Well I have been struggling to find/create a circular buffer(list to be exact) on erlang, but I’m still not satisfied, I have hacked this:
-
[_P | T ] = lists:reverse(O),
-
N = [ V | lists:reverse(T)],
To insert V in the head and remove P from the tail. But can you see the problem with that: Two list reverse operations, that to me seems like expensive operations, maybe I’m not understanding fully the erlang’s lists module but I can’t find other way to do this using lists.
I know several other ways to implement circular buffers using arrays, one even might think using queues for what I need this buffer, but I need to remove any index from the buffer so lists makes a lot more sense than any other data structure.
Well I’ll be glad if someone enlightens me with another solution
EOP
Tags: erlang · programmingView Comments
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)].
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.
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.
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
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.