listtupleserlang

# How to convert List to List of tuples?

I want to convert `[z,z,a,z,z,a,a,z]` to `[{z,2},{a,1},{z,2},{a,2},{z,1}]`. How can I do it?

So, I need to accumulate previous value, counter of it and list of tuples.

I've create record

``````-record(acc, {previous, counter, tuples}).
``````

Redefined

``````listToTuples([]) -> [];
listToTuples([H | Tail]) ->
Acc = #acc{previous=H, counter=1},
listToTuples([Tail], Acc).
``````

But then I have some trouble

``````listToTuples([H | Tail], Acc) ->
case H == Acc#acc.previous of
true  ->
false ->
end.
``````

Solution

• if you build up your answer (Acc) in reverse, the previous will be the head of that list.

here's how i would do it --

``````list_pairs(List) -> list_pairs(List, []).

list_pairs([], Acc) -> lists:reverse(Acc);
list_pairs([H|T], [{H, Count}|Acc]) -> list_pairs(T, [{H, Count+1}|Acc]);
list_pairs([H|T], Acc) -> list_pairs(T, [{H, 1}|Acc]).
``````

(i expect someone will now follow with a one-line list comprehension version..)