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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
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3 | @c Copyright (C) 1999, 2002, 2003, 2004, 2005, |
4 | @c 2006 Free Software Foundation, Inc. | |
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5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/hash | |
7 | @node Hash Tables, Symbols, Sequences Arrays Vectors, Top | |
8 | @chapter Hash Tables | |
9 | @cindex hash tables | |
10 | ||
11 | A hash table is a very fast kind of lookup table, somewhat like | |
12 | an alist in that it maps keys to corresponding values. It differs | |
13 | from an alist in these ways: | |
14 | ||
15 | @itemize @bullet | |
16 | @item | |
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17 | Lookup in a hash table is extremely fast for large tables---in fact, the |
18 | time required is essentially @emph{independent} of how many elements are | |
19 | stored in the table. For smaller tables (a few tens of elements) | |
20 | alists may still be faster because hash tables have a more-or-less | |
21 | constant overhead. | |
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22 | |
23 | @item | |
24 | The correspondences in a hash table are in no particular order. | |
25 | ||
26 | @item | |
27 | There is no way to share structure between two hash tables, | |
28 | the way two alists can share a common tail. | |
29 | @end itemize | |
30 | ||
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31 | Emacs Lisp provides a general-purpose hash table data type, along |
32 | with a series of functions for operating on them. Hash tables have no | |
33 | read syntax, and print in hash notation, like this: | |
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34 | |
35 | @example | |
36 | (make-hash-table) | |
37 | @result{} #<hash-table 'eql nil 0/65 0x83af980> | |
38 | @end example | |
39 | ||
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40 | @noindent |
41 | (The term ``hash notation'' refers to the initial @samp{#} | |
42 | character---@pxref{Printed Representation}---and has nothing to do with | |
43 | the term ``hash table.'') | |
44 | ||
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45 | Obarrays are also a kind of hash table, but they are a different type |
46 | of object and are used only for recording interned symbols | |
47 | (@pxref{Creating Symbols}). | |
48 | ||
49 | @menu | |
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50 | * Creating Hash:: Functions to create hash tables. |
51 | * Hash Access:: Reading and writing the hash table contents. | |
52 | * Defining Hash:: Defining new comparison methods | |
53 | * Other Hash:: Miscellaneous. | |
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54 | @end menu |
55 | ||
56 | @node Creating Hash | |
57 | @section Creating Hash Tables | |
58 | ||
59 | The principal function for creating a hash table is | |
60 | @code{make-hash-table}. | |
61 | ||
62 | @tindex make-hash-table | |
63 | @defun make-hash-table &rest keyword-args | |
64 | This function creates a new hash table according to the specified | |
65 | arguments. The arguments should consist of alternating keywords | |
66 | (particular symbols recognized specially) and values corresponding to | |
67 | them. | |
68 | ||
69 | Several keywords make sense in @code{make-hash-table}, but the only two | |
711331aa | 70 | that you really need to know about are @code{:test} and @code{:weakness}. |
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71 | |
72 | @table @code | |
73 | @item :test @var{test} | |
74 | This specifies the method of key lookup for this hash table. The | |
75 | default is @code{eql}; @code{eq} and @code{equal} are other | |
76 | alternatives: | |
77 | ||
78 | @table @code | |
79 | @item eql | |
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80 | Keys which are numbers are ``the same'' if they are @code{equal}, that |
81 | is, if they are equal in value and either both are integers or both | |
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82 | are floating point numbers; otherwise, two distinct objects are never |
83 | ``the same''. | |
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84 | |
85 | @item eq | |
86 | Any two distinct Lisp objects are ``different'' as keys. | |
87 | ||
88 | @item equal | |
89 | Two Lisp objects are ``the same'', as keys, if they are equal | |
90 | according to @code{equal}. | |
91 | @end table | |
92 | ||
93 | You can use @code{define-hash-table-test} (@pxref{Defining Hash}) to | |
94 | define additional possibilities for @var{test}. | |
95 | ||
96 | @item :weakness @var{weak} | |
97 | The weakness of a hash table specifies whether the presence of a key or | |
98 | value in the hash table preserves it from garbage collection. | |
99 | ||
100 | The value, @var{weak}, must be one of @code{nil}, @code{key}, | |
18925e78 | 101 | @code{value}, @code{key-or-value}, @code{key-and-value}, or @code{t} |
1c673658 | 102 | which is an alias for @code{key-and-value}. If @var{weak} is @code{key} |
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103 | then the hash table does not prevent its keys from being collected as |
104 | garbage (if they are not referenced anywhere else); if a particular key | |
105 | does get collected, the corresponding association is removed from the | |
106 | hash table. | |
107 | ||
108 | If @var{weak} is @code{value}, then the hash table does not prevent | |
109 | values from being collected as garbage (if they are not referenced | |
110 | anywhere else); if a particular value does get collected, the | |
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111 | corresponding association is removed from the hash table. |
112 | ||
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113 | If @var{weak} is @code{key-and-value} or @code{t}, both the key and |
114 | the value must be live in order to preserve the association. Thus, | |
115 | the hash table does not protect either keys or values from garbage | |
116 | collection; if either one is collected as garbage, that removes the | |
117 | association. | |
a9749dab | 118 | |
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119 | If @var{weak} is @code{key-or-value}, either the key or |
120 | the value can preserve the association. Thus, associations are | |
121 | removed from the hash table when both their key and value would be | |
122 | collected as garbage (if not for references from weak hash tables). | |
18925e78 | 123 | |
7d15d35d | 124 | The default for @var{weak} is @code{nil}, so that all keys and values |
2c6d3eef | 125 | referenced in the hash table are preserved from garbage collection. |
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126 | |
127 | @item :size @var{size} | |
128 | This specifies a hint for how many associations you plan to store in the | |
129 | hash table. If you know the approximate number, you can make things a | |
711331aa | 130 | little more efficient by specifying it this way. If you specify too |
7d15d35d | 131 | small a size, the hash table will grow automatically when necessary, but |
00510a6b | 132 | doing that takes some extra time. |
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133 | |
134 | The default size is 65. | |
135 | ||
136 | @item :rehash-size @var{rehash-size} | |
137 | When you add an association to a hash table and the table is ``full,'' | |
138 | it grows automatically. This value specifies how to make the hash table | |
139 | larger, at that time. | |
140 | ||
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141 | If @var{rehash-size} is an integer, it should be positive, and the hash |
142 | table grows by adding that much to the nominal size. If | |
143 | @var{rehash-size} is a floating point number, it had better be greater | |
144 | than 1, and the hash table grows by multiplying the old size by that | |
145 | number. | |
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146 | |
147 | The default value is 1.5. | |
148 | ||
149 | @item :rehash-threshold @var{threshold} | |
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150 | This specifies the criterion for when the hash table is ``full'' (so |
151 | it should be made larger). The value, @var{threshold}, should be a | |
152 | positive floating point number, no greater than 1. The hash table is | |
153 | ``full'' whenever the actual number of entries exceeds this fraction | |
154 | of the nominal size. The default for @var{threshold} is 0.8. | |
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155 | @end table |
156 | @end defun | |
157 | ||
158 | @tindex makehash | |
159 | @defun makehash &optional test | |
160 | This is equivalent to @code{make-hash-table}, but with a different style | |
161 | argument list. The argument @var{test} specifies the method | |
162 | of key lookup. | |
163 | ||
b02495f1 | 164 | This function is obsolete. Use @code{make-hash-table} instead. |
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165 | @end defun |
166 | ||
167 | @node Hash Access | |
168 | @section Hash Table Access | |
169 | ||
170 | This section describes the functions for accessing and storing | |
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171 | associations in a hash table. In general, any Lisp object can be used |
172 | as a hash key, unless the comparison method imposes limits. Any Lisp | |
173 | object can also be used as the value. | |
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174 | |
175 | @tindex gethash | |
176 | @defun gethash key table &optional default | |
177 | This function looks up @var{key} in @var{table}, and returns its | |
178 | associated @var{value}---or @var{default}, if @var{key} has no | |
179 | association in @var{table}. | |
180 | @end defun | |
181 | ||
182 | @tindex puthash | |
177c0ea7 | 183 | @defun puthash key value table |
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184 | This function enters an association for @var{key} in @var{table}, with |
185 | value @var{value}. If @var{key} already has an association in | |
186 | @var{table}, @var{value} replaces the old associated value. | |
187 | @end defun | |
188 | ||
189 | @tindex remhash | |
190 | @defun remhash key table | |
191 | This function removes the association for @var{key} from @var{table}, if | |
192 | there is one. If @var{key} has no association, @code{remhash} does | |
193 | nothing. | |
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194 | |
195 | @b{Common Lisp note:} In Common Lisp, @code{remhash} returns | |
196 | non-@code{nil} if it actually removed an association and @code{nil} | |
197 | otherwise. In Emacs Lisp, @code{remhash} always returns @code{nil}. | |
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198 | @end defun |
199 | ||
200 | @tindex clrhash | |
201 | @defun clrhash table | |
202 | This function removes all the associations from hash table @var{table}, | |
203 | so that it becomes empty. This is also called @dfn{clearing} the hash | |
204 | table. | |
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205 | |
206 | @b{Common Lisp note:} In Common Lisp, @code{clrhash} returns the empty | |
207 | @var{table}. In Emacs Lisp, it returns @code{nil}. | |
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208 | @end defun |
209 | ||
210 | @tindex maphash | |
211 | @defun maphash function table | |
7baeca0c | 212 | @anchor{Definition of maphash} |
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213 | This function calls @var{function} once for each of the associations in |
214 | @var{table}. The function @var{function} should accept two | |
215 | arguments---a @var{key} listed in @var{table}, and its associated | |
38bf67d3 | 216 | @var{value}. @code{maphash} returns @code{nil}. |
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217 | @end defun |
218 | ||
219 | @node Defining Hash | |
220 | @section Defining Hash Comparisons | |
221 | @cindex hash code | |
222 | ||
223 | You can define new methods of key lookup by means of | |
224 | @code{define-hash-table-test}. In order to use this feature, you need | |
225 | to understand how hash tables work, and what a @dfn{hash code} means. | |
226 | ||
227 | You can think of a hash table conceptually as a large array of many | |
228 | slots, each capable of holding one association. To look up a key, | |
229 | @code{gethash} first computes an integer, the hash code, from the key. | |
230 | It reduces this integer modulo the length of the array, to produce an | |
231 | index in the array. Then it looks in that slot, and if necessary in | |
232 | other nearby slots, to see if it has found the key being sought. | |
233 | ||
234 | Thus, to define a new method of key lookup, you need to specify both a | |
235 | function to compute the hash code from a key, and a function to compare | |
236 | two keys directly. | |
237 | ||
238 | @tindex define-hash-table-test | |
239 | @defun define-hash-table-test name test-fn hash-fn | |
240 | This function defines a new hash table test, named @var{name}. | |
241 | ||
242 | After defining @var{name} in this way, you can use it as the @var{test} | |
243 | argument in @code{make-hash-table}. When you do that, the hash table | |
244 | will use @var{test-fn} to compare key values, and @var{hash-fn} to compute | |
245 | a ``hash code'' from a key value. | |
246 | ||
247 | The function @var{test-fn} should accept two arguments, two keys, and | |
248 | return non-@code{nil} if they are considered ``the same.'' | |
249 | ||
250 | The function @var{hash-fn} should accept one argument, a key, and return | |
251 | an integer that is the ``hash code'' of that key. For good results, the | |
252 | function should use the whole range of integer values for hash codes, | |
253 | including negative integers. | |
254 | ||
255 | The specified functions are stored in the property list of @var{name} | |
256 | under the property @code{hash-table-test}; the property value's form is | |
257 | @code{(@var{test-fn} @var{hash-fn})}. | |
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258 | @end defun |
259 | ||
260 | @tindex sxhash | |
261 | @defun sxhash obj | |
262 | This function returns a hash code for Lisp object @var{obj}. | |
263 | This is an integer which reflects the contents of @var{obj} | |
264 | and the other Lisp objects it points to. | |
265 | ||
266 | If two objects @var{obj1} and @var{obj2} are equal, then @code{(sxhash | |
267 | @var{obj1})} and @code{(sxhash @var{obj2})} are the same integer. | |
268 | ||
269 | If the two objects are not equal, the values returned by @code{sxhash} | |
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270 | are usually different, but not always; once in a rare while, by luck, |
271 | you will encounter two distinct-looking objects that give the same | |
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272 | result from @code{sxhash}. |
273 | @end defun | |
7d15d35d | 274 | |
a9749dab | 275 | This example creates a hash table whose keys are strings that are |
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276 | compared case-insensitively. |
277 | ||
278 | @example | |
279 | (defun case-fold-string= (a b) | |
280 | (compare-strings a nil nil b nil nil t)) | |
281 | ||
282 | (defun case-fold-string-hash (a) | |
283 | (sxhash (upcase a))) | |
284 | ||
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285 | (define-hash-table-test 'case-fold |
286 | 'case-fold-string= 'case-fold-string-hash) | |
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287 | |
288 | (make-hash-table :test 'case-fold) | |
289 | @end example | |
7d15d35d | 290 | |
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291 | Here is how you could define a hash table test equivalent to the |
292 | predefined test value @code{equal}. The keys can be any Lisp object, | |
293 | and equal-looking objects are considered the same key. | |
7d15d35d | 294 | |
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295 | @example |
296 | (define-hash-table-test 'contents-hash 'equal 'sxhash) | |
7d15d35d | 297 | |
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298 | (make-hash-table :test 'contents-hash) |
299 | @end example | |
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300 | |
301 | @node Other Hash | |
302 | @section Other Hash Table Functions | |
303 | ||
304 | Here are some other functions for working with hash tables. | |
305 | ||
306 | @tindex hash-table-p | |
307 | @defun hash-table-p table | |
308 | This returns non-@code{nil} if @var{table} is a hash table object. | |
309 | @end defun | |
310 | ||
311 | @tindex copy-hash-table | |
312 | @defun copy-hash-table table | |
313 | This function creates and returns a copy of @var{table}. Only the table | |
314 | itself is copied---the keys and values are shared. | |
315 | @end defun | |
316 | ||
317 | @tindex hash-table-count | |
318 | @defun hash-table-count table | |
319 | This function returns the actual number of entries in @var{table}. | |
320 | @end defun | |
321 | ||
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322 | @tindex hash-table-test |
323 | @defun hash-table-test table | |
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324 | This returns the @var{test} value that was given when @var{table} was |
325 | created, to specify how to hash and compare keys. See | |
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326 | @code{make-hash-table} (@pxref{Creating Hash}). |
327 | @end defun | |
328 | ||
329 | @tindex hash-table-weakness | |
330 | @defun hash-table-weakness table | |
331 | This function returns the @var{weak} value that was specified for hash | |
332 | table @var{table}. | |
333 | @end defun | |
334 | ||
335 | @tindex hash-table-rehash-size | |
336 | @defun hash-table-rehash-size table | |
337 | This returns the rehash size of @var{table}. | |
338 | @end defun | |
339 | ||
340 | @tindex hash-table-rehash-threshold | |
341 | @defun hash-table-rehash-threshold table | |
342 | This returns the rehash threshold of @var{table}. | |
343 | @end defun | |
344 | ||
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345 | @tindex hash-table-size |
346 | @defun hash-table-size table | |
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347 | This returns the current nominal size of @var{table}. |
348 | @end defun | |
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349 | |
350 | @ignore | |
351 | arch-tag: 3b5107f9-d2f0-47d5-ad61-3498496bea0e | |
352 | @end ignore |