erlang; cons operator does not flatten

Question

I wrote the function:

rotate_bin_list_right([H|T]) ->
  erlang:display(H),
  erlang:display(T),
  erlang:display([T|H]),
  [T|H].

When called with [<<"2">>,<<"3">>,<<"4">>,<<"2">>,<<"3">>,<<"4">>], it prints:

 <<"2">>
[<<"3">>,<<"4">>,<<"2">>,<<"3">>,<<"4">>]
[[<<"3">>,<<"4">>,<<"2">>,<<"3">>,<<"4">>]|<<"2">>]

I was expecting a "flattened list":

[<<"3">>,<<"4">>,<<"2">>,<<"3">>,<<"4">>,<<"2">>]

What is happening?

further to what Steve has mentioned in his answer, to get a flattened list as teh return, you must make the 'H' a list, and use the flatten function, like so: `erlang:display(lists:flatten([T|[H]])).` — liam_g, Feb 10 '16 at 17:56
@liam_g: It's unnecessarily slow implementation. `lists:flatten/1` is Erlang implemented `++/2` is Bif. — Hynek -Pichi- Vychodil, Feb 10 '16 at 19:32

score 3 · Answer 1 · edited May 23 '17 at 12:15

3

T is a list and H is not a list, so the construct [T|H] creates an improper list, where the tail is not a list.

edited May 23 '17 at 12:15

Community

answered Feb 10 '16 at 17:50

Steve Vinoski

No, @Tommy, `[[H]|T]` gives you a list with a single-element list containing `H` as the head and `T` as the tail. – Steve Vinoski Feb 10 '16 at 19:31
3

@Tommy, see @Hynek-Pichi-Vychodil's answer for the correct solution. Your fears of `++` are unfounded, as he explains. Also note that my answer doesn't include a solution because that's not what you asked; you merely asked "what's happening". – Steve Vinoski Feb 10 '16 at 19:42

score 1 · Accepted Answer · answered Feb 10 '16 at 17:59

1

Just for sake of completeness, this is correct implementation:

rotate_bin_list_right([H|T]) -> T ++ [H].

answered Feb 10 '16 at 17:59

I am new to Erlang, and from what I have read, `++` is very bad as opposed to prepending and reversing because it can lead to quadratic recursive functions. – Tommy Feb 10 '16 at 19:23
2

@Tommy: I see. You are new to Erlang. So take advice from someone who is not. It is the correct implementation of `rotate_list_right/1` if you like this operation. This is not recursive function so it can't lead to any recursive function including quadratic ones. – Hynek -Pichi- Vychodil Feb 10 '16 at 19:29
Thanks for your help. So is there a way to do this in O(1)? – Tommy Feb 11 '16 at 01:27
@Tommy: No, there is not a way to do it in O(1) but O(N) is good when you consider it is the immutable single-linked list. – Hynek -Pichi- Vychodil Feb 11 '16 at 17:28

2 Answers2