/usr/lib/swi-prolog/doc/Manual/pairs.html

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<H2><A NAME="sec:A.17"><SPAN class="sec-nr">A.17</SPAN> <SPAN class="sec-title">library(pairs): 
Operations on key-value lists</SPAN></A></H2>

<P><A NAME="sec:pairs"></A>

<DL>
<DT><B>author</B><DD> Jan Wielemaker
<DT><B>See also</B><DD>
<A class="pred" href="builtinlist.html#keysort/2">keysort/2</A>, 
library(assoc)
</DL>

<P>This module implements common operations on Key-Value lists, also 
known as <I>Pairs</I>. Pairs have great practical value, especially due 
to
<A class="pred" href="builtinlist.html#keysort/2">keysort/2</A> and the 
library <CODE>assoc.pl</CODE>.

<P>This library is based on disussion in the SWI-Prolog mailinglist, 
including specifications from Quintus and a library proposal by Richard 
O'Keefe.

<DL class="latex">
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="pairs_keys_values/3"><STRONG>pairs_keys_values</STRONG>(<VAR>?Pairs, 
?Keys, ?Values</VAR>)</A></DT>
<DD class="defbody">
True if <VAR>Keys</VAR> holds the keys of <VAR>Pairs</VAR> and <VAR>Values</VAR> 
the values.

<P>Deterministic if any argument is instantiated to a finite list and 
the others are either free or finite lists. All three lists are in the 
same order.

<DL>
<DT><B>See also</B><DD>
<A class="pred" href="pairs.html#pairs_values/2">pairs_values/2</A> and <A class="pred" href="pairs.html#pairs_keys/2">pairs_keys/2</A>.
</DL>

</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="pairs_values/2"><STRONG>pairs_values</STRONG>(<VAR>+Pairs, 
-Values</VAR>)</A></DT>
<DD class="defbody">
Remove the keys from a list of Key-Value pairs. Same as 
pairs_keys_values(<VAR>Pairs</VAR>, _, <VAR>Values</VAR>)</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="pairs_keys/2"><STRONG>pairs_keys</STRONG>(<VAR>+Pairs, 
-Keys</VAR>)</A></DT>
<DD class="defbody">
Remove the values from a list of Key-Value pairs. Same as 
pairs_keys_values(<VAR>Pairs</VAR>, <VAR>Keys</VAR>, _)</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="group_pairs_by_key/2"><STRONG>group_pairs_by_key</STRONG>(<VAR>+Pairs, 
-Joined:list(Key-Values)</VAR>)</A></DT>
<DD class="defbody">
Group values with the same key. <VAR>Pairs</VAR> must be a key-sorted 
list. For example:

<PRE class="code">
?- group_pairs_by_key([a-2, a-1, b-4], X).

X = [a-[2,1], b-[4]]
</PRE>

<TABLE class="paramlist">
<TR VALIGN=top><TD><VAR>Pairs</VAR> </TD><TD><VAR>Key</VAR>-Value list, 
sorted to the standard order of terms (as <A class="pred" href="builtinlist.html#keysort/2">keysort/2</A> 
does) </TD></TR>
<TR VALIGN=top><TD><VAR>Joined</VAR> </TD><TD>List of <VAR>Key</VAR>-Group, 
where Group is the list of <VAR>Values</VAR> associated with <VAR>Key</VAR>. </TD></TR>
</TABLE>

</DD>
<DT class="pubdef"><span class="pred-tag">[det]</span><A NAME="transpose_pairs/2"><STRONG>transpose_pairs</STRONG>(<VAR>+Pairs, 
-Transposed</VAR>)</A></DT>
<DD class="defbody">
Swap Key-Value to Value-Key and sort the result on Value (the new key) 
using <A class="pred" href="builtinlist.html#keysort/2">keysort/2</A>.</DD>
<DT class="pubdef"><A NAME="map_list_to_pairs/3"><STRONG>map_list_to_pairs</STRONG>(<VAR>:Function, 
+List, -Keyed</VAR>)</A></DT>
<DD class="defbody">
Create a key-value list by mapping each element of <VAR>List</VAR>. For 
example, if we have a list of lists we can create a list of Length-<VAR>List</VAR> 
using

<PRE class="code">
        map_list_to_pairs(length, ListOfLists, Pairs),
</PRE>

<P></DD>
</DL>

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