Comments on: Writting Circular Lists in Erlang

By: deepspawn

deepspawn — Fri, 27 Mar 2009 21:45:21 +0000

Thanks, I'll give it a try and do a benchmark to see if It's better to
the other solution in the above comments.

By: nonme

nonme — Fri, 27 Mar 2009 21:33:04 +0000

keep two lists {A, B} one in normal order the other reversed
To move forward do this

forward({[H|T],B}) -> {T, [H|B]};
forward({[], B}) -> forward({reverse(B),[]})

bawards is the opposite

By: zaphar

zaphar — Sun, 22 Mar 2009 21:53:49 +0000

yeah ++ is less efficient. [Push | drop_last(List)] would be better. That's what I get for coding when I'm tired. but the basic concept is the same.

By: deepspawn

deepspawn — Fri, 20 Mar 2009 01:24:26 +0000

Nice, I'm just repeating what I have read so far but

[Push] ++ drop_last(List)

Wouldn't it be better written as:

[ Push | drop_last(List)]

Anyway, ++ is discouraged since it makes a copy of the left operator and with long lists it would be highly inefficient, in this case it wouldn't be much of a problem but I still like more how the second one looks.

Thank you a lot for this idea, I'm guessing it's probably the best way to write it on erlang.

By: zaphar

zaphar — Wed, 18 Mar 2009 22:07:56 +0000

The key is to process the list in one pass. So if I read your intent above correctly then the following should work:

%% append the Push element to the beginning of the list while dropping the last element
circular(List, Push) ->
[Push] ++ drop_last(List).

%% drop the last element from the list
drop_last([_]) ->
[];
drop_last([H|T]) ->
[H | drop_last(T)].