# [erlang-questions] Circular list

Ulf Wiger (TN/EAB) ulf.wiger@REDACTED
Mon Aug 25 15:33:17 CEST 2008

```yoursurrogategod@REDACTED skrev:
> Doing the below is pretty inefficient, as Programming Erlang by Joe
> Armstrong pg. 64 mentions.
>
> List ++ [Foo],
>
> I'm curious why doing this doesn't work at all.
>
> 4> L3 = [L2|"F"].
> [["B","C","D","E"],70]

The pattern [A|B] means "a list with the element A followed by
the list B".

In this case A = L2 and B = "F". So it *does* work - it's
just doesn't do the same thing as ++.  ;-)

Using [Head|Tail] patterns, the Erlang equivalent to ++
is:

'++'([H|T], L2) ->
[H|'++'(T, L2)];
'++'([], L2) ->
L2.

That is, you need to iterate to the end of the first list
in order to append another list to it.

The '++'(A,B) function above is less efficient than A++B,
since the latter is a BIF.

One place where you could use [L1|L2], or even [L1,L2]
in place of append, is when building an "IO list". When
sending a "deep" list through a port, or converting it
to a binary, using list_to_binary/1 or iolist_to_binary/1,
it is automatically and efficiently flattened.

BR,
Ulf W

>
> I'm not too keen on how lists work, so this is puzzling to me :) .
>
> On Aug 22, 9:34 am, "Colm Dougan" <colm.dou...@REDACTED> wrote:
>> Hi,
>>
>> If I have a list of a fixed size (say 10,000 elements).  What is the
>> most efficient way to add a new item to the list and force out the old
>> first element.  By most efficient I mostly mean without having to copy
>> the entire list each time, if that is even possible.
>>
>> I'm currently doing a double reverse trick, e.g. :
>>
>> Eshell V5.6.3  (abort with ^G)
>> 1> L1 = ["A", "B", "C", "D", "E"].
>> ["A","B","C","D","E"]
>> 2> [_ | L2] = L1.
>> ["A","B","C","D","E"]
>> 3> lists:reverse(["F" | lists:reverse(L2)]).
>> ["B","C","D","E","F"]
>>
>> Is there a more efficient way?  I tried a benchmark and the reverse
>> seems more costly than I expected.
>>
>> Colm
>> _______________________________________________
>> erlang-questions mailing list
>> erlang-questi...@REDACTED://www.erlang.org/mailman/listinfo/erlang-questions
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```

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