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.

Answer 1

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]).