     equivalent implemented in erlang.
>
>     http://www.erlang.org/doc/man/dict.html#merge-3
>
>     Why does the maps module not have a similar merge/3?  Is it
>     because writing our own erlang implementation is at fast as it gets?
>
>     Below is an implementation which seems to work.  Can it be made
>     faster?
>
>
>     merge_maps(Fun, M1, M2) ->
>         maps:fold(fun(K, V1, Acc) ->
>                           case maps:is_key(K, Acc) of
>                               false ->
>                                   maps:put(K, V1, Acc);
>                               true ->
>                                   V2 = maps:get(K, Acc),
>                                   maps:put(K, Fun(K, V1, V2), Acc)
>                           end
>                   end,
>                   M1,
>                   M2).
>
>     merge_maps_test() ->
>         merge_maps(fun(_K, V1, V2) -> V1 + V2 end,
>                    #{a => 1, b => 2, c => 3},
>                    #{b => 2, c => 3, d => 4}) =:=
>             #{a => 1, b => 4, c => 6, d => 4}.
>
>
>     With thanks and best wishes
>
>     Ivan
>
>     -- 
>     ============================================================
>     Ivan A. Uemlianin PhD
>     Llaisdy
>     Speech Technology Research and Development
>
>     ivan@REDACTED <mailto:ivan@REDACTED>
>                             @llaisdy
>     llaisdy.wordpress.com <http://llaisdy.wordpress.com>
>     github.com/llaisdy <http://github.com/llaisdy>
>     www.linkedin.com/in/ivanuemlianin
>     <http://www.linkedin.com/in/ivanuemlianin>
>
>                             festina lente
>     ============================================================
>
>     _______________________________________________
>     erlang-questions mailing list
>     erlang-questions@REDACTED <mailto:erlang-questions@REDACTED>
>     http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
>
> -- 
> Петровский Александр / Alexander Petrovsky,
>
> Skype: askjuise
> Phone: +7 914 8 820 815
>

-- 
============================================================
Ivan A. Uemlianin PhD
Llaisdy
Speech Technology Research and Development

                     ivan@REDACTED
                         @llaisdy
                          llaisdy.wordpress.com
               github.com/llaisdy
                      www.linkedin.com/in/ivanuemlianin

                         festina lente
============================================================

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20150826/60837446/attachment.htm>