Question

I’m trying to create a list and print it out, counting down from N to 1. This is my attempt:

%% Create a list counting down from N to 1 %%
-module(list).
-export([create_list/1]).

create_list(N) when length(N)<hd(N) ->
 lists:append([N],lists:last([N])-1),
 create_list(lists:last([N])-1); 
create_list(N) ->
 N.

This works when N is 1, but otherwise I get this error:

172> list:create_list([2]).
** exception error: an error occurred when evaluating an arithmetic expression
     in function  list:create_list/1 (list.erl, line 6)

Any help would be appreciated.

Answer 1

The error is lists:last([N])-1. Since N is an array as your input, lists:last([N]) will return N itself. Not a number you expect. And if you see the warning when compiling your code, there is another bug: lists:append will not append the element into N itself, but in the return value. In functional programming, the value of a variable cannot be changed.

Here’s my implementation:

create_list(N) ->
    create_list_iter(N, []).

create_list_iter(N, Acc) ->
    case N > 0 of
        true -> NewAcc = lists:append(Acc, [N]),
                create_list_iter(N-1, NewAcc);
        false -> Acc
    end.