2 @c This is part of the XEmacs Lisp Reference Manual.
3 @c Copyright (C) 1996 Ben Wing.
4 @c See the file lispref.texi for copying conditions.
5 @setfilename ../../info/hash-tables.info
6 @node Hash Tables, Range Tables, Display, top
10 @defun hash-table-p object
11 This function returns @code{t} if @var{object} is a hash table, else @code{nil}.
15 * Introduction to Hash Tables:: Hash tables are fast data structures for
16 implementing simple tables (i.e. finite
17 mappings from keys to values).
18 * Working With Hash Tables:: Hash table functions.
19 * Weak Hash Tables:: Hash tables with special garbage-collection
23 @node Introduction to Hash Tables
24 @section Introduction to Hash Tables
26 A @dfn{hash table} is a data structure that provides mappings from
27 arbitrary Lisp objects called @dfn{keys} to other arbitrary Lisp objects
28 called @dfn{values}. A key/value pair is sometimes called an
29 @dfn{entry} in the hash table. There are many ways other than hash
30 tables of implementing the same sort of mapping, e.g. association lists
31 (@pxref{Association Lists}) and property lists (@pxref{Property Lists}),
32 but hash tables provide much faster lookup when there are many entries
33 in the mapping. Hash tables are an implementation of the abstract data
34 type @dfn{dictionary}, also known as @dfn{associative array}.
36 Internally, hash tables are hashed using the @dfn{linear probing} hash
37 table implementation method. This method hashes each key to a
38 particular spot in the hash table, and then scans forward sequentially
39 until a blank entry is found. To look up a key, hash to the appropriate
40 spot, then search forward for the key until either a key is found or a
41 blank entry stops the search. This method is used in preference to
42 double hashing because of changes in recent hardware. The penalty for
43 non-sequential access to memory has been increasing, and this
44 compensates for the problem of clustering that linear probing entails.
46 When hash tables are created, the user may (but is not required to)
47 specify initial properties that influence performance.
49 Use the @code{:size} parameter to specify the number of entries that are
50 likely to be stored in the hash table, to avoid the overhead of resizing
51 the table. But if the pre-allocated space for the entries is never
52 used, it is simply wasted and makes XEmacs slower. Excess unused hash
53 table entries exact a small continuous performance penalty, since they
54 must be scanned at every garbage collection. If the number of entries
55 in the hash table is unknown, simply avoid using the @code{:size}
58 Use the @code{:rehash-size} and @code{:rehash-threshold} keywords to
59 adjust the algorithm for deciding when to rehash the hash table. For
60 temporary hash tables that are going to be very heavily used, use a
61 small rehash threshold, for example, 0.4 and a large rehash size, for
62 example 2.0. For permanent hash tables that will be infrequently used,
63 specify a large rehash threshold, for example 0.8.
65 Hash tables can also be created by the lisp reader using structure
68 #s(hash-table size 20 data (foo 1 bar 2))
71 The structure syntax accepts the same keywords as @code{make-hash-table}
72 (without the @code{:} character), as well as the additional keyword
73 @code{data}, which specifies the initial hash table contents.
75 @defun make-hash-table &key @code{test} @code{size} @code{rehash-size} @code{rehash-threshold} @code{weakness}
76 This function returns a new empty hash table object.
78 Keyword @code{:test} can be @code{eq}, @code{eql} (default) or @code{equal}.
79 Comparison between keys is done using this function.
80 If speed is important, consider using @code{eq}.
81 When storing strings in the hash table, you will likely need to use @code{equal}.
83 Keyword @code{:size} specifies the number of keys likely to be inserted.
84 This number of entries can be inserted without enlarging the hash table.
86 Keyword @code{:rehash-size} must be a float greater than 1.0, and specifies
87 the factor by which to increase the size of the hash table when enlarging.
89 Keyword @code{:rehash-threshold} must be a float between 0.0 and 1.0,
90 and specifies the load factor of the hash table which triggers enlarging.
92 Non-standard keyword @code{:weakness} can be @code{nil} (default),
93 @code{t}, @code{key-and-value}, @code{key}, @code{value} or
94 @code{key-or-value}. @code{t} is an alias for @code{key-and-value}.
96 A key-and-value-weak hash table, also known as a fully-weak or simply
97 as a weak hash table, is one whose pointers do not count as GC
98 referents: for any key-value pair in the hash table, if the only
99 remaining pointer to either the key or the value is in a weak hash
100 table, then the pair will be removed from the hash table, and the key
101 and value collected. A non-weak hash table (or any other pointer)
102 would prevent the object from being collected.
104 A key-weak hash table is similar to a fully-weak hash table except that
105 a key-value pair will be removed only if the key remains unmarked
106 outside of weak hash tables. The pair will remain in the hash table if
107 the key is pointed to by something other than a weak hash table, even
110 A value-weak hash table is similar to a fully-weak hash table except
111 that a key-value pair will be removed only if the value remains
112 unmarked outside of weak hash tables. The pair will remain in the
113 hash table if the value is pointed to by something other than a weak
114 hash table, even if the key is not.
116 A key-or-value-weak hash table is similar to a fully-weak hash table except
117 that a key-value pair will be removed only if the value and the key remain
118 unmarked outside of weak hash tables. The pair will remain in the
119 hash table if the value or key are pointed to by something other than a weak
120 hash table, even if the other is not.
123 @defun copy-hash-table hash-table
124 This function returns a new hash table which contains the same keys and
125 values as @var{hash-table}. The keys and values will not themselves be
129 @defun hash-table-count hash-table
130 This function returns the number of entries in @var{hash-table}.
133 @defun hash-table-test hash-table
134 This function returns the test function of @var{hash-table}.
135 This can be one of @code{eq}, @code{eql} or @code{equal}.
138 @defun hash-table-size hash-table
139 This function returns the current number of slots in @var{hash-table},
140 whether occupied or not.
143 @defun hash-table-rehash-size hash-table
144 This function returns the current rehash size of @var{hash-table}.
145 This is a float greater than 1.0; the factor by which @var{hash-table}
146 is enlarged when the rehash threshold is exceeded.
149 @defun hash-table-rehash-threshold hash-table
150 This function returns the current rehash threshold of @var{hash-table}.
151 This is a float between 0.0 and 1.0; the maximum @dfn{load factor} of
152 @var{hash-table}, beyond which the @var{hash-table} is enlarged by rehashing.
155 @defun hash-table-weakness hash-table
156 This function returns the weakness of @var{hash-table}.
157 This can be one of @code{nil}, @code{t}, @code{key} or @code{value}.
160 @node Working With Hash Tables
161 @section Working With Hash Tables
163 @defun puthash key value hash-table
164 This function hashes @var{key} to @var{value} in @var{hash-table}.
167 @defun gethash key hash-table &optional default
168 This function finds the hash value for @var{key} in @var{hash-table}.
169 If there is no entry for @var{key} in @var{hash-table}, @var{default} is
170 returned (which in turn defaults to @code{nil}).
173 @defun remhash key hash-table
174 This function removes the entry for @var{key} from @var{hash-table}.
175 Does nothing if there is no entry for @var{key} in @var{hash-table}.
178 @defun clrhash hash-table
179 This function removes all entries from @var{hash-table}, leaving it empty.
182 @defun maphash function hash-table
183 This function maps @var{function} over entries in @var{hash-table},
184 calling it with two args, each key and value in the hash table.
186 @var{function} may not modify @var{hash-table}, with the one exception
187 that @var{function} may remhash or puthash the entry currently being
188 processed by @var{function}.
192 @node Weak Hash Tables
193 @section Weak Hash Tables
194 @cindex hash table, weak
195 @cindex weak hash table
197 A @dfn{weak hash table} is a special variety of hash table whose
198 elements do not count as GC referents. For any key-value pair in such a
199 hash table, if either the key or value (or in some cases, if one
200 particular one of the two) has no references to it outside of weak hash
201 tables (and similar structures such as weak lists), the pair will be
202 removed from the table, and the key and value collected. A non-weak
203 hash table (or any other pointer) would prevent the objects from being
206 Weak hash tables are useful for keeping track of information in a
207 non-obtrusive way, for example to implement caching. If the cache
208 contains objects such as buffers, markers, image instances, etc. that
209 will eventually disappear and get garbage-collected, using a weak hash
210 table ensures that these objects are collected normally rather than
211 remaining around forever, long past their actual period of use.
212 (Otherwise, you'd have to explicitly map over the hash table every so
213 often and remove unnecessary elements.)
215 There are four types of weak hash tables:
218 @item key-and-value-weak hash tables
219 In these hash tables, also known as fully weak or simply as weak hash
220 tables, a pair disappears if either the key or the value is unreferenced
221 outside of the table.
222 @item key-weak hash tables
223 In these hash tables, a pair disappears if the key is unreferenced outside
224 of the table, regardless of how the value is referenced.
225 @item value-weak hash tables
226 In these hash tables, a pair disappears if the value is unreferenced outside
227 of the table, regardless of how the key is referenced.
228 @item key-or-value-weak hash tables
229 In these hash tables, a pair disappears if both the key and the value
230 are unreferenced outside of the table.
233 Also see @ref{Weak Lists}.
235 Weak hash tables are created by specifying the @code{:weakness} keyword to
236 @code{make-hash-table}.