# [erlang-questions] is this a well written function

Roelof Wobben r.wobben@REDACTED
Thu Feb 12 08:03:22 CET 2015

```Richard A. O'Keefe schreef op 12-2-2015 om 3:25:
> On 11/02/2015, at 8:45 pm, Roelof Wobben <r.wobben@REDACTED> wrote:
>> -export([display_numbers/1]).
>>
>> %%  displays(N) displays the numbers from  1,2,...+N when N is a non-negative integer.
>> %%  It is not defined for other arguments.
>> %%  When N is a non-negative number, a helper function is called so it prints out
>> %%  the numbers in the right order. This is a exercise from the Erlang Programming book
>> %%  where I have to practice side-effects.
> The comment disagrees with the code.
> The comment says the function call is 'displays(_)','
> but the code says it is 'display_numbers(_)'.
>
> The comment disagrees with the code.
> The comment says "when N is a NON-NEGATIVE INTEGER",
> but the code insists on a STRICTLY POSITIVE number of
> any kind.  So the comment would accept 0 and reject 1.2,
> while the code rejects 0 and accepts 1.2.
>
>> display_numbers(Number) when Number > 0 ->
>>   display_numbers_loop(Number, 1).
> You would do better to stick with the comment,
> and to count UP from 0, so that the control argument
> actually means something: it counts how many numbers
> already printed.  Also, control arguments are
> conventionally written first.
>
> display_numbers(N) when is_integer(N), N >= 0 ->  % 1; A,B
>      display_numbers_loop(0, N).                   % 2; C
>
> %   In display_numbers_loop(N0, N), N is how many numbers
> %   we want to print altogether, and N0 is how many
> %   numbers we have already printed.
>
> display_numbers_loop(N, N) ->                     % 3; C
>      ok;                                           % 4; F
> display_numbers_loop(N0, N) ->                    % 5;
>      N1 = N0 + 1,                                  % 6; C,D
>      io:format("Number: ~p~n", [N1]),              % 7; D
>      display_numbers_loop(N1, N).                  % 8;
>
>

Thanks for all the remarks. I changed it all but one remark about your
code ;

When I do N1 = N0 + 1 then I get a error because N1 is imutable and
cannot be changed.

So the new code will be :

-module(side_effects).

-export([display_numbers/1]).

%%  display_numbers(N) displays the numbers from  1,2,...+N when N is a
non-negative integer.
%%  It is not defined for other arguments.
%%  When N is a non-negative number, a helper function is called so it
prints out
%%  the numbers in the right order. This is a exercise from the Erlang
Programming book
%%  where I have to practice side-effects.
display_numbers(Number) when is_integer(Number), Number > 0 ->
display_numbers_loop(0, Number ).

%%  When the control argument)(second argument) is equal to the number
%%  the user has given, the end is reached and the last number is printed
display_numbers_loop(Number, Number) ->
ok;

%%  When the contro argument(second argument) is not equal to the number
%%  the user has given the control argument is increased by one and the
%%  display_numbers_loop function is called again with the new arguments.
display_numbers_loop(Current, Number) ->
io:format("Number:~p~n",[Current + 1]),
display_numbers_loop(Current +1, Number ).

Roelof

```