[erlang-questions] optimal way to append an element in a list inside a map

[Brujo Benavides]

I think this might be what you are looking for:

append(Key, Value, Map) ->
	maps:update_with(Key, fun(Tail) -> [Value | Tail] end, [], Map).

Brujo Benavides <http://about.me/elbrujohalcon>



> On Aug 29, 2017, at 15:19, Dmitry Kolesnikov <dmkolesnikov@REDACTED> wrote:
> 
> Hello,
> 
> Premature optimisation is an evil ;-)
> 
> I would use the following syntax:
> ```
> append(Key, Value, Map) ->
>   List = case Map of
>      #{Key := Tail} -> 
>         [Value | Tail];
>      _ ->
>         [Value]
>   end,
>   Map#{Key => List}.
> ```
> 
> Lists are not copied they are referenced. Maps… Hmm, I am not sure. I hope the implementation is smart enough to keep reference as well.
> 
> - Dmitry
> 
> 
>> On 29 Aug 2017, at 20.34, Caragea Silviu <silviu.cpp@REDACTED> wrote:
>> 
>> Hello,
>> 
>> Having a map where the value of each element it's a list :
>> 
>> #{ 1 => [], 2 => [], ... n => []} 
>> 
>> and you need to append elements in the list for a specific key, what's the most optimal way to do this without copying the lists and the map inside the VM lot of times ?
>> 
>> Anything better than the following solution:
>> 
>> append_element(Key, Value, Map) ->
>>    case maps:find(Key, Map) of
>>        {ok, V} ->
>>            maps:put(Key, [Value | V], Map);
>>        _ ->
>>            maps:put(Key, [Value], Map)
>>    end.
>> 
>> Kind regards,
>> Silviu
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> 
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