List creation in Erlang

Question

From "Erlang Programming" by Cesarini exercise 3-2

As I go through "Erlang Programming" I get weird list creation issues. From exercise 3-2 I wrote two similar functions.

create( 0 ) -> [];
create( N ) when N > 0 -> [ N | create( N-1 ) ].

reverse_create( 0 ) -> [];
reverse_create( N ) when N > 0 -> [ reverse_create( N-1 ) | N ].

so create(3) generates as I'd expect.

exercise3:create(3).
[3,2,1]

but reverse_create does not generate the list I expect.

exercise3:reverse_create(3).
[[[[]|1]|2]|3]

What do I need to change so that reverse_create(3) returns [1,2,3]? Thanks for explaining.

OK, got the example working. Any clue why the two similar functions above return such different answers? Why does create return a clean list while reverse_create returns lists of lists? — Superpolock, Nov 20 '10 at 08:43
you can read this for explanation: http://learnyousomeerlang.com/starting-out-for-real#lists and also look at these: http://stackoverflow.com/questions/3232786/how-to-concat-lists-in-erlang-without-creating-nested-lists and http://stackoverflow.com/questions/1919097/functional-programming-what-is-an-improper-list — Peer Stritzinger, Nov 20 '10 at 18:56

Chandra Patni · Accepted Answer · 2010-12-01T08:48:15.127

5

reverse_create returns a list and you using that as head element to create the list which is resulting in nested lists. Try this solution:

reverse_create( 0 ) -> [];
reverse_create( N ) when N > 0 -> reverse_create( N-1 ) ++ [N].

EDIT: A much better implementation would be:

reverse_create2(N) -> reverse_create_helper(N, []).

reverse_create_helper(0, Acc) ->
    Acc;
reverse_create_helper(N, Acc) ->
    reverse_create_helper(N-1, [N|Acc]).

edited Dec 01 '10 at 08:48

answered Nov 20 '10 at 08:29

Chandra Patni

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It makes nice O(N^2) from O(N) task. – Hynek -Pichi- Vychodil Nov 30 '10 at 13:33
I have made the edits as you correctly pointed out since ++ copies the left operand. – Chandra Patni Dec 01 '10 at 08:49

score 3 · Answer 2 · answered Nov 21 '10 at 04:44

Normally a function such as reverse_create would be done in a tail-recursive fashion with an accumulator.

reverse_create(N) ->
    reverse_create(N, []).

reverse_create(0, Acc) ->
    Acc;
reverse_create(N, Acc) when N > 0 ->
    reverse_create(N - 1, [N | Acc]).

score 2 · Answer 3 · answered Nov 20 '10 at 09:37

2

Of course, you could always do:

reverse_create(N) -> lists:reverse(create(N)).

This would actually run faster. But that obviously isn't the intent of the exercise. :)

answered Nov 20 '10 at 09:37

OJ.

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score 1 · Answer 4 · answered Nov 10 '11 at 05:13

1

I'm reading the same book, so I'm no more expert than you, but this worked for me...

create(0) -> [];
create(N) when N > 0 -> create(N-1) ++ [N].

reverse_create(0) -> [];
reverse_create(N) when N > 0 -> [N|create(N-1)].

answered Nov 10 '11 at 05:13

Felix

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score 0 · Answer 5 · answered Sep 11 '13 at 09:02

0

This is



    reverse_create(0) -> [];
    reverse_create(N) ->
        list_create_1(1, N, []).

    list_create_1(I, N, List) when N >= I ->
        list_create_1(I + 1, N, [I | List]);
    list_create_1(_, _, List) -> List.

answered Sep 11 '13 at 09:02

ololo

1

List creation in Erlang

5 Answers5