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<chapter>
  <header>
    <copyright>
      <year>2001</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>List Handling</title>
    <prepared>Bjorn Gustavsson</prepared>
    <docno></docno>
    <date>2007-11-16</date>
    <rev></rev>
    <file>listHandling.xml</file>
  </header>

  <section>
    <title>Creating a List</title>
    
    <p>Lists can only be built starting from the end and attaching list
    elements at the beginning. If you use the "<c>++</c>" operator as
    follows, a new list is created that is a copy of the elements in
    <c>List1</c>, followed by <c>List2</c>:</p>

    <code type="erl">
List1 ++ List2</code>

    <p>Looking at how <c>lists:append/1</c> or <c>++</c> would be
    implemented in plain Erlang, clearly the first list is copied:</p>

    <code type="erl">
append([H|T], Tail) ->
    [H|append(T, Tail)];
append([], Tail) ->
    Tail.</code>

    <p>When recursing and building a list, it is important to ensure
    that you attach the new elements to the beginning of the list. In
    this way, you will build <em>one</em> list, not hundreds or thousands
    of copies of the growing result list.</p>

    <p>Let us first see how it is not to be done:</p>

    <p><em>DO NOT</em></p>
    <code type="erl"><![CDATA[
bad_fib(N) ->
    bad_fib(N, 0, 1, []).

bad_fib(0, _Current, _Next, Fibs) ->
    Fibs;
bad_fib(N, Current, Next, Fibs) -> 
    bad_fib(N - 1, Next, Current + Next, Fibs ++ [Current]).]]></code>

    <p>Here more than one list is built. In each iteration step a new list
    is created that is one element longer than the new previous list.</p>

    <p>To avoid copying the result in each iteration, build the list in
    reverse order and reverse the list when you are done:</p>

    <p><em>DO</em></p>
    <code type="erl"><![CDATA[
tail_recursive_fib(N) ->
    tail_recursive_fib(N, 0, 1, []).

tail_recursive_fib(0, _Current, _Next, Fibs) ->
    lists:reverse(Fibs);
tail_recursive_fib(N, Current, Next, Fibs) -> 
    tail_recursive_fib(N - 1, Next, Current + Next, [Current|Fibs]).]]></code>

  </section>

  <section>
    <title>List Comprehensions</title>

    <p>Lists comprehensions still have a reputation for being slow.
    They used to be implemented using funs, which used to be slow.</p>

    <p>A list comprehension:</p>

    <code type="erl"><![CDATA[
[Expr(E) || E <- List]]]></code>

    <p>is basically translated to a local function:</p>

    <code type="erl">
'lc^0'([E|Tail], Expr) ->
    [Expr(E)|'lc^0'(Tail, Expr)];
'lc^0'([], _Expr) -> [].</code>

    <p>If the result of the list comprehension will <em>obviously</em>
    not be used, a list will not be constructed. For example, in this code:</p>
    
    <code type="erl"><![CDATA[
[io:put_chars(E) || E <- List],
ok.]]></code>
 
    <p>or in this code:</p>
   
    <code type="erl"><![CDATA[
...
case Var of
    ... ->
        [io:put_chars(E) || E <- List];
    ... ->
end,
some_function(...),
...]]></code>
    
    <p>the value is not assigned to a variable, not passed to another function,
    and not returned. This means that there is no need to construct a list and
    the compiler will simplify the code for the list comprehension to:</p>

    <code type="erl">
'lc^0'([E|Tail], Expr) ->
    Expr(E),
    'lc^0'(Tail, Expr);
'lc^0'([], _Expr) -> [].</code>

    <p>The compiler also understands that assigning to '_' means that
    the value will not used. Therefore, the code in the following example
    will also be optimized:</p>

    <code type="erl"><![CDATA[
_ = [io:put_chars(E) || E <- List],
ok.]]></code>

  </section>

  <section>
    <title>Deep and Flat Lists</title>

    <p><seemfa marker="stdlib:lists#flatten/1">lists:flatten/1</seemfa>
    builds an entirely new list. It is therefore expensive, and even
    <em>more</em> expensive than the <c>++</c> operator (which copies its
    left argument, but not its right argument).</p>

    <p>In the following situations, you can easily avoid calling
    <c>lists:flatten/1</c>:</p>

    <list type="bulleted">
      <item>When sending data to a port. Ports understand deep lists
       so there is no reason to flatten the list before sending it to
       the port.</item>
      <item>When calling BIFs that accept deep lists, such as
      <seemfa marker="erts:erlang#list_to_binary/1">list_to_binary/1</seemfa> or
      <seemfa marker="erts:erlang#iolist_to_binary/1">iolist_to_binary/1</seemfa>.</item>
      <item>When you know that your list is only one level deep, you can use
      <seemfa marker="stdlib:lists#append/1">lists:append/1</seemfa>.</item>
    </list>

    <section>
      <title>Port Example</title>

    <p><em>DO</em></p>
    <pre>
      ...
      port_command(Port, DeepList)
      ...</pre>

    <p><em>DO NOT</em></p>
    <pre>
      ...
      port_command(Port, lists:flatten(DeepList))
      ...</pre>

    <p>A common way to send a zero-terminated string to a port is the following:</p>

    <p><em>DO NOT</em></p>
    <pre>
      ...
      TerminatedStr = String ++ [0], % String="foo" => [$f, $o, $o, 0]
      port_command(Port, TerminatedStr)
      ...</pre>

    <p>Instead:</p>

    <p><em>DO</em></p>
    <pre>
      ...
      TerminatedStr = [String, 0], % String="foo" => [[$f, $o, $o], 0]
      port_command(Port, TerminatedStr) 
      ...</pre>
    </section>

    <section>
      <title>Append Example</title>

    <p><em>DO</em></p>
    <pre>
      > lists:append([[1], [2], [3]]).
      [1,2,3]
      ></pre>

    <p><em>DO NOT</em></p>
    <pre>
      > lists:flatten([[1], [2], [3]]).
      [1,2,3]
      ></pre>
    </section>
  </section>

  <section>
    <title>Recursive List Functions</title>

    <p>In section about myths, the following myth was exposed:
    <seeguide marker="myths#tail_recursive">Tail-Recursive Functions
    are Much Faster Than Recursive Functions</seeguide>.</p>

    <p>There is usually not much difference between
    a body-recursive list function and tail-recursive function that reverses
    the list at the end. Therefore, concentrate on writing beautiful code
    and forget about the performance of your list functions. In the
    time-critical parts of your code (and only there), <em>measure</em>
    before rewriting your code.</p>

    <note><p>This section is about list functions that <em>construct</em>
    lists. A tail-recursive function that does not construct a list runs
    in constant space, while the corresponding body-recursive function
    uses stack space proportional to the length of the list.</p></note>

    <p>For example, a function that sums a list of integers, is
    <em>not</em> to be written as follows:</p>
    
    <p><em>DO NOT</em></p>
    <code type="erl">
recursive_sum([H|T]) -> H+recursive_sum(T);
recursive_sum([])    -> 0.</code>

    <p>Instead:</p>

    <p><em>DO</em></p>
    <code type="erl">
sum(L) -> sum(L, 0).

sum([H|T], Sum) -> sum(T, Sum + H);
sum([], Sum)    -> Sum.</code>
  </section>
</chapter>

