path: root/lib/stdlib/doc/src/lists.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>lists</title>
    <prepared>Robert Virding</prepared>
    <docno>1</docno>
    <date>96-09-28</date>
    <rev>A</rev>
  </header>
  <module>lists</module>
  <modulesummary>List Processing Functions</modulesummary>
  <description>
    <p>This module contains functions for list processing.</p>

    <p>Unless otherwise stated, all functions assume that position
      numbering starts at 1. That is, the first element of a list is at
      position 1.</p>

    <p>Two terms <c>T1</c> and <c>T2</c> compare equal if
      <c>T1&nbsp;==&nbsp;T2</c> evaluates to <c>true</c>. They match
      if <c>T1&nbsp;=:=&nbsp;T2</c> evaluates to <c>true</c>.</p>

    <p>Whenever an <marker
      id="ordering_function"></marker><em>ordering function</em>
      <c>F</c> is expected as argument, it is assumed that the
      following properties hold of <c>F</c> for all x, y and z:</p>
    <list type="bulleted">
      <item><p>if x <c>F</c> y and y <c>F</c> x then x = y (<c>F</c>
        is antisymmetric);</p>
      </item>
      <item><p>if x <c>F</c> y and y <c>F</c> z then x <c>F</c> z
        (<c>F</c> is transitive);</p>
      </item>
      <item><p>x <c>F</c> y or y <c>F</c> x (<c>F</c> is total).</p>
      </item>
    </list>
    <p>An example of a typical ordering function is less than or equal
      to, <c>=&lt;/2</c>.</p>

  </description>
  <funcs>
    <func>
      <name name="all" arity="2"/>
      <fsummary>Return true if all elements in the list satisfy<c>Pred</c></fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns
          <c>true</c> for all elements <c><anno>Elem</anno></c> in <c><anno>List</anno></c>,
          otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="any" arity="2"/>
      <fsummary>Return true if any of the elements in the list satisfies<c>Pred</c></fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns
          <c>true</c> for at least one element <c><anno>Elem</anno></c> in
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="append" arity="1"/>
      <fsummary>Append a list of lists</fsummary>
      <desc>
        <p>Returns a list in which all the sub-lists of
          <c><anno>ListOfLists</anno></c> have been appended. For example:</p>
        <pre>
> <input>lists:append([[1, 2, 3], [a, b], [4, 5, 6]]).</input>
[1,2,3,a,b,4,5,6]</pre>
      </desc>
    </func>
    <func>
      <name name="append" arity="2"/>
      <fsummary>Append two lists</fsummary>
      <desc>
        <p>Returns a new list <c><anno>List3</anno></c> which is made from
          the elements of <c><anno>List1</anno></c> followed by the elements of
          <c><anno>List2</anno></c>. For example:</p>
        <pre>
> <input>lists:append("abc", "def").</input>
"abcdef"</pre>
        <p><c>lists:append(A, B)</c> is equivalent to <c>A ++ B</c>.</p>
      </desc>
    </func>
    <func>
      <name name="concat" arity="1"/>
      <fsummary>Concatenate a list of atoms</fsummary>
      <desc>
        <p>Concatenates the text representation of the elements
          of <c><anno>Things</anno></c>. The elements of <c><anno>Things</anno></c> can be atoms,
          integers, floats or strings.</p>
        <pre>
> <input>lists:concat([doc, '/', file, '.', 3]).</input>
"doc/file.3"</pre>
      </desc>
    </func>
    <func>
      <name name="delete" arity="2"/>
      <fsummary>Delete an element from a list</fsummary>
      <desc>
        <p>Returns a copy of <c><anno>List1</anno></c> where the first element
          matching <c><anno>Elem</anno></c> is deleted, if there is such an
          element.</p>
      </desc>
    </func>
    <func>
      <name name="droplast" arity="1"/>
      <fsummary>Drop the last element of a list</fsummary>
      <desc>
        <p>Drops the last element of a <c><anno>List</anno></c>. The list should
           be non-empty, otherwise the function will crash with a <c>function_clause</c></p>
      </desc>
    </func>
    <func>
      <name name="dropwhile" arity="2"/>
      <fsummary>Drop elements from a list while a predicate is true</fsummary>
      <desc>
        <p>Drops elements <c><anno>Elem</anno></c> from <c><anno>List1</anno></c> while
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c> and returns
          the remaining list.</p>
      </desc>
    </func>
    <func>
      <name name="duplicate" arity="2"/>
      <fsummary>Make N copies of element</fsummary>
      <desc>
        <p>Returns a list which contains <c><anno>N</anno></c> copies of the term
          <c><anno>Elem</anno></c>. For example:</p>
        <pre>
> <input>lists:duplicate(5, xx).</input>
[xx,xx,xx,xx,xx]</pre>
      </desc>
    </func>
    <func>
      <name name="filter" arity="2"/>
      <fsummary>Choose elements which satisfy a predicate</fsummary>
      <desc>
        <p><c><anno>List2</anno></c> is a list of all elements <c><anno>Elem</anno></c> in
          <c><anno>List1</anno></c> for which <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns
          <c>true</c>.</p>
      </desc>
    </func>
    <func>
      <name name="filtermap" arity="2"/>
      <fsummary>Filter and map elements which satisfy a function</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>)</c> on successive elements <c>Elem</c>
          of <c><anno>List1</anno></c>. <c><anno>Fun</anno>/2</c> must return either a boolean
          or a tuple <c>{true, <anno>Value</anno>}</c>. The function returns the list of elements
          for which <c><anno>Fun</anno></c> returns a new value, where a value of <c>true</c>
          is synonymous with <c>{true, <anno>Elem</anno>}</c>.</p>
        <p>That is, <c>filtermap</c> behaves as if it had been defined as follows:</p>
        <code type="none">
filtermap(Fun, List1) ->
    lists:foldr(fun(Elem, Acc) ->
                       case Fun(Elem) of
                           false -> Acc;
                           true -> [Elem|Acc];
                           {true,Value} -> [Value|Acc]
                       end
                end, [], List1).</code>
        <p>Example:</p>
        <pre>
> <input>lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X div 2}; _ -> false end end, [1,2,3,4,5]).</input>
[1,2]</pre>
      </desc>
    </func>
    <func>
      <name name="flatlength" arity="1"/>
      <fsummary>Length of flattened deep list</fsummary>
      <desc>
        <p>Equivalent to <c>length(flatten(<anno>DeepList</anno>))</c>, but more
          efficient.</p>
      </desc>
    </func>
    <func>
      <name name="flatmap" arity="2"/>
      <fsummary>Map and flatten in one pass</fsummary>
      <desc>
        <p>Takes a function from <c><anno>A</anno></c>s to lists of <c><anno>B</anno></c>s, and a
          list of <c><anno>A</anno></c>s (<c><anno>List1</anno></c>) and produces a list of
          <c><anno>B</anno></c>s by applying the function to every element in
          <c><anno>List1</anno></c> and appending the resulting lists.</p>
        <p>That is, <c>flatmap</c> behaves as if it had been defined as
          follows:</p>
        <code type="none">
flatmap(Fun, List1) ->
    append(map(Fun, List1)).</code>
        <p>Example:</p>
        <pre>
> <input>lists:flatmap(fun(X)->[X,X] end, [a,b,c]).</input>
[a,a,b,b,c,c]</pre>
      </desc>
    </func>
    <func>
      <name name="flatten" arity="1"/>
      <fsummary>Flatten a deep list</fsummary>
      <desc>
        <p>Returns a flattened version of <c><anno>DeepList</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="flatten" arity="2"/>
      <fsummary>Flatten a deep list</fsummary>
      <desc>
        <p>Returns a flattened version of <c><anno>DeepList</anno></c> with the tail
          <c><anno>Tail</anno></c> appended.</p>
      </desc>
    </func>
    <func>
      <name name="foldl" arity="3"/>
      <fsummary>Fold a function over a list</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>, <anno>AccIn</anno>)</c> on successive elements <c>A</c>
          of <c><anno>List</anno></c>, starting with <c><anno>AccIn</anno> == <anno>Acc0</anno></c>.
          <c><anno>Fun</anno>/2</c> must return a new accumulator which is passed to
          the next call. The function returns the final value of
          the accumulator. <c><anno>Acc0</anno></c> is returned if the list is empty.
          For example:</p>
        <pre>
> <input>lists:foldl(fun(X, Sum) -> X + Sum end, 0, [1,2,3,4,5]).</input>
15
> <input>lists:foldl(fun(X, Prod) -> X * Prod end, 1, [1,2,3,4,5]).</input>
120</pre>
      </desc>
    </func>
    <func>
      <name name="foldr" arity="3"/>
      <fsummary>Fold a function over a list</fsummary>
      <desc>
        <p>Like <c>foldl/3</c>, but the list is traversed from right to
          left. For example:</p>
        <pre>
> <input>P = fun(A, AccIn) -> io:format("~p ", [A]), AccIn end.</input>
#Fun&lt;erl_eval.12.2225172&gt;
> <input>lists:foldl(P, void, [1,2,3]).</input>
1 2 3 void
> <input>lists:foldr(P, void, [1,2,3]).</input>
3 2 1 void</pre>
        <p><c>foldl/3</c> is tail recursive and would usually be
          preferred to <c>foldr/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="foreach" arity="2"/>
      <fsummary>Apply a function to each element of a list</fsummary>
      <desc>
        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>)</c> for each element <c><anno>Elem</anno></c> in
          <c><anno>List</anno></c>. This function is used for its side effects and
          the evaluation order is defined to be the same as the order
          of the elements in the list.</p>
      </desc>
    </func>
    <func>
      <name name="keydelete" arity="3"/>
      <fsummary>Delete an element from a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a tuple whose <c><anno>N</anno></c>th element compares equal to
          <c><anno>Key</anno></c> is deleted, if there is such a tuple.</p>
      </desc>
    </func>
    <func>
      <name name="keyfind" arity="3"/>
      <fsummary>Search for an element in a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList</anno></c> for a
          tuple whose <c><anno>N</anno></c>th element compares equal to <c><anno>Key</anno></c>.
          Returns <c><anno>Tuple</anno></c> if such a tuple is found,
          otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="keymap" arity="3"/>
      <fsummary>Map a function over a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list of tuples where, for each tuple in
          <c><anno>TupleList1</anno></c>, the <c><anno>N</anno></c>th element <c><anno>Term1</anno></c> of the tuple
          has been replaced with the result of calling
          <c><anno>Fun</anno>(<anno>Term1</anno>)</c>.</p>
        <p>Examples:</p>
        <pre>
> <input>Fun = fun(Atom) -> atom_to_list(Atom) end.</input>
#Fun&lt;erl_eval.6.10732646&gt;
2> <input>lists:keymap(Fun, 2, [{name,jane,22},{name,lizzie,20},{name,lydia,15}]).</input>
[{name,"jane",22},{name,"lizzie",20},{name,"lydia",15}]</pre>
      </desc>
    </func>
    <func>
      <name name="keymember" arity="3"/>
      <fsummary>Test for membership of a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns <c>true</c> if there is a tuple in <c><anno>TupleList</anno></c>
          whose <c><anno>N</anno></c>th element compares equal to <c><anno>Key</anno></c>, otherwise
          <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="keymerge" arity="3"/>
      <fsummary>Merge two key-sorted lists of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>TupleList1</anno></c>
          and <c><anno>TupleList2</anno></c>. The merge is performed on
          the <c><anno>N</anno></c>th element of each tuple. Both <c><anno>TupleList1</anno></c> and
          <c><anno>TupleList2</anno></c> must be key-sorted prior to evaluating this
          function. When two tuples compare equal, the tuple from
          <c><anno>TupleList1</anno></c> is picked before the tuple from
          <c><anno>TupleList2</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="keyreplace" arity="4"/>
      <fsummary>Replace an element in a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a <c>T</c> tuple whose <c><anno>N</anno></c>th element
          compares equal to <c><anno>Key</anno></c> is replaced with
          <c><anno>NewTuple</anno></c>, if there is such a tuple <c>T</c>.</p>
      </desc>
    </func>
    <func>
      <name name="keysearch" arity="3"/>
      <fsummary>Search for an element in a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList</anno></c> for a
          tuple whose <c><anno>N</anno></c>th element compares equal to <c><anno>Key</anno></c>.
          Returns <c>{value, <anno>Tuple</anno>}</c> if such a tuple is found,
          otherwise <c>false</c>.</p>
      <note><p>This function is retained for backward compatibility.
      The function <c>lists:keyfind/3</c> (introduced in R13A)
      is in most cases more convenient.</p></note>
      </desc>
    </func>
    <func>
      <name name="keysort" arity="2"/>
      <fsummary>Sort a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list containing the sorted elements of the list
          <c><anno>TupleList1</anno></c>. Sorting is performed on the <c><anno>N</anno></c>th
          element of the tuples. The sort is stable.</p>
      </desc>
    </func>
    <func>
      <name name="keystore" arity="4"/>
      <fsummary>Store an element in a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a copy of <c><anno>TupleList1</anno></c> where the first
          occurrence of a tuple <c>T</c> whose <c><anno>N</anno></c>th element
          compares equal to <c><anno>Key</anno></c> is replaced with
          <c><anno>NewTuple</anno></c>, if there is such a tuple <c>T</c>. If there
          is no such tuple <c>T</c> a copy of <c><anno>TupleList1</anno></c> where
          [<c><anno>NewTuple</anno></c>] has been appended to the end is
          returned.</p>
      </desc>
    </func>
    <func>
      <name name="keytake" arity="3"/>
      <fsummary>Extract an element from a list of tuples</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Searches the list of tuples <c><anno>TupleList1</anno></c> for a tuple
          whose <c><anno>N</anno></c>th element compares equal to <c><anno>Key</anno></c>.
          Returns <c>{value, <anno>Tuple</anno>, <anno>TupleList2</anno>}</c> if such a tuple is
          found, otherwise <c>false</c>. <c><anno>TupleList2</anno></c> is a copy
          of <c><anno>TupleList1</anno></c> where the first occurrence of
          <c><anno>Tuple</anno></c> has been removed.</p>
      </desc>
    </func>
    <func>
      <name name="last" arity="1"/>
      <fsummary>Return last element in a list</fsummary>
      <desc>
        <p>Returns the last element in <c><anno>List</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="map" arity="2"/>
      <fsummary>Map a function over a list</fsummary>
      <desc>
        <p>Takes a function from <c><anno>A</anno></c>s to <c><anno>B</anno></c>s, and a list of
          <c><anno>A</anno></c>s and produces a list of <c><anno>B</anno></c>s by applying
          the function to every element in the list. This function is
          used to obtain the return values. The evaluation order is
          implementation dependent.</p>
      </desc>
    </func>
    <func>
      <name name="mapfoldl" arity="3"/>
      <fsummary>Map and fold in one pass</fsummary>
      <desc>
        <p><c>mapfoldl</c> combines the operations of <c>map/2</c> and
          <c>foldl/3</c> into one pass. An example, summing
          the elements in a list and double them at the same time:</p>
        <pre>
> <input>lists:mapfoldl(fun(X, Sum) -> {2*X, X+Sum} end,</input>
<input>0, [1,2,3,4,5]).</input>
{[2,4,6,8,10],15}</pre>
      </desc>
    </func>
    <func>
      <name name="mapfoldr" arity="3"/>
      <fsummary>Map and fold in one pass</fsummary>
      <desc>
        <p><c>mapfoldr</c> combines the operations of <c>map/2</c> and
          <c>foldr/3</c> into one pass.</p>
      </desc>
    </func>
    <func>
      <name name="max" arity="1"/>
      <fsummary>Return maximum element of a list</fsummary>
      <desc>
        <p>Returns the first element of <c><anno>List</anno></c> that compares
          greater than or equal to all other elements of
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="member" arity="2"/>
      <fsummary>Test for membership of a list</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>Elem</anno></c> matches some element of
          <c><anno>List</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="merge" arity="1"/>
      <fsummary>Merge a list of sorted lists</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging all the sub-lists
          of <c><anno>ListOfLists</anno></c>. All sub-lists must be sorted prior to
          evaluating this function. When two elements compare equal,
          the element from the sub-list with the lowest position in
          <c><anno>ListOfLists</anno></c> is picked before the other element.</p>
      </desc>
    </func>
    <func>
      <name name="merge" arity="2"/>
      <fsummary>Merge two sorted lists</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and <c><anno>List2</anno></c> must be
          sorted prior to evaluating this function. When two elements
          compare equal, the element from <c><anno>List1</anno></c> is picked
          before the element from <c><anno>List2</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="merge" arity="3"/>
      <fsummary>Merge two sorted list</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and <c><anno>List2</anno></c> must be
          sorted according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c> prior to evaluating this function. <c><anno>Fun</anno>(<anno>A</anno>,
          <anno>B</anno>)</c> should return <c>true</c> if <c><anno>A</anno></c> compares less
          than or equal to <c><anno>B</anno></c> in the ordering, <c>false</c>
          otherwise. When two elements compare equal, the element from
          <c><anno>List1</anno></c> is picked before the element from
          <c><anno>List2</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="merge3" arity="3"/>
      <fsummary>Merge three sorted lists</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c> and <c><anno>List3</anno></c>. All of <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c> and <c><anno>List3</anno></c> must be sorted prior to
          evaluating this function. When two elements compare equal,
          the element from <c><anno>List1</anno></c>, if there is such an element,
          is picked before the other element, otherwise the element
          from <c><anno>List2</anno></c> is picked before the element from
          <c><anno>List3</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="min" arity="1"/>
      <fsummary>Return minimum element of a list</fsummary>
      <desc>
        <p>Returns the first element of <c><anno>List</anno></c> that compares
          less than or equal to all other elements of
          <c><anno>List</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="nth" arity="2"/>
      <fsummary>Return the Nth element of a list</fsummary>
      <type_desc variable="N">1..length(<anno>List</anno>)</type_desc>
      <desc>
        <p>Returns the <c><anno>N</anno></c>th element of <c><anno>List</anno></c>. For example:</p>
        <pre>
> <input>lists:nth(3, [a, b, c, d, e]).</input>
c</pre>
      </desc>
    </func>
    <func>
      <name name="nthtail" arity="2"/>
      <fsummary>Return the Nth tail of a list</fsummary>
      <type_desc variable="N">0..length(<anno>List</anno>)</type_desc>
      <desc>
        <p>Returns the <c><anno>N</anno></c>th tail of <c><anno>List</anno></c>, that is, the sublist of
          <c><anno>List</anno></c> starting at <c><anno>N</anno>+1</c> and continuing up to
          the end of the list. For example:</p>
        <pre>
> <input>lists:nthtail(3, [a, b, c, d, e]).</input>
[d,e]
> <input>tl(tl(tl([a, b, c, d, e]))).</input>
[d,e]
> <input>lists:nthtail(0, [a, b, c, d, e]).</input>
[a,b,c,d,e]
> <input>lists:nthtail(5, [a, b, c, d, e]).</input>
[]</pre>
      </desc>
    </func>
    <func>
      <name name="partition" arity="2"/>
      <fsummary>Partition a list into two lists based on a predicate</fsummary>
      <desc>
        <p>Partitions <c><anno>List</anno></c> into two lists, where the first list
          contains all elements for which <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns
          <c>true</c>, and the second list contains all elements for
          which <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>false</c>.</p>
        <p>Examples:</p>
        <pre>
> <input>lists:partition(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).</input>
{[1,3,5,7],[2,4,6]}
> <input>lists:partition(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).</input>
{[a,b,c,d,e],[1,2,3,4]}</pre>
        <p>See also <c>splitwith/2</c> for a different way to partition
          a list.</p>
      </desc>
    </func>
    <func>
      <name name="prefix" arity="2"/>
      <fsummary>Test for list prefix</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>List1</anno></c> is a prefix of
          <c><anno>List2</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="reverse" arity="1"/>
      <fsummary>Reverse a list</fsummary>
      <desc>
        <p>Returns a list with the elements in <c><anno>List1</anno></c>
          in reverse order.</p>
      </desc>
    </func>
    <func>
      <name name="reverse" arity="2"/>
      <fsummary>Reverse a list appending a tail</fsummary>
      <desc>
        <p>Returns a list with the elements in <c><anno>List1</anno></c>
          in reverse order, with the tail <c><anno>Tail</anno></c> appended. For
          example:</p>
        <pre>
> <input>lists:reverse([1, 2, 3, 4], [a, b, c]).</input>
[4,3,2,1,a,b,c]</pre>
      </desc>
    </func>
    <func>
      <name name="seq" arity="2"/>
      <name name="seq" arity="3"/>
      <fsummary>Generate a sequence of integers</fsummary>
      <desc>
        <p>Returns a sequence of integers which starts with <c><anno>From</anno></c>
          and contains the successive results of adding <c><anno>Incr</anno></c> to
          the previous element, until <c><anno>To</anno></c> has been reached or
          passed (in the latter case, <c><anno>To</anno></c> is not an element of
          the sequence). <c><anno>Incr</anno></c> defaults to 1.</p>
        <p>Failure: If <c><anno>To</anno>&lt;<anno>From</anno>-<anno>Incr</anno></c> and <c><anno>Incr</anno></c>
          is positive, or if <c><anno>To</anno>><anno>From</anno>-<anno>Incr</anno></c> and <c><anno>Incr</anno></c> is
          negative, or if <c><anno>Incr</anno>==0</c> and <c><anno>From</anno>/=<anno>To</anno></c>.</p>
        <p>The following equalities hold for all sequences:</p>
        <code type="none">
length(lists:seq(From, To)) == To-From+1
length(lists:seq(From, To, Incr)) == (To-From+Incr) div Incr</code>
        <p>Examples:</p>
        <pre>
> <input>lists:seq(1, 10).</input>
[1,2,3,4,5,6,7,8,9,10]
> <input>lists:seq(1, 20, 3).</input>
[1,4,7,10,13,16,19]
> <input>lists:seq(1, 0, 1).</input>
[]
> <input>lists:seq(10, 6, 4).</input>
[]
> <input>lists:seq(1, 1, 0).</input>
[1]</pre>
      </desc>
    </func>
    <func>
      <name name="sort" arity="1"/>
      <fsummary>Sort a list</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="sort" arity="2"/>
      <fsummary>Sort a list</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c>, according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c>. <c><anno>Fun</anno>(<anno>A</anno>, <anno>B</anno>)</c> should return <c>true</c> if
          <c><anno>A</anno></c> compares less than or equal to <c><anno>B</anno></c> in the
          ordering, <c>false</c> otherwise.</p>
      </desc>
    </func>
    <func>
      <name name="split" arity="2"/>
      <fsummary>Split a list into two lists</fsummary>
      <type_desc variable="N">0..length(<anno>List1</anno>)</type_desc>
      <desc>
        <p>Splits <c><anno>List1</anno></c> into <c><anno>List2</anno></c> and <c><anno>List3</anno></c>.
          <c><anno>List2</anno></c> contains the first <c><anno>N</anno></c> elements and
          <c><anno>List3</anno></c> the rest of the elements (the <c><anno>N</anno></c>th tail).</p>
      </desc>
    </func>
    <func>
      <name name="splitwith" arity="2"/>
      <fsummary>Split a list into two lists based on a predicate</fsummary>
      <desc>
        <p>Partitions <c><anno>List</anno></c> into two lists according to
          <c><anno>Pred</anno></c>. <c>splitwith/2</c> behaves as if it is defined
          as follows:</p>
        <code type="none">
splitwith(Pred, List) ->
    {takewhile(Pred, List), dropwhile(Pred, List)}.</code>
        <p>Examples:</p>
        <pre>
> <input>lists:splitwith(fun(A) -> A rem 2 == 1 end, [1,2,3,4,5,6,7]).</input>
{[1],[2,3,4,5,6,7]}
> <input>lists:splitwith(fun(A) -> is_atom(A) end, [a,b,1,c,d,2,3,4,e]).</input>
{[a,b],[1,c,d,2,3,4,e]}</pre>
        <p>See also <c>partition/2</c> for a different way to partition
          a list.</p>
      </desc>
    </func>
    <func>
      <name name="sublist" arity="2"/>
      <fsummary>Return a sub-list of a certain length, starting at the first position</fsummary>
      <desc>
        <p>Returns the sub-list of <c><anno>List1</anno></c> starting at position 1
          and with (max) <c><anno>Len</anno></c> elements. It is not an error for
          <c><anno>Len</anno></c> to exceed the length of the list, in that case
          the whole list is returned.</p>
      </desc>
    </func>
    <func>
      <name name="sublist" arity="3"/>
      <fsummary>Return a sub-list starting at a given position and with a given number of elements</fsummary>
      <type_desc variable="Start">1..(length(<anno>List1</anno>)+1)</type_desc>
      <desc>
        <p>Returns the sub-list of <c><anno>List1</anno></c> starting at <c><anno>Start</anno></c>
          and with (max) <c><anno>Len</anno></c> elements. It is not an error for
          <c><anno>Start</anno>+<anno>Len</anno></c> to exceed the length of the list.</p>
        <pre>
> <input>lists:sublist([1,2,3,4], 2, 2).</input>
[2,3]
> <input>lists:sublist([1,2,3,4], 2, 5).</input>
[2,3,4]
> <input>lists:sublist([1,2,3,4], 5, 2).</input>
[]</pre>
      </desc>
    </func>
    <func>
      <name name="subtract" arity="2"/>
      <fsummary>Subtract the element in one list from another list</fsummary>
      <desc>
        <p>Returns a new list <c><anno>List3</anno></c> which is a copy of
          <c><anno>List1</anno></c>, subjected to the following procedure: for each
          element in <c><anno>List2</anno></c>, its first occurrence in <c><anno>List1</anno></c>
          is deleted. For example:</p>
        <pre>
> <input>lists:subtract("123212", "212").</input>
"312".</pre>
        <p><c>lists:subtract(A, B)</c> is equivalent to <c>A -- B</c>.</p>
      </desc>
    </func>
    <func>
      <name name="suffix" arity="2"/>
      <fsummary>Test for list suffix</fsummary>
      <desc>
        <p>Returns <c>true</c> if <c><anno>List1</anno></c> is a suffix of
          <c><anno>List2</anno></c>, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="sum" arity="1"/>
      <fsummary>Return sum of elements in a list</fsummary>
      <desc>
        <p>Returns the sum of the elements in <c><anno>List</anno></c>.</p>
      </desc>
    </func>
    <func>
      <name name="takewhile" arity="2"/>
      <fsummary>Take elements from a list while a predicate is true</fsummary>
      <desc>
        <p>Takes elements <c><anno>Elem</anno></c> from <c><anno>List1</anno></c> while
          <c><anno>Pred</anno>(<anno>Elem</anno>)</c> returns <c>true</c>, that is,
          the function returns the longest prefix of the list for which
          all elements satisfy the predicate.</p>
      </desc>
    </func>
    <func>
      <name name="ukeymerge" arity="3"/>
      <fsummary>Merge two key-sorted lists of tuples, removing duplicates</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>TupleList1</anno></c>
          and <c><anno>TupleList2</anno></c>. The merge is performed on the
          <c><anno>N</anno></c>th element of each tuple. Both <c><anno>TupleList1</anno></c> and
          <c><anno>TupleList2</anno></c> must be key-sorted without duplicates
          prior to evaluating this function. When two tuples compare
          equal, the tuple from <c><anno>TupleList1</anno></c> is picked and the
          one from <c><anno>TupleList2</anno></c> deleted.</p>
      </desc>
    </func>
    <func>
      <name name="ukeysort" arity="2"/>
      <fsummary>Sort a list of tuples, removing duplicates</fsummary>
      <type_desc variable="N">1..tuple_size(<anno>Tuple</anno>)</type_desc>
      <desc>
        <p>Returns a list containing the sorted elements of the list
          <c><anno>TupleList1</anno></c> where all but the first tuple of the
          tuples comparing equal have been deleted. Sorting is
          performed on the <c><anno>N</anno></c>th element of the tuples.</p>
      </desc>
    </func>
    <func>
      <name name="umerge" arity="1"/>
      <fsummary>Merge a list of sorted lists, removing duplicates</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging all the sub-lists
          of <c><anno>ListOfLists</anno></c>. All sub-lists must be sorted and
          contain no duplicates prior to evaluating this function.
          When two elements compare equal, the element from the
          sub-list with the lowest position in <c><anno>ListOfLists</anno></c> is
          picked and the other one deleted.</p>
      </desc>
    </func>
    <func>
      <name name="umerge" arity="2"/>
      <fsummary>Merge two sorted lists, removing duplicates</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and <c><anno>List2</anno></c> must be
          sorted and contain no duplicates prior to evaluating this
          function. When two elements compare equal, the element from
          <c><anno>List1</anno></c> is picked and the one from <c><anno>List2</anno></c>
          deleted.</p>
      </desc>
    </func>
    <func>
      <name name="umerge" arity="3"/>
      <fsummary>Merge two sorted lists, removing duplicates</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c> and
          <c><anno>List2</anno></c>. Both <c><anno>List1</anno></c> and <c><anno>List2</anno></c> must be
          sorted according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c>Fun</c> and contain no duplicates prior to evaluating
          this function. <c><anno>Fun</anno>(<anno>A</anno>, <anno>B</anno>)</c> should return <c>true</c> if
          <c><anno>A</anno></c> compares less than or equal to <c><anno>B</anno></c> in the
          ordering, <c>false</c> otherwise. When two elements compare
          equal, the element from
          <c><anno>List1</anno></c> is picked and the one from <c><anno>List2</anno></c>
          deleted.</p>
      </desc>
    </func>
    <func>
      <name name="umerge3" arity="3"/>
      <fsummary>Merge three sorted lists, removing duplicates</fsummary>
      <desc>
        <p>Returns the sorted list formed by merging <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c> and <c><anno>List3</anno></c>. All of <c><anno>List1</anno></c>,
          <c><anno>List2</anno></c> and <c><anno>List3</anno></c> must be sorted and contain no
          duplicates prior to evaluating this function. When two
          elements compare equal, the element from <c><anno>List1</anno></c> is
          picked if there is such an element, otherwise the element
          from <c><anno>List2</anno></c> is picked, and the other one deleted.</p>
      </desc>
    </func>
    <func>
      <name name="unzip" arity="1"/>
      <fsummary>Unzip a list of two-tuples into two lists</fsummary>
      <desc>
        <p>"Unzips" a list of two-tuples into two lists, where the first
          list contains the first element of each tuple, and the second
          list contains the second element of each tuple.</p>
      </desc>
    </func>
    <func>
      <name name="unzip3" arity="1"/>
      <fsummary>Unzip a list of three-tuples into three lists</fsummary>
      <desc>
        <p>"Unzips" a list of three-tuples into three lists, where
          the first list contains the first element of each tuple,
          the second list contains the second element of each tuple, and
          the third list contains the third element of each tuple.</p>
      </desc>
    </func>
    <func>
      <name name="usort" arity="1"/>
      <fsummary>Sort a list, removing duplicates</fsummary>
      <desc>
        <p>Returns a list containing the sorted elements of
          <c><anno>List1</anno></c> where all but the first element of the elements
          comparing equal have been deleted.</p>
      </desc>
    </func>
    <func>
      <name name="usort" arity="2"/>
      <fsummary>Sort a list, removing duplicates</fsummary>
      <desc>
        <p>Returns a list which contains the sorted elements of
          <c><anno>List1</anno></c> where all but the first element of the elements
          comparing equal according to the <seealso
          marker="#ordering_function">ordering function</seealso>
          <c><anno>Fun</anno></c> have been deleted. <c><anno>Fun</anno>(A, B)</c> should return
          <c>true</c> if <c>A</c> compares less than or equal to
          <c>B</c> in the ordering, <c>false</c> otherwise.</p>
      </desc>
    </func>
    <func>
      <name name="zip" arity="2"/>
      <fsummary>Zip two lists into a list of two-tuples</fsummary>
      <desc>
        <p>"Zips" two lists of equal length into one list of two-tuples,
          where the first element of each tuple is taken from the first
          list and the second element is taken from corresponding
          element in the second list.</p>
      </desc>
    </func>
    <func>
      <name name="zip3" arity="3"/>
      <fsummary>Zip three lists into a list of three-tuples</fsummary>
      <desc>
        <p>"Zips" three lists of equal length into one list of
          three-tuples, where the first element of each tuple is taken
          from the first list, the second element is taken from
          corresponding element in the second list, and the third
          element is taken from the corresponding element in the third
          list.</p>
      </desc>
    </func>
    <func>
      <name name="zipwith" arity="3"/>
      <fsummary>Zip two lists into one list according to a fun</fsummary>
      <desc>
        <p>Combine the elements of two lists of equal length into one
          list. For each pair <c><anno>X</anno>, <anno>Y</anno></c> of list elements from the two
          lists, the element in the result list will be
          <c><anno>Combine</anno>(<anno>X</anno>, <anno>Y</anno>)</c>.</p>
        <p><c>zipwith(fun(X, Y) -> {X,Y} end, List1, List2)</c> is
          equivalent to <c>zip(List1, List2)</c>.</p>
        <p>Example:</p>
        <pre>
> <input>lists:zipwith(fun(X, Y) -> X+Y end, [1,2,3], [4,5,6]).</input>
[5,7,9]</pre>
      </desc>
    </func>
    <func>
      <name name="zipwith3" arity="4"/>
      <fsummary>Zip three lists into one list according to a fun</fsummary>
      <desc>
        <p>Combine the elements of three lists of equal length into one
          list. For each triple <c><anno>X</anno>, <anno>Y</anno>, <anno>Z</anno></c> of list elements from
          the three lists, the element in the result list will be
          <c><anno>Combine</anno>(<anno>X</anno>, <anno>Y</anno>, <anno>Z</anno>)</c>.</p>
        <p><c>zipwith3(fun(X, Y, Z) -> {X,Y,Z} end, List1, List2, List3)</c> is equivalent to <c>zip3(List1, List2, List3)</c>.</p>
        <p>Examples:</p>
        <pre>
> <input>lists:zipwith3(fun(X, Y, Z) -> X+Y+Z end, [1,2,3], [4,5,6], [7,8,9]).</input>
[12,15,18]
> <input>lists:zipwith3(fun(X, Y, Z) -> [X,Y,Z] end, [a,b,c], [x,y,z], [1,2,3]).</input>
[[a,x,1],[b,y,2],[c,z,3]]</pre>
      </desc>
    </func>
  </funcs>
</erlref>