120 lines
6.5 KiB
HTML
120 lines
6.5 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>16. Hash Tables</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Hash Tables">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html3447 HREF="node155.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3445 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3439 HREF="node153.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3449 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3450 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3448 HREF="node155.html"> Hash Table Functions</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3446 HREF="clm.html">Common Lisp the Language</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3440 HREF="node153.html"> Association Lists</A>
|
||
|
|
<HR> <P>
|
||
|
|
<H1><A NAME=SECTION002000000000000000000>16. Hash Tables</A></H1>
|
||
|
|
<P>
|
||
|
|
<A NAME=HASH>A</A>
|
||
|
|
hash table is a Lisp object that can efficiently map a given
|
||
|
|
Lisp object to another Lisp object.
|
||
|
|
Each hash table has a set of <i>entries</i>, each of which associates a
|
||
|
|
particular <i>key</i> with a particular <i>value</i>. The basic functions
|
||
|
|
that deal with hash tables can create entries, delete entries, and find
|
||
|
|
the value that is associated with a given key. Finding the value is
|
||
|
|
very fast, even if there are many entries, because hashing is used; this
|
||
|
|
is an important advantage of hash tables over property lists.
|
||
|
|
<P>
|
||
|
|
A given hash table can associate only one <i>value</i> with a given
|
||
|
|
<i>key</i>; if you try to add a second <i>value</i>, it will replace the
|
||
|
|
first. Also, adding a value to a hash table is a destructive operation;
|
||
|
|
the hash table is modified. By contrast, association lists can be
|
||
|
|
augmented non-destructively.
|
||
|
|
<P>
|
||
|
|
Hash tables come in three kinds, the difference being whether the keys
|
||
|
|
are compared with <tt>eq</tt>, <tt>eql</tt>, or <tt>equal</tt>. In other words, there
|
||
|
|
are hash tables that hash on Lisp <i>objects</i> (using <tt>eq</tt> or <tt>eql</tt>)
|
||
|
|
and there are hash tables that hash on <i>tree structure</i>
|
||
|
|
(using <tt>equal</tt>).
|
||
|
|
<P>
|
||
|
|
Hash tables are created with the function
|
||
|
|
<tt>make-hash-table</tt>, which takes various options, including
|
||
|
|
which kind of hash table to make (the default being the <tt>eql</tt> kind).
|
||
|
|
To look up a key and find
|
||
|
|
the associated value, use <tt>gethash</tt>.
|
||
|
|
New entries are added
|
||
|
|
to hash tables using <tt>setf</tt> with <tt>gethash</tt>.
|
||
|
|
To remove an entry, use <tt>remhash</tt>. Here is a simple example.
|
||
|
|
<P><pre>
|
||
|
|
(setq a (make-hash-table))
|
||
|
|
(setf (gethash 'color a) 'brown)
|
||
|
|
(setf (gethash 'name a) 'fred)
|
||
|
|
(gethash 'color a) => brown
|
||
|
|
(gethash 'name a) => fred
|
||
|
|
(gethash 'pointy a) => <tt>nil</tt>
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
In this example, the symbols <tt>color</tt> and <tt>name</tt> are being used as
|
||
|
|
keys, and the symbols <tt>brown</tt> and <tt>fred</tt> are being used as the
|
||
|
|
associated values. The hash table has two items in it, one of which
|
||
|
|
associates from <tt>color</tt> to <tt>brown</tt>, and the other of which
|
||
|
|
associates from <tt>name</tt> to <tt>fred</tt>.
|
||
|
|
<P>
|
||
|
|
Keys do not have to be symbols; they can be any Lisp object. Similarly,
|
||
|
|
values can be any Lisp object.
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
|
||
|
|
When a hash table is first created, it has a <i>size</i>, which is the
|
||
|
|
maximum number of entries it can hold. Usually the actual capacity of
|
||
|
|
the table is somewhat less, since the hashing is not perfectly
|
||
|
|
collision-free. With the maximum possible bad luck, the capacity could
|
||
|
|
be very much less, but this rarely happens. If so many entries are
|
||
|
|
added that the capacity is exceeded, the hash table will automatically
|
||
|
|
grow, and the entries will be <i>rehashed</i> (new hash values will be
|
||
|
|
recomputed, and everything will be rearranged so that the fast hash
|
||
|
|
lookup still works). This is transparent to the caller; it all happens
|
||
|
|
automatically.
|
||
|
|
<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
There is a discrepancy between the preceding description of the
|
||
|
|
size of a hash table and the description of the <tt>:size</tt> argument
|
||
|
|
in the specification below
|
||
|
|
of <tt>make-hash-table</tt>.
|
||
|
|
<P>
|
||
|
|
X3J13 voted in June 1989 (HASH-TABLE-SIZE) <A NAME=17310> </A> to regard the
|
||
|
|
latter description as definitive: the <tt>:size</tt> argument
|
||
|
|
is approximately the number of entries that can be inserted
|
||
|
|
without having to enlarge the hash table. This definition is certainly
|
||
|
|
more convenient for the user.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<hr>
|
||
|
|
<b>Compatibility note:</b> This hash table facility is compatible with Lisp Machine Lisp. It
|
||
|
|
is similar to the hasharray facility of Interlisp, and some of the
|
||
|
|
function names are the same. However, it is <i>not</i> compatible with
|
||
|
|
Interlisp. The exact details and the order of arguments are designed to
|
||
|
|
be consistent with the rest of MacLisp rather than with
|
||
|
|
Interlisp. For instance, the order of arguments to <tt>maphash</tt> is
|
||
|
|
different, there is no ``system hash table,'' and there is not
|
||
|
|
the Interlisp restriction that keys and values may not be <tt>nil</tt>.
|
||
|
|
<hr>
|
||
|
|
<P>
|
||
|
|
<HR>
|
||
|
|
<UL>
|
||
|
|
<LI> <A NAME=tex2html3451 HREF="node155.html#SECTION002010000000000000000"> Hash Table Functions</A>
|
||
|
|
<LI> <A NAME=tex2html3452 HREF="node156.html#SECTION002020000000000000000"> Primitive Hash Function</A>
|
||
|
|
</UL>
|
||
|
|
<BR> <HR><A NAME=tex2html3447 HREF="node155.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3445 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3439 HREF="node153.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3449 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3450 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3448 HREF="node155.html"> Hash Table Functions</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3446 HREF="clm.html">Common Lisp the Language</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3440 HREF="node153.html"> Association Lists</A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|