Erlang - merging maps - erlang

Question

I am trying to figure out how to merge two maps in a way that allows me to process elements with the same keys.
For example, merging
#{"Ala" => 1,"kota" => 3}
with
#{"kota" => 4}
should result in:
#{"Ala" => 1,"kota" => 7}

Answer 1

There's no builtin function in Erlang that does exactly this but it can be done with maps:fold/3 and maps:update_with/4 like this:
1> A = #{"Ala" => 1,"kota" => 3}.
#{"Ala" => 1,"kota" => 3}
2> B = #{"kota" => 4}.
#{"kota" => 4}
3> maps:fold(fun(K, V, Map) -> maps:update_with(K, fun(X) -> X + V end, V, Map) end, A, B).
#{"Ala" => 1,"kota" => 7}
The code basically does this: for each item in B, if the same key exists in A, it gets the value (V) and adds the current value(X). If it doesn't exist, it sets the value to V

Answer 2

This function will merge two maps, running a function (Fun) where the key exists in both maps. It also handled the situation where Map1 is bigger than Map2 and vice-versa.
map_merge(Map1, Map2, Fun) ->
Size1 = maps:size(Map1),
Size2 = maps:size(Map2),
if
Size1 > Size2 ->
Folder = fun(K, V1, Map) ->
maps:update_with(K, fun(V2) -> Fun(K, V1, V2) end, V1, Map)
end,
maps:fold(Folder, Map1, Map2);
true ->
Folder = fun(K, V1, Map) ->
maps:update_with(K, fun(V2) -> Fun(K, V2, V1) end, V1, Map)
end,
maps:fold(Folder, Map2, Map1)
end.
Example usage in the erl shell:
1> my_module:map_merge(#{"a" => 10, "b" => 2}, #{"a" => 1}, fun(K, V1, V2) -> V1 + V2 end).
#{"a" => 11,"b" => 2}
2> my_module:map_merge(#{"a" => 10}, #{"a" => 1, "b" => 2}, fun(K, V1, V2) -> V1 + V2 end).
#{"a" => 11,"b" => 2}
3> my_module:map_merge(#{"a" => 10},#{"a" => 10}, fun(K, V1, V2) -> V1 + V2 end).
#{"a" => 20}