Combine/Merge Two Erlang lists

Question

How to combine tuple lists in erlang? I have lists:

L1 = [{k1, 10}, {k2, 20}, {k3, 30}, {k4, 20.9}, {k6, "Hello world"}],

and

L2 = [{k1, 90}, {k2, 210}, {k3, 60}, {k4, 66.9}, {k6, "Hello universe"}],

now I want a combined list as :

L3 = [
       {k1, [10, 90]},
       {k2, [20, 210]},
       {K3, [30, 60]},
       {k4, [20.9, 66.9]},
       {K6, ["Hello world", "Hello universe"]}
     ].

Answer 1

Something shorter, and the lists don't even have to posses the same keys, and can be unordered:

merge(In1,In2) ->
    Combined = In1 ++ In2,
    Fun      = fun(Key) -> {Key,proplists:get_all_values(Key,Combined)} end,
    lists:map(Fun,proplists:get_keys(Combined)).

Fun could be written directly in the lists:map/2 function, but this makes it readable.

Output, with data from example:

1> test:merge(L1,L2).
[{k1,"\nZ"},
 {k2,[20,210]},
 {k3,[30,60]},
 {k4,[20.9,66.9]},
 {k6,["Hello world","Hello universe"]}]

"\nZ" is because erlang interprets [10,90] as a string (which are, in fact, lists). Don't bother.