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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1999, 2001, 2002, 2003, 2004, 2005, | |
114f9c96 | 4 | @c 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/hash |
b8d4c8d0 GM |
7 | @node Hash Tables, Symbols, Sequences Arrays Vectors, Top |
8 | @chapter Hash Tables | |
9 | @cindex hash tables | |
10 | @cindex lookup tables | |
11 | ||
12 | A hash table is a very fast kind of lookup table, somewhat like an | |
13 | alist (@pxref{Association Lists}) in that it maps keys to | |
14 | corresponding values. It differs from an alist in these ways: | |
15 | ||
16 | @itemize @bullet | |
17 | @item | |
18 | Lookup in a hash table is extremely fast for large tables---in fact, the | |
19 | time required is essentially @emph{independent} of how many elements are | |
20 | stored in the table. For smaller tables (a few tens of elements) | |
21 | alists may still be faster because hash tables have a more-or-less | |
22 | constant overhead. | |
23 | ||
24 | @item | |
25 | The correspondences in a hash table are in no particular order. | |
26 | ||
27 | @item | |
28 | There is no way to share structure between two hash tables, | |
29 | the way two alists can share a common tail. | |
30 | @end itemize | |
31 | ||
32 | Emacs Lisp provides a general-purpose hash table data type, along | |
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33 | with a series of functions for operating on them. Hash tables have a |
34 | special printed representation, which consists of @samp{#s} followed | |
35 | by a list specifying the hash table properties and contents. | |
36 | @xref{Creating Hash}. (Note that the term ``hash notation'', which | |
37 | refers to the initial @samp{#} character used in the printed | |
38 | representations of objects with no read representation, has nothing to | |
39 | do with the term ``hash table''. @xref{Printed Representation}.) | |
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40 | |
41 | Obarrays are also a kind of hash table, but they are a different type | |
42 | of object and are used only for recording interned symbols | |
43 | (@pxref{Creating Symbols}). | |
44 | ||
45 | @menu | |
46 | * Creating Hash:: Functions to create hash tables. | |
47 | * Hash Access:: Reading and writing the hash table contents. | |
c3cbbb40 | 48 | * Defining Hash:: Defining new comparison methods. |
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49 | * Other Hash:: Miscellaneous. |
50 | @end menu | |
51 | ||
52 | @node Creating Hash | |
53 | @section Creating Hash Tables | |
54 | @cindex creating hash tables | |
55 | ||
56 | The principal function for creating a hash table is | |
57 | @code{make-hash-table}. | |
58 | ||
59 | @defun make-hash-table &rest keyword-args | |
60 | This function creates a new hash table according to the specified | |
61 | arguments. The arguments should consist of alternating keywords | |
62 | (particular symbols recognized specially) and values corresponding to | |
63 | them. | |
64 | ||
65 | Several keywords make sense in @code{make-hash-table}, but the only two | |
66 | that you really need to know about are @code{:test} and @code{:weakness}. | |
67 | ||
68 | @table @code | |
69 | @item :test @var{test} | |
70 | This specifies the method of key lookup for this hash table. The | |
71 | default is @code{eql}; @code{eq} and @code{equal} are other | |
72 | alternatives: | |
73 | ||
74 | @table @code | |
75 | @item eql | |
76 | Keys which are numbers are ``the same'' if they are @code{equal}, that | |
77 | is, if they are equal in value and either both are integers or both | |
78 | are floating point numbers; otherwise, two distinct objects are never | |
79 | ``the same.'' | |
80 | ||
81 | @item eq | |
82 | Any two distinct Lisp objects are ``different'' as keys. | |
83 | ||
84 | @item equal | |
85 | Two Lisp objects are ``the same,'' as keys, if they are equal | |
86 | according to @code{equal}. | |
87 | @end table | |
88 | ||
89 | You can use @code{define-hash-table-test} (@pxref{Defining Hash}) to | |
90 | define additional possibilities for @var{test}. | |
91 | ||
92 | @item :weakness @var{weak} | |
93 | The weakness of a hash table specifies whether the presence of a key or | |
94 | value in the hash table preserves it from garbage collection. | |
95 | ||
96 | The value, @var{weak}, must be one of @code{nil}, @code{key}, | |
97 | @code{value}, @code{key-or-value}, @code{key-and-value}, or @code{t} | |
98 | which is an alias for @code{key-and-value}. If @var{weak} is @code{key} | |
99 | then the hash table does not prevent its keys from being collected as | |
100 | garbage (if they are not referenced anywhere else); if a particular key | |
101 | does get collected, the corresponding association is removed from the | |
102 | hash table. | |
103 | ||
104 | If @var{weak} is @code{value}, then the hash table does not prevent | |
105 | values from being collected as garbage (if they are not referenced | |
106 | anywhere else); if a particular value does get collected, the | |
107 | corresponding association is removed from the hash table. | |
108 | ||
109 | If @var{weak} is @code{key-and-value} or @code{t}, both the key and | |
110 | the value must be live in order to preserve the association. Thus, | |
111 | the hash table does not protect either keys or values from garbage | |
112 | collection; if either one is collected as garbage, that removes the | |
113 | association. | |
114 | ||
115 | If @var{weak} is @code{key-or-value}, either the key or | |
116 | the value can preserve the association. Thus, associations are | |
117 | removed from the hash table when both their key and value would be | |
118 | collected as garbage (if not for references from weak hash tables). | |
119 | ||
120 | The default for @var{weak} is @code{nil}, so that all keys and values | |
121 | referenced in the hash table are preserved from garbage collection. | |
122 | ||
123 | @item :size @var{size} | |
124 | This specifies a hint for how many associations you plan to store in the | |
125 | hash table. If you know the approximate number, you can make things a | |
126 | little more efficient by specifying it this way. If you specify too | |
127 | small a size, the hash table will grow automatically when necessary, but | |
128 | doing that takes some extra time. | |
129 | ||
130 | The default size is 65. | |
131 | ||
132 | @item :rehash-size @var{rehash-size} | |
133 | When you add an association to a hash table and the table is ``full,'' | |
134 | it grows automatically. This value specifies how to make the hash table | |
135 | larger, at that time. | |
136 | ||
137 | If @var{rehash-size} is an integer, it should be positive, and the hash | |
138 | table grows by adding that much to the nominal size. If | |
139 | @var{rehash-size} is a floating point number, it had better be greater | |
140 | than 1, and the hash table grows by multiplying the old size by that | |
141 | number. | |
142 | ||
143 | The default value is 1.5. | |
144 | ||
145 | @item :rehash-threshold @var{threshold} | |
146 | This specifies the criterion for when the hash table is ``full'' (so | |
147 | it should be made larger). The value, @var{threshold}, should be a | |
148 | positive floating point number, no greater than 1. The hash table is | |
149 | ``full'' whenever the actual number of entries exceeds this fraction | |
150 | of the nominal size. The default for @var{threshold} is 0.8. | |
151 | @end table | |
152 | @end defun | |
153 | ||
154 | @defun makehash &optional test | |
155 | This is equivalent to @code{make-hash-table}, but with a different style | |
156 | argument list. The argument @var{test} specifies the method | |
157 | of key lookup. | |
158 | ||
159 | This function is obsolete. Use @code{make-hash-table} instead. | |
160 | @end defun | |
161 | ||
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162 | You can also create a new hash table using the printed representation |
163 | for hash tables. The Lisp reader can read this printed | |
164 | representation, provided each element in the specified hash table has | |
165 | a valid read syntax (@pxref{Printed Representation}). For instance, | |
166 | the following specifies a new hash table containing the keys | |
167 | @code{key1} and @code{key2} (both symbols) associated with @code{val1} | |
168 | (a symbol) and @code{300} (a number) respectively. | |
169 | ||
170 | @example | |
171 | #s(hash-table size 30 data (key1 val1 key2 300)) | |
172 | @end example | |
173 | ||
174 | @noindent | |
175 | The printed representation for a hash table consists of @samp{#s} | |
176 | followed by a list beginning with @samp{hash-table}. The rest of the | |
177 | list should consist of zero or more property-value pairs specifying | |
178 | the hash table's properties and initial contents. The properties and | |
179 | values are read literally. Valid property names are @code{size}, | |
180 | @code{test}, @code{weakness}, @code{rehash-size}, | |
181 | @code{rehash-threshold}, and @code{data}. The @code{data} property | |
182 | should be a list of key-value pairs for the initial contents; the | |
183 | other properties have the same meanings as the matching | |
184 | @code{make-hash-table} keywords (@code{:size}, @code{:test}, etc.), | |
185 | described above. | |
186 | ||
187 | Note that you cannot specify a hash table whose initial contents | |
188 | include objects that have no read syntax, such as buffers and frames. | |
189 | Such objects may be added to the hash table after it is created. | |
190 | ||
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191 | @node Hash Access |
192 | @section Hash Table Access | |
193 | ||
194 | This section describes the functions for accessing and storing | |
195 | associations in a hash table. In general, any Lisp object can be used | |
196 | as a hash key, unless the comparison method imposes limits. Any Lisp | |
197 | object can also be used as the value. | |
198 | ||
199 | @defun gethash key table &optional default | |
200 | This function looks up @var{key} in @var{table}, and returns its | |
201 | associated @var{value}---or @var{default}, if @var{key} has no | |
202 | association in @var{table}. | |
203 | @end defun | |
204 | ||
205 | @defun puthash key value table | |
206 | This function enters an association for @var{key} in @var{table}, with | |
207 | value @var{value}. If @var{key} already has an association in | |
208 | @var{table}, @var{value} replaces the old associated value. | |
209 | @end defun | |
210 | ||
211 | @defun remhash key table | |
212 | This function removes the association for @var{key} from @var{table}, if | |
213 | there is one. If @var{key} has no association, @code{remhash} does | |
214 | nothing. | |
215 | ||
216 | @b{Common Lisp note:} In Common Lisp, @code{remhash} returns | |
217 | non-@code{nil} if it actually removed an association and @code{nil} | |
218 | otherwise. In Emacs Lisp, @code{remhash} always returns @code{nil}. | |
219 | @end defun | |
220 | ||
221 | @defun clrhash table | |
222 | This function removes all the associations from hash table @var{table}, | |
223 | so that it becomes empty. This is also called @dfn{clearing} the hash | |
224 | table. | |
225 | ||
226 | @b{Common Lisp note:} In Common Lisp, @code{clrhash} returns the empty | |
227 | @var{table}. In Emacs Lisp, it returns @code{nil}. | |
228 | @end defun | |
229 | ||
230 | @defun maphash function table | |
231 | @anchor{Definition of maphash} | |
232 | This function calls @var{function} once for each of the associations in | |
233 | @var{table}. The function @var{function} should accept two | |
234 | arguments---a @var{key} listed in @var{table}, and its associated | |
235 | @var{value}. @code{maphash} returns @code{nil}. | |
236 | @end defun | |
237 | ||
238 | @node Defining Hash | |
239 | @section Defining Hash Comparisons | |
240 | @cindex hash code | |
241 | @cindex define hash comparisons | |
242 | ||
243 | You can define new methods of key lookup by means of | |
244 | @code{define-hash-table-test}. In order to use this feature, you need | |
245 | to understand how hash tables work, and what a @dfn{hash code} means. | |
246 | ||
247 | You can think of a hash table conceptually as a large array of many | |
248 | slots, each capable of holding one association. To look up a key, | |
249 | @code{gethash} first computes an integer, the hash code, from the key. | |
250 | It reduces this integer modulo the length of the array, to produce an | |
251 | index in the array. Then it looks in that slot, and if necessary in | |
252 | other nearby slots, to see if it has found the key being sought. | |
253 | ||
254 | Thus, to define a new method of key lookup, you need to specify both a | |
255 | function to compute the hash code from a key, and a function to compare | |
256 | two keys directly. | |
257 | ||
258 | @defun define-hash-table-test name test-fn hash-fn | |
259 | This function defines a new hash table test, named @var{name}. | |
260 | ||
261 | After defining @var{name} in this way, you can use it as the @var{test} | |
262 | argument in @code{make-hash-table}. When you do that, the hash table | |
263 | will use @var{test-fn} to compare key values, and @var{hash-fn} to compute | |
264 | a ``hash code'' from a key value. | |
265 | ||
266 | The function @var{test-fn} should accept two arguments, two keys, and | |
267 | return non-@code{nil} if they are considered ``the same.'' | |
268 | ||
269 | The function @var{hash-fn} should accept one argument, a key, and return | |
270 | an integer that is the ``hash code'' of that key. For good results, the | |
271 | function should use the whole range of integer values for hash codes, | |
272 | including negative integers. | |
273 | ||
274 | The specified functions are stored in the property list of @var{name} | |
275 | under the property @code{hash-table-test}; the property value's form is | |
276 | @code{(@var{test-fn} @var{hash-fn})}. | |
277 | @end defun | |
278 | ||
279 | @defun sxhash obj | |
280 | This function returns a hash code for Lisp object @var{obj}. | |
281 | This is an integer which reflects the contents of @var{obj} | |
282 | and the other Lisp objects it points to. | |
283 | ||
284 | If two objects @var{obj1} and @var{obj2} are equal, then @code{(sxhash | |
285 | @var{obj1})} and @code{(sxhash @var{obj2})} are the same integer. | |
286 | ||
287 | If the two objects are not equal, the values returned by @code{sxhash} | |
288 | are usually different, but not always; once in a rare while, by luck, | |
289 | you will encounter two distinct-looking objects that give the same | |
290 | result from @code{sxhash}. | |
291 | @end defun | |
292 | ||
293 | This example creates a hash table whose keys are strings that are | |
294 | compared case-insensitively. | |
295 | ||
296 | @example | |
297 | (defun case-fold-string= (a b) | |
298 | (compare-strings a nil nil b nil nil t)) | |
299 | (defun case-fold-string-hash (a) | |
300 | (sxhash (upcase a))) | |
301 | ||
302 | (define-hash-table-test 'case-fold | |
303 | 'case-fold-string= 'case-fold-string-hash) | |
304 | ||
305 | (make-hash-table :test 'case-fold) | |
306 | @end example | |
307 | ||
308 | Here is how you could define a hash table test equivalent to the | |
309 | predefined test value @code{equal}. The keys can be any Lisp object, | |
310 | and equal-looking objects are considered the same key. | |
311 | ||
312 | @example | |
313 | (define-hash-table-test 'contents-hash 'equal 'sxhash) | |
314 | ||
315 | (make-hash-table :test 'contents-hash) | |
316 | @end example | |
317 | ||
318 | @node Other Hash | |
319 | @section Other Hash Table Functions | |
320 | ||
321 | Here are some other functions for working with hash tables. | |
322 | ||
323 | @defun hash-table-p table | |
324 | This returns non-@code{nil} if @var{table} is a hash table object. | |
325 | @end defun | |
326 | ||
327 | @defun copy-hash-table table | |
328 | This function creates and returns a copy of @var{table}. Only the table | |
329 | itself is copied---the keys and values are shared. | |
330 | @end defun | |
331 | ||
332 | @defun hash-table-count table | |
333 | This function returns the actual number of entries in @var{table}. | |
334 | @end defun | |
335 | ||
336 | @defun hash-table-test table | |
337 | This returns the @var{test} value that was given when @var{table} was | |
338 | created, to specify how to hash and compare keys. See | |
339 | @code{make-hash-table} (@pxref{Creating Hash}). | |
340 | @end defun | |
341 | ||
342 | @defun hash-table-weakness table | |
343 | This function returns the @var{weak} value that was specified for hash | |
344 | table @var{table}. | |
345 | @end defun | |
346 | ||
347 | @defun hash-table-rehash-size table | |
348 | This returns the rehash size of @var{table}. | |
349 | @end defun | |
350 | ||
351 | @defun hash-table-rehash-threshold table | |
352 | This returns the rehash threshold of @var{table}. | |
353 | @end defun | |
354 | ||
355 | @defun hash-table-size table | |
356 | This returns the current nominal size of @var{table}. | |
357 | @end defun | |
358 | ||
359 | @ignore | |
360 | arch-tag: 3b5107f9-d2f0-47d5-ad61-3498496bea0e | |
361 | @end ignore |