# [erlang-questions] Additions to lists module

Gleb Peregud <>
Wed Nov 26 14:46:27 CET 2008

```Hi,

Isn't #myTable.field_name enough to determine the position of the
field in record's tuple? Have I understood this correctly?

BR

On 11/26/08, Serge Aleynikov <> wrote:
> Say you have a mnesia table with a structure being record_info(fields,
> myTable).  The pos/2 function would help you find a position of a given
> field in the table.
>
> Mazen Harake wrote:
>> What is the idea behind pos?
>>
>> Curious because I have never been in a situation where I need to know
>> the actual position of an element since I always assume that the order
>> in a list is always undefined. Perhaps I missed something... do you have
>> a practical example?
>>
>> /M
>>
>> Serge Aleynikov wrote:
>>> I'd like to propose addition of these two functions to the lists
>>> module.   Very often I find the need for this functionality and feel
>>> that it belongs to the standard library.
>>>
>>> Serge
>>>
>>>
>>>
>>> %% @spec (List::list(), Ele) -> integer()
>>> %% @doc Returns the position of `Ele' in the `List'. 0 is returned
>>> %% @end
>>> pos(List, Ele) ->
>>>      pos(List, Ele, 1).
>>> pos([Ele | Tail], Ele, Pos) ->
>>>      Pos;
>>> pos([_ | Tail], Ele, Pos) ->
>>>      pos(Tail, Ele, Pos+1);
>>> pos([], _Ele, _) ->
>>>      0.
>>>
>>> %% @spec (Pred, Acc, List::list()) -> Acc1
>>> %%          Pred = (Ele, Acc) -> Acc1
>>> %% @doc A combination of foldl/3 and takewhile/2. Accumulate the result
>>> %%      while the predicate function returns true.
>>> %% @end
>>> foldlwhile(Pred, Acc0, [H|T]) ->
>>>      case Pred(H, Acc0) of
>>>      {true, Acc} ->
>>>          foldwhile(Fun, Acc, T);
>>>      {false, Acc} ->
>>>          Acc;
>>>      end;
>>> foldlwhile(_Pred, Acc, []) ->
>>>      Acc.
>>> _______________________________________________
>>> erlang-questions mailing list
>>>
>>> http://www.erlang.org/mailman/listinfo/erlang-questions
>>>
>>
>>
>
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>
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--
Gleb Peregud
http://gleber.pl/

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