By: Carlo

Carlo — Fri, 25 Feb 2011 16:44:31 +0000

Hi, I’m studying erlang too.
My version of concatenate use only two functions, but instead, the helper has one more case:

concatenate(Lists) ->
reverse(concatenateHelper(Lists, [])).

concatenateHelper([], Acc) -> Acc;
concatenateHelper([H | T], Acc) ->
Acc2 = concatenateHelper(H, Acc),
concatenateHelper(T, Acc2);
concatenateHelper(X, Acc) ->
[X | Acc].

Thinking about flatten, I realized that this can also be used to flatten a list.

concatenate([[1,[2,[3],[]]], [[[4]]], [5,6]]).
[1,2,3,4,5,6]

Do you think that my reasoning is correct?

By: Andrei Marius

Andrei Marius — Tue, 14 Dec 2010 17:02:53 +0000

Hi,
Nice job with the exercises
My observation is that the exercise with the concatenation is not tail-recursive.
I like to see your way of resolving after I first tried on my own.
So here is the tail-recursive version I wrote, since, probably, a lot of people is reading your site when they get to the exercises. I have tested it with the sample from the book.

concatenate([]) ->
[];
concatenate(ListOfLists) ->
concatenate_acc(ListOfLists, []).

concatenate_acc([], ConList) ->
lists:reverse(ConList);
concatenate_acc([H|T], ConList) ->
concatenate_acc(H, T, ConList).
concatenate_acc([H|T], Lists, ConList) ->
concatenate_acc(T, Lists, [H|ConList]);
concatenate_acc([], Lists, ConList) ->
concatenate_acc(Lists, ConList).

Andrei Marius

Comments on: Erlang Programming Exercise: 3-5

By: Carlo

By: Andrei Marius