# [erlang-questions] how: Another library flatten function?

Robert Virding rvirding@REDACTED
Fri Feb 26 23:31:34 CET 2010

```Here are two very similar versions based on the code in lists.erl.
They are both coded in a very straight forward style (I think) without
reverse or append, but I haven't measured them.

-module(ft).

-export([fl1/2, fl2/2]).

fl1([[C|_]=H|T], Tail) when is_integer(C) ->
[H|fl1(T, Tail)];
fl1([H|T], Tail) when is_list(H) ->
fl1(H, fl1(T, Tail));
fl1([H|T], Tail) ->
[H|fl1(T, Tail)];
fl1([], Tail) -> Tail.

fl2([[C|_]=H|T], Tail) when not is_integer(C) ->
fl2(H, fl2(T, Tail));
fl2([[]|T], Tail) -> fl2(T, Tail);
fl2([H|T], Tail) ->
[H|fl2(T, Tail)];
fl2([], Tail) -> Tail.

Robert

2010/2/26 Dmitry Belayev <rumata-estor@REDACTED>:
> How do you propose to remove lists:reverse? To use A ++ [I] in the second
> clause? I don't think that would be optimal solution.
>
> Robert Virding wrote:
>>
>> I personally think this is the best solution, write your own function
>> that is, as it is such a simple function to start with. Of course, I
>> wouldn't use reverse but that is just a matter of detail. :-)
>>
>> Robert
>>
>> On 26 February 2010 16:50, Dmitry Belayev <rumata-estor@REDACTED> wrote:
>>
>>>
>>> You can write your own:
>>>
>>> f(List) ->
>>>  lists:reverse(f(List, [])).
>>>
>>> f([], A) ->
>>>  A;
>>> f([L|_]=I, A) when is_number(L) ->
>>>  [I | A];
>>> f([H|Tail], A) ->
>>>  f(Tail, f(H, A)).
>>>
>>> Bengt Kleberg wrote:
>>>
>>>>
>>>> Greetings,
>>>>
>>>> I have the following list: ["asd",[["Flow","kalle"]],["Sub","F"]]
>>>> I would like to flatten it to: ["asd","Flow","kalle","Sub","F"]
>>>> lists:flatten/1 is too effective. It gives me: "asdFlowkalleSubF"
>>>>
>>>> Is there another flatten somewhere?
>>>>
>>>>
>>>> bengt
>>>>
>>>>
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>>>>
>>>>
>>>>
>>>
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>>>
>>
>>
>
>
```