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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
816c421e | 3 | @c Copyright (C) 1999, 2003 Free Software Foundation, Inc. |
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4 | @c See the file elisp.texi for copying conditions. |
5 | @setfilename ../info/hash | |
6 | @node Hash Tables, Symbols, Sequences Arrays Vectors, Top | |
7 | @chapter Hash Tables | |
8 | @cindex hash tables | |
9 | ||
10 | A hash table is a very fast kind of lookup table, somewhat like | |
11 | an alist in that it maps keys to corresponding values. It differs | |
12 | from an alist in these ways: | |
13 | ||
14 | @itemize @bullet | |
15 | @item | |
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16 | Lookup in a hash table is extremely fast for large tables---in fact, the |
17 | time required is essentially @emph{independent} of how many elements are | |
18 | stored in the table. For smaller tables (a few tens of elements) | |
19 | alists may still be faster because hash tables have a more-or-less | |
20 | constant overhead. | |
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21 | |
22 | @item | |
23 | The correspondences in a hash table are in no particular order. | |
24 | ||
25 | @item | |
26 | There is no way to share structure between two hash tables, | |
27 | the way two alists can share a common tail. | |
28 | @end itemize | |
29 | ||
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30 | Emacs Lisp provides a general-purpose hash table data type, along |
31 | with a series of functions for operating on them. Hash tables have no | |
32 | read syntax, and print in hash notation, like this: | |
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33 | |
34 | @example | |
35 | (make-hash-table) | |
36 | @result{} #<hash-table 'eql nil 0/65 0x83af980> | |
37 | @end example | |
38 | ||
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39 | @noindent |
40 | (The term ``hash notation'' refers to the initial @samp{#} | |
41 | character---@pxref{Printed Representation}---and has nothing to do with | |
42 | the term ``hash table.'') | |
43 | ||
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44 | Obarrays are also a kind of hash table, but they are a different type |
45 | of object and are used only for recording interned symbols | |
46 | (@pxref{Creating Symbols}). | |
47 | ||
48 | @menu | |
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49 | * Creating Hash:: Functions to create hash tables. |
50 | * Hash Access:: Reading and writing the hash table contents. | |
51 | * Defining Hash:: Defining new comparison methods | |
52 | * Other Hash:: Miscellaneous. | |
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53 | @end menu |
54 | ||
55 | @node Creating Hash | |
56 | @section Creating Hash Tables | |
57 | ||
58 | The principal function for creating a hash table is | |
59 | @code{make-hash-table}. | |
60 | ||
61 | @tindex make-hash-table | |
62 | @defun make-hash-table &rest keyword-args | |
63 | This function creates a new hash table according to the specified | |
64 | arguments. The arguments should consist of alternating keywords | |
65 | (particular symbols recognized specially) and values corresponding to | |
66 | them. | |
67 | ||
68 | Several keywords make sense in @code{make-hash-table}, but the only two | |
711331aa | 69 | that you really need to know about are @code{:test} and @code{:weakness}. |
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70 | |
71 | @table @code | |
72 | @item :test @var{test} | |
73 | This specifies the method of key lookup for this hash table. The | |
74 | default is @code{eql}; @code{eq} and @code{equal} are other | |
75 | alternatives: | |
76 | ||
77 | @table @code | |
78 | @item eql | |
b02495f1 LT |
79 | Keys which are numbers are ``the same'' if they are @code{equal}, that |
80 | is, if they are equal in value and either both are integers or both | |
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81 | are floating point numbers; otherwise, two distinct objects are never |
82 | ``the same''. | |
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83 | |
84 | @item eq | |
85 | Any two distinct Lisp objects are ``different'' as keys. | |
86 | ||
87 | @item equal | |
88 | Two Lisp objects are ``the same'', as keys, if they are equal | |
89 | according to @code{equal}. | |
90 | @end table | |
91 | ||
92 | You can use @code{define-hash-table-test} (@pxref{Defining Hash}) to | |
93 | define additional possibilities for @var{test}. | |
94 | ||
95 | @item :weakness @var{weak} | |
96 | The weakness of a hash table specifies whether the presence of a key or | |
97 | value in the hash table preserves it from garbage collection. | |
98 | ||
99 | The value, @var{weak}, must be one of @code{nil}, @code{key}, | |
18925e78 | 100 | @code{value}, @code{key-or-value}, @code{key-and-value}, or @code{t} |
1c673658 | 101 | which is an alias for @code{key-and-value}. If @var{weak} is @code{key} |
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102 | then the hash table does not prevent its keys from being collected as |
103 | garbage (if they are not referenced anywhere else); if a particular key | |
104 | does get collected, the corresponding association is removed from the | |
105 | hash table. | |
106 | ||
107 | If @var{weak} is @code{value}, then the hash table does not prevent | |
108 | values from being collected as garbage (if they are not referenced | |
109 | anywhere else); if a particular value does get collected, the | |
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110 | corresponding association is removed from the hash table. |
111 | ||
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112 | If @var{weak} is @code{key-and-value} or @code{t}, both the key and |
113 | the value must be live in order to preserve the association. Thus, | |
114 | the hash table does not protect either keys or values from garbage | |
115 | collection; if either one is collected as garbage, that removes the | |
116 | association. | |
a9749dab | 117 | |
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118 | If @var{weak} is @code{key-or-value}, either the key or |
119 | the value can preserve the association. Thus, associations are | |
120 | removed from the hash table when both their key and value would be | |
121 | collected as garbage (if not for references from weak hash tables). | |
18925e78 | 122 | |
7d15d35d | 123 | The default for @var{weak} is @code{nil}, so that all keys and values |
2c6d3eef | 124 | referenced in the hash table are preserved from garbage collection. |
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125 | |
126 | @item :size @var{size} | |
127 | This specifies a hint for how many associations you plan to store in the | |
128 | hash table. If you know the approximate number, you can make things a | |
711331aa | 129 | little more efficient by specifying it this way. If you specify too |
7d15d35d | 130 | small a size, the hash table will grow automatically when necessary, but |
00510a6b | 131 | doing that takes some extra time. |
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132 | |
133 | The default size is 65. | |
134 | ||
135 | @item :rehash-size @var{rehash-size} | |
136 | When you add an association to a hash table and the table is ``full,'' | |
137 | it grows automatically. This value specifies how to make the hash table | |
138 | larger, at that time. | |
139 | ||
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140 | If @var{rehash-size} is an integer, it should be positive, and the hash |
141 | table grows by adding that much to the nominal size. If | |
142 | @var{rehash-size} is a floating point number, it had better be greater | |
143 | than 1, and the hash table grows by multiplying the old size by that | |
144 | number. | |
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145 | |
146 | The default value is 1.5. | |
147 | ||
148 | @item :rehash-threshold @var{threshold} | |
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149 | This specifies the criterion for when the hash table is ``full'' (so |
150 | it should be made larger). The value, @var{threshold}, should be a | |
151 | positive floating point number, no greater than 1. The hash table is | |
152 | ``full'' whenever the actual number of entries exceeds this fraction | |
153 | of the nominal size. The default for @var{threshold} is 0.8. | |
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154 | @end table |
155 | @end defun | |
156 | ||
157 | @tindex makehash | |
158 | @defun makehash &optional test | |
159 | This is equivalent to @code{make-hash-table}, but with a different style | |
160 | argument list. The argument @var{test} specifies the method | |
161 | of key lookup. | |
162 | ||
b02495f1 | 163 | This function is obsolete. Use @code{make-hash-table} instead. |
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164 | @end defun |
165 | ||
166 | @node Hash Access | |
167 | @section Hash Table Access | |
168 | ||
169 | This section describes the functions for accessing and storing | |
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170 | associations in a hash table. In general, any Lisp object can be used |
171 | as a hash key, unless the comparison method imposes limits. Any Lisp | |
172 | object can also be used as the value. | |
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173 | |
174 | @tindex gethash | |
175 | @defun gethash key table &optional default | |
176 | This function looks up @var{key} in @var{table}, and returns its | |
177 | associated @var{value}---or @var{default}, if @var{key} has no | |
178 | association in @var{table}. | |
179 | @end defun | |
180 | ||
181 | @tindex puthash | |
177c0ea7 | 182 | @defun puthash key value table |
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183 | This function enters an association for @var{key} in @var{table}, with |
184 | value @var{value}. If @var{key} already has an association in | |
185 | @var{table}, @var{value} replaces the old associated value. | |
186 | @end defun | |
187 | ||
188 | @tindex remhash | |
189 | @defun remhash key table | |
190 | This function removes the association for @var{key} from @var{table}, if | |
191 | there is one. If @var{key} has no association, @code{remhash} does | |
192 | nothing. | |
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193 | |
194 | @b{Common Lisp note:} In Common Lisp, @code{remhash} returns | |
195 | non-@code{nil} if it actually removed an association and @code{nil} | |
196 | otherwise. In Emacs Lisp, @code{remhash} always returns @code{nil}. | |
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197 | @end defun |
198 | ||
199 | @tindex clrhash | |
200 | @defun clrhash table | |
201 | This function removes all the associations from hash table @var{table}, | |
202 | so that it becomes empty. This is also called @dfn{clearing} the hash | |
203 | table. | |
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204 | |
205 | @b{Common Lisp note:} In Common Lisp, @code{clrhash} returns the empty | |
206 | @var{table}. In Emacs Lisp, it returns @code{nil}. | |
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207 | @end defun |
208 | ||
209 | @tindex maphash | |
210 | @defun maphash function table | |
7baeca0c | 211 | @anchor{Definition of maphash} |
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212 | This function calls @var{function} once for each of the associations in |
213 | @var{table}. The function @var{function} should accept two | |
214 | arguments---a @var{key} listed in @var{table}, and its associated | |
38bf67d3 | 215 | @var{value}. @code{maphash} returns @code{nil}. |
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216 | @end defun |
217 | ||
218 | @node Defining Hash | |
219 | @section Defining Hash Comparisons | |
220 | @cindex hash code | |
221 | ||
222 | You can define new methods of key lookup by means of | |
223 | @code{define-hash-table-test}. In order to use this feature, you need | |
224 | to understand how hash tables work, and what a @dfn{hash code} means. | |
225 | ||
226 | You can think of a hash table conceptually as a large array of many | |
227 | slots, each capable of holding one association. To look up a key, | |
228 | @code{gethash} first computes an integer, the hash code, from the key. | |
229 | It reduces this integer modulo the length of the array, to produce an | |
230 | index in the array. Then it looks in that slot, and if necessary in | |
231 | other nearby slots, to see if it has found the key being sought. | |
232 | ||
233 | Thus, to define a new method of key lookup, you need to specify both a | |
234 | function to compute the hash code from a key, and a function to compare | |
235 | two keys directly. | |
236 | ||
237 | @tindex define-hash-table-test | |
238 | @defun define-hash-table-test name test-fn hash-fn | |
239 | This function defines a new hash table test, named @var{name}. | |
240 | ||
241 | After defining @var{name} in this way, you can use it as the @var{test} | |
242 | argument in @code{make-hash-table}. When you do that, the hash table | |
243 | will use @var{test-fn} to compare key values, and @var{hash-fn} to compute | |
244 | a ``hash code'' from a key value. | |
245 | ||
246 | The function @var{test-fn} should accept two arguments, two keys, and | |
247 | return non-@code{nil} if they are considered ``the same.'' | |
248 | ||
249 | The function @var{hash-fn} should accept one argument, a key, and return | |
250 | an integer that is the ``hash code'' of that key. For good results, the | |
251 | function should use the whole range of integer values for hash codes, | |
252 | including negative integers. | |
253 | ||
254 | The specified functions are stored in the property list of @var{name} | |
255 | under the property @code{hash-table-test}; the property value's form is | |
256 | @code{(@var{test-fn} @var{hash-fn})}. | |
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257 | @end defun |
258 | ||
259 | @tindex sxhash | |
260 | @defun sxhash obj | |
261 | This function returns a hash code for Lisp object @var{obj}. | |
262 | This is an integer which reflects the contents of @var{obj} | |
263 | and the other Lisp objects it points to. | |
264 | ||
265 | If two objects @var{obj1} and @var{obj2} are equal, then @code{(sxhash | |
266 | @var{obj1})} and @code{(sxhash @var{obj2})} are the same integer. | |
267 | ||
268 | If the two objects are not equal, the values returned by @code{sxhash} | |
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269 | are usually different, but not always; once in a rare while, by luck, |
270 | you will encounter two distinct-looking objects that give the same | |
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271 | result from @code{sxhash}. |
272 | @end defun | |
7d15d35d | 273 | |
a9749dab | 274 | This example creates a hash table whose keys are strings that are |
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275 | compared case-insensitively. |
276 | ||
277 | @example | |
278 | (defun case-fold-string= (a b) | |
279 | (compare-strings a nil nil b nil nil t)) | |
280 | ||
281 | (defun case-fold-string-hash (a) | |
282 | (sxhash (upcase a))) | |
283 | ||
177c0ea7 | 284 | (define-hash-table-test 'case-fold 'case-fold-string= |
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285 | 'case-fold-string-hash)) |
286 | ||
287 | (make-hash-table :test 'case-fold) | |
288 | @end example | |
7d15d35d | 289 | |
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290 | Here is how you could define a hash table test equivalent to the |
291 | predefined test value @code{equal}. The keys can be any Lisp object, | |
292 | and equal-looking objects are considered the same key. | |
7d15d35d | 293 | |
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294 | @example |
295 | (define-hash-table-test 'contents-hash 'equal 'sxhash) | |
7d15d35d | 296 | |
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297 | (make-hash-table :test 'contents-hash) |
298 | @end example | |
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299 | |
300 | @node Other Hash | |
301 | @section Other Hash Table Functions | |
302 | ||
303 | Here are some other functions for working with hash tables. | |
304 | ||
305 | @tindex hash-table-p | |
306 | @defun hash-table-p table | |
307 | This returns non-@code{nil} if @var{table} is a hash table object. | |
308 | @end defun | |
309 | ||
310 | @tindex copy-hash-table | |
311 | @defun copy-hash-table table | |
312 | This function creates and returns a copy of @var{table}. Only the table | |
313 | itself is copied---the keys and values are shared. | |
314 | @end defun | |
315 | ||
316 | @tindex hash-table-count | |
317 | @defun hash-table-count table | |
318 | This function returns the actual number of entries in @var{table}. | |
319 | @end defun | |
320 | ||
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321 | @tindex hash-table-test |
322 | @defun hash-table-test table | |
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323 | This returns the @var{test} value that was given when @var{table} was |
324 | created, to specify how to hash and compare keys. See | |
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325 | @code{make-hash-table} (@pxref{Creating Hash}). |
326 | @end defun | |
327 | ||
328 | @tindex hash-table-weakness | |
329 | @defun hash-table-weakness table | |
330 | This function returns the @var{weak} value that was specified for hash | |
331 | table @var{table}. | |
332 | @end defun | |
333 | ||
334 | @tindex hash-table-rehash-size | |
335 | @defun hash-table-rehash-size table | |
336 | This returns the rehash size of @var{table}. | |
337 | @end defun | |
338 | ||
339 | @tindex hash-table-rehash-threshold | |
340 | @defun hash-table-rehash-threshold table | |
341 | This returns the rehash threshold of @var{table}. | |
342 | @end defun | |
343 | ||
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344 | @tindex hash-table-size |
345 | @defun hash-table-size table | |
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346 | This returns the current nominal size of @var{table}. |
347 | @end defun | |
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348 | |
349 | @ignore | |
350 | arch-tag: 3b5107f9-d2f0-47d5-ad61-3498496bea0e | |
351 | @end ignore |