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<h2 id="sec:builtinlist"><a id="sec:4.28"><span class="sec-nr">4.28</span> <span class="sec-title">Built-in
list operations</span></a></h2>
<a id="sec:builtinlist"></a>
<p>Most list operations are defined in the library <code>library(lists)</code>
described in <a class="sec" href="lists.html">section A.14</a>. Some
that are implemented with more low-level primitives are built-in and
described here.
<dl class="latex">
<dt class="pubdef"><a id="is_list/1"><strong>is_list</strong>(<var>+Term</var>)</a></dt>
<dd class="defbody">
True if <var>Term</var> is bound to the empty list (<code>[]</code>) or
a term with functor `<code>'[|]'</code>'<sup class="fn">105<span class="fn-text">The
traditional list functor is the dot (<code>'.'</code>). This is still
the case of the command line option <strong>--traditional</strong> is
given. See also <a class="sec" href="ext-lists.html">section 5.1</a>.</span></sup>
and arity 2 and the second argument is a list.<sup class="fn">106<span class="fn-text">In
versions before 5.0.1, <a id="idx:islist1:1222"></a><a class="pred" href="builtinlist.html#is_list/1">is_list/1</a>
just checked for <code>[]</code> or <code>[_|_]</code> and proper_list/1
had the role of the current <a id="idx:islist1:1223"></a><a class="pred" href="builtinlist.html#is_list/1">is_list/1</a>.
The current definition conforms to the de facto standard. Assuming
proper coding standards, there should only be very few cases where a
quick-and-dirty <a id="idx:islist1:1224"></a><a class="pred" href="builtinlist.html#is_list/1">is_list/1</a>
is a good choice. Richard O'Keefe pointed at this issue.</span></sup>
This predicate acts as if defined by the definition below on
<em>acyclic</em> terms. The implementation <em>fails</em> safely if
<var>Term</var> represents a cyclic list.
<pre class="code">
is_list(X) :-
var(X), !,
fail.
is_list([]).
is_list([_|T]) :-
is_list(T).
</pre>
</dd>
<dt class="pubdef"><span class="pred-tag">[semidet]</span><a id="memberchk/2"><strong>memberchk</strong>(<var>?Elem,
+List</var>)</a></dt>
<dd class="defbody">
True when <var>Elem</var> is an element of <var>List</var>. This `chk'
variant of
<a id="idx:member2:1225"></a><a class="pred" href="lists.html#member/2">member/2</a>
is semi deterministic and typically used to test membership of a list.
Raises a <code>type</code> error if scanning <var>List</var> encounters
a non-list. Note that <a id="idx:memberchk2:1226"></a><a class="pred" href="builtinlist.html#memberchk/2">memberchk/2</a>
does <em>not</em> perform a full list typecheck. For example, <code>memberchk(a,
[a|b])</code> succeeds without error and <a id="idx:memberchk2:1227"></a><a class="pred" href="builtinlist.html#memberchk/2">memberchk/2</a>
loops on a cyclic list if <var>Elem</var> is not a member of <var>List</var>.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="length/2"><strong>length</strong>(<var>?List,
?Int</var>)</a></dt>
<dd class="defbody">
True if <var>Int</var> represents the number of elements in <var>List</var>.
This predicate is a true relation and can be used to find the length of
a list or produce a list (holding variables) of length <var>Int</var>.
The predicate is non-deterministic, producing lists of increasing length
if
<var>List</var> is a <em>partial list</em> and <var>Int</var> is
unbound. It raises errors if
<p>
<ul class="latex">
<li><var>Int</var> is bound to a non-integer.
<li><var>Int</var> is a negative integer.
<li><var>List</var> is neither a list nor a partial list. This error
condition includes cyclic lists.<sup class="fn">107<span class="fn-text">ISO
demands failure here. We think an error is more appropriate.</span></sup>
</ul>
<p>This predicate fails if the tail of <var>List</var> is equivalent to
<var>Int</var> (e.g., <code>length(L,L)</code>).<sup class="fn">108<span class="fn-text">This
is logically correct. An exception would be more appropriate, but to our
best knowledge, current practice in Prolog does not describe a suitable
candidate exception term.</span></sup></dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="sort/2"><strong>sort</strong>(<var>+List,
-Sorted</var>)</a></dt>
<dd class="defbody">
True if <var>Sorted</var> can be unified with a list holding the
elements of <var>List</var>, sorted to the standard order of terms (see
<a class="sec" href="compare.html">section 4.7</a>). Duplicates are
removed. The implementation is in C, using <em>natural merge sort</em>.<sup class="fn">109<span class="fn-text">Contributed
by Richard O'Keefe.</span></sup> The <a id="idx:sort2:1228"></a><a class="pred" href="builtinlist.html#sort/2">sort/2</a>
predicate can sort a cyclic list, returning a non-cyclic version with
the same elements.</dd>
<dt class="pubdef"><a id="sort/4"><strong>sort</strong>(<var>+Key,
+Order, +List, -Sorted</var>)</a></dt>
<dd class="defbody">
True when <var>Sorted</var> can be unified with a list holding the
element of
<var>List</var>. <var>Key</var> determines which part of each element in
<var>List</var> is used for comparing two term and <var>Order</var>
describes the relation between each set of consecutive elements in
<var>Sorted</var>.<sup class="fn">110<span class="fn-text">The
definition of this predicate was established after discussion with
Joachim Schimpf from the ECLiPSe team. ECLiPSe currently only accepts <code><code><</code></code>, <code><code>=<</code></code>, <code><code>></code></code>
and <code><code>>=</code></code> for the <var>Order</var> argument
but this is likely to change. SWI-Prolog extends this predicate to deal
with dicts.</span></sup>
<p>If <var>Key</var> is the integer zero (0), the entire term is used to
compare two elements. Using <var>Key</var>=0 can be used to sort
arbitrary Prolog terms. Other values for <var>Key</var> can only be used
with compound terms or dicts (see <a class="sec" href="dicts.html">section
5.4</a>). An integer key extracts the <var>Key</var>-th argument from a
compound term. An integer or atom key extracts the value from a dict
that is associated with the given key. A type_error is raised if the
list element is of the wrong type and an existence_error is raised if
the compound has not enough argument or the dict does not contain the
requested key.
<p>Deeper nested elements of structures can be selected by using a list
of keys for the <var>Key</var> argument.
<p>The <var>Order</var> argument is described in the table below<sup class="fn">111<span class="fn-text">For
compatibility with ECLiPSe, the values <code><code><</code></code>, <code><code>=<</code></code>, <code><code>></code></code>
and <code><code>>=</code></code> are allowed as synonyms.</span></sup>
<p><table class="latex frame-void center">
<tr><td>Order</td><td>Ordering</td><td>Duplicate handling </td></tr>
<tr class="hline"><td><code><code>@<</code></code> </td><td>ascending</td><td>remove </td></tr>
<tr><td><code><code>@=<</code></code> </td><td>ascending</td><td>keep </td></tr>
<tr><td><code><code>@></code></code> </td><td>descending</td><td>remove </td></tr>
<tr><td><code><code>@>=</code></code> </td><td>descending</td><td>keep </td></tr>
</table>
<p>The sort is <em>stable</em>, which implies that, if duplicates are
kept, the order of duplicates is not changed. If duplicates are removed,
only the first element of a sequence of duplicates appears in <var>Sorted</var>.
<p>This predicate supersedes most of the other sorting primitives, for
example:
<pre class="code">
sort(List, Sorted) :- sort(0, @<, List, Sorted).
msort(List, Sorted) :- sort(0, @=<, List, Sorted).
keysort(Pairs, Sorted) :- sort(1, @=<, Pairs, Sorted).
</pre>
<p>The following example sorts a list of rows, for example resulting
from
<a id="idx:csvreadfile2:1229"></a><a class="pred" href="csv.html#csv_read_file/2">csv_read_file/2</a>)
ascending on the 3th column and descending on the 4th column:
<pre class="code">
sort(4, @>=, Rows0, Rows1),
sort(3, @=<, Rows1, Sorted).
</pre>
<p>See also <a id="idx:sort2:1230"></a><a class="pred" href="builtinlist.html#sort/2">sort/2</a>
(ISO), <a id="idx:msort2:1231"></a><a class="pred" href="builtinlist.html#msort/2">msort/2</a>, <a id="idx:keysort2:1232"></a><a class="pred" href="builtinlist.html#keysort/2">keysort/2</a>, <a id="idx:predsort3:1233"></a><a class="pred" href="builtinlist.html#predsort/3">predsort/3</a>
and <a id="idx:orderby2:1234"></a><a class="pred" href="solutionsequences.html#order_by/2">order_by/2</a>.</dd>
<dt class="pubdef"><a id="msort/2"><strong>msort</strong>(<var>+List,
-Sorted</var>)</a></dt>
<dd class="defbody">
Equivalent to <a id="idx:sort2:1235"></a><a class="pred" href="builtinlist.html#sort/2">sort/2</a>,
but does not remove duplicates. Raises a
<code>type_error</code> if <var>List</var> is a cyclic list or not a
list.</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="keysort/2"><strong>keysort</strong>(<var>+List,
-Sorted</var>)</a></dt>
<dd class="defbody">
Sort a list of <em>pairs</em>. <var>List</var> must be a list of
<code><var>Key</var>-<var>Value</var></code> pairs, terms whose
principal functor is (-)/2. <var>List</var> is sorted on <var>Key</var>
according to the standard order of terms (see <a class="sec" href="compare.html">section
4.7.1</a>). Duplicates are <em>not</em> removed. Sorting is <em>stable</em>
with regard to the order of the
<var>Values</var>, i.e., the order of multiple elements that have the
same
<var>Key</var> is not changed.
<p>The <a id="idx:keysort2:1236"></a><a class="pred" href="builtinlist.html#keysort/2">keysort/2</a>
predicate is often used together with library
<code>library(pairs)</code>. It can be used to sort lists on different
or multiple criteria. For example, the following predicates sorts a list
of atoms according to their length, maintaining the initial order for
atoms that have the same length.
<pre class="code">
:- use_module(library(pairs)).
sort_atoms_by_length(Atoms, ByLength) :-
map_list_to_pairs(atom_length, Atoms, Pairs),
keysort(Pairs, Sorted),
pairs_values(Sorted, ByLength).
</pre>
</dd>
<dt class="pubdef"><a id="predsort/3"><strong>predsort</strong>(<var>+Pred,
+List, -Sorted</var>)</a></dt>
<dd class="defbody">
Sorts similar to <a id="idx:sort2:1237"></a><a class="pred" href="builtinlist.html#sort/2">sort/2</a>,
but determines the order of two terms by calling <var>Pred</var>(-<var>Delta</var>,
+<var>E1</var>, +<var>E2</var>) . This call must unify <var>Delta</var>
with one of <code><code><</code></code>, <code><code>></code></code>
or
<code><code>=</code></code>. If the built-in predicate <a id="idx:compare3:1238"></a><a class="pred" href="compare.html#compare/3">compare/3</a>
is used, the result is the same as <a id="idx:sort2:1239"></a><a class="pred" href="builtinlist.html#sort/2">sort/2</a>.
See also <a id="idx:keysort2:1240"></a><a class="pred" href="builtinlist.html#keysort/2">keysort/2</a>.
</dd>
</dl>
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