| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. |
| 4 | @c See the file elisp.texi for copying conditions. |
| 5 | @setfilename ../info/lists |
| 6 | @node Lists, Sequences Arrays Vectors, Strings and Characters, Top |
| 7 | @chapter Lists |
| 8 | @cindex list |
| 9 | @cindex element (of list) |
| 10 | |
| 11 | A @dfn{list} represents a sequence of zero or more elements (which may |
| 12 | be any Lisp objects). The important difference between lists and |
| 13 | vectors is that two or more lists can share part of their structure; in |
| 14 | addition, you can insert or delete elements in a list without copying |
| 15 | the whole list. |
| 16 | |
| 17 | @menu |
| 18 | * Cons Cells:: How lists are made out of cons cells. |
| 19 | * Lists as Boxes:: Graphical notation to explain lists. |
| 20 | * List-related Predicates:: Is this object a list? Comparing two lists. |
| 21 | * List Elements:: Extracting the pieces of a list. |
| 22 | * Building Lists:: Creating list structure. |
| 23 | * Modifying Lists:: Storing new pieces into an existing list. |
| 24 | * Sets And Lists:: A list can represent a finite mathematical set. |
| 25 | * Association Lists:: A list can represent a finite relation or mapping. |
| 26 | @end menu |
| 27 | |
| 28 | @node Cons Cells |
| 29 | @section Lists and Cons Cells |
| 30 | @cindex lists and cons cells |
| 31 | @cindex @code{nil} and lists |
| 32 | |
| 33 | Lists in Lisp are not a primitive data type; they are built up from |
| 34 | @dfn{cons cells}. A cons cell is a data object that represents an |
| 35 | ordered pair. It records two Lisp objects, one labeled as the @sc{car}, |
| 36 | and the other labeled as the @sc{cdr}. These names are traditional; see |
| 37 | @ref{Cons Cell Type}. @sc{cdr} is pronounced ``could-er.'' |
| 38 | |
| 39 | A list is a series of cons cells chained together, one cons cell per |
| 40 | element of the list. By convention, the @sc{car}s of the cons cells are |
| 41 | the elements of the list, and the @sc{cdr}s are used to chain the list: |
| 42 | the @sc{cdr} of each cons cell is the following cons cell. The @sc{cdr} |
| 43 | of the last cons cell is @code{nil}. This asymmetry between the |
| 44 | @sc{car} and the @sc{cdr} is entirely a matter of convention; at the |
| 45 | level of cons cells, the @sc{car} and @sc{cdr} slots have the same |
| 46 | characteristics. |
| 47 | |
| 48 | @cindex list structure |
| 49 | Because most cons cells are used as part of lists, the phrase |
| 50 | @dfn{list structure} has come to mean any structure made out of cons |
| 51 | cells. |
| 52 | |
| 53 | The symbol @code{nil} is considered a list as well as a symbol; it is |
| 54 | the list with no elements. For convenience, the symbol @code{nil} is |
| 55 | considered to have @code{nil} as its @sc{cdr} (and also as its |
| 56 | @sc{car}). |
| 57 | |
| 58 | The @sc{cdr} of any nonempty list @var{l} is a list containing all the |
| 59 | elements of @var{l} except the first. |
| 60 | |
| 61 | @node Lists as Boxes |
| 62 | @comment node-name, next, previous, up |
| 63 | @section Lists as Linked Pairs of Boxes |
| 64 | @cindex box representation for lists |
| 65 | @cindex lists represented as boxes |
| 66 | @cindex cons cell as box |
| 67 | |
| 68 | A cons cell can be illustrated as a pair of boxes. The first box |
| 69 | represents the @sc{car} and the second box represents the @sc{cdr}. |
| 70 | Here is an illustration of the two-element list, @code{(tulip lily)}, |
| 71 | made from two cons cells: |
| 72 | |
| 73 | @example |
| 74 | @group |
| 75 | --------------- --------------- |
| 76 | | car | cdr | | car | cdr | |
| 77 | | tulip | o---------->| lily | nil | |
| 78 | | | | | | | |
| 79 | --------------- --------------- |
| 80 | @end group |
| 81 | @end example |
| 82 | |
| 83 | Each pair of boxes represents a cons cell. Each box ``refers to'', |
| 84 | ``points to'' or ``contains'' a Lisp object. (These terms are |
| 85 | synonymous.) The first box, which is the @sc{car} of the first cons |
| 86 | cell, contains the symbol @code{tulip}. The arrow from the @sc{cdr} of |
| 87 | the first cons cell to the second cons cell indicates that the @sc{cdr} |
| 88 | of the first cons cell points to the second cons cell. |
| 89 | |
| 90 | The same list can be illustrated in a different sort of box notation |
| 91 | like this: |
| 92 | |
| 93 | @example |
| 94 | @group |
| 95 | ___ ___ ___ ___ |
| 96 | |___|___|--> |___|___|--> nil |
| 97 | | | |
| 98 | | | |
| 99 | --> tulip --> lily |
| 100 | @end group |
| 101 | @end example |
| 102 | |
| 103 | Here is a more complex illustration, showing the three-element list, |
| 104 | @code{((pine needles) oak maple)}, the first element of which is a |
| 105 | two-element list: |
| 106 | |
| 107 | @example |
| 108 | @group |
| 109 | ___ ___ ___ ___ ___ ___ |
| 110 | |___|___|--> |___|___|--> |___|___|--> nil |
| 111 | | | | |
| 112 | | | | |
| 113 | | --> oak --> maple |
| 114 | | |
| 115 | | ___ ___ ___ ___ |
| 116 | --> |___|___|--> |___|___|--> nil |
| 117 | | | |
| 118 | | | |
| 119 | --> pine --> needles |
| 120 | @end group |
| 121 | @end example |
| 122 | |
| 123 | The same list represented in the first box notation looks like this: |
| 124 | |
| 125 | @example |
| 126 | @group |
| 127 | -------------- -------------- -------------- |
| 128 | | car | cdr | | car | cdr | | car | cdr | |
| 129 | | o | o------->| oak | o------->| maple | nil | |
| 130 | | | | | | | | | | | |
| 131 | -- | --------- -------------- -------------- |
| 132 | | |
| 133 | | |
| 134 | | -------------- ---------------- |
| 135 | | | car | cdr | | car | cdr | |
| 136 | ------>| pine | o------->| needles | nil | |
| 137 | | | | | | | |
| 138 | -------------- ---------------- |
| 139 | @end group |
| 140 | @end example |
| 141 | |
| 142 | @xref{Cons Cell Type}, for the read and print syntax of cons cells and |
| 143 | lists, and for more ``box and arrow'' illustrations of lists. |
| 144 | |
| 145 | @node List-related Predicates |
| 146 | @section Predicates on Lists |
| 147 | |
| 148 | The following predicates test whether a Lisp object is an atom, is a |
| 149 | cons cell or is a list, or whether it is the distinguished object |
| 150 | @code{nil}. (Many of these predicates can be defined in terms of the |
| 151 | others, but they are used so often that it is worth having all of them.) |
| 152 | |
| 153 | @defun consp object |
| 154 | This function returns @code{t} if @var{object} is a cons cell, @code{nil} |
| 155 | otherwise. @code{nil} is not a cons cell, although it @emph{is} a list. |
| 156 | @end defun |
| 157 | |
| 158 | @defun atom object |
| 159 | @cindex atoms |
| 160 | This function returns @code{t} if @var{object} is an atom, @code{nil} |
| 161 | otherwise. All objects except cons cells are atoms. The symbol |
| 162 | @code{nil} is an atom and is also a list; it is the only Lisp object |
| 163 | that is both. |
| 164 | |
| 165 | @example |
| 166 | (atom @var{object}) @equiv{} (not (consp @var{object})) |
| 167 | @end example |
| 168 | @end defun |
| 169 | |
| 170 | @defun listp object |
| 171 | This function returns @code{t} if @var{object} is a cons cell or |
| 172 | @code{nil}. Otherwise, it returns @code{nil}. |
| 173 | |
| 174 | @example |
| 175 | @group |
| 176 | (listp '(1)) |
| 177 | @result{} t |
| 178 | @end group |
| 179 | @group |
| 180 | (listp '()) |
| 181 | @result{} t |
| 182 | @end group |
| 183 | @end example |
| 184 | @end defun |
| 185 | |
| 186 | @defun nlistp object |
| 187 | This function is the opposite of @code{listp}: it returns @code{t} if |
| 188 | @var{object} is not a list. Otherwise, it returns @code{nil}. |
| 189 | |
| 190 | @example |
| 191 | (listp @var{object}) @equiv{} (not (nlistp @var{object})) |
| 192 | @end example |
| 193 | @end defun |
| 194 | |
| 195 | @defun null object |
| 196 | This function returns @code{t} if @var{object} is @code{nil}, and |
| 197 | returns @code{nil} otherwise. This function is identical to @code{not}, |
| 198 | but as a matter of clarity we use @code{null} when @var{object} is |
| 199 | considered a list and @code{not} when it is considered a truth value |
| 200 | (see @code{not} in @ref{Combining Conditions}). |
| 201 | |
| 202 | @example |
| 203 | @group |
| 204 | (null '(1)) |
| 205 | @result{} nil |
| 206 | @end group |
| 207 | @group |
| 208 | (null '()) |
| 209 | @result{} t |
| 210 | @end group |
| 211 | @end example |
| 212 | @end defun |
| 213 | |
| 214 | @need 2000 |
| 215 | |
| 216 | @node List Elements |
| 217 | @section Accessing Elements of Lists |
| 218 | @cindex list elements |
| 219 | |
| 220 | @defun car cons-cell |
| 221 | This function returns the value pointed to by the first pointer of the |
| 222 | cons cell @var{cons-cell}. Expressed another way, this function |
| 223 | returns the @sc{car} of @var{cons-cell}. |
| 224 | |
| 225 | As a special case, if @var{cons-cell} is @code{nil}, then @code{car} |
| 226 | is defined to return @code{nil}; therefore, any list is a valid argument |
| 227 | for @code{car}. An error is signaled if the argument is not a cons cell |
| 228 | or @code{nil}. |
| 229 | |
| 230 | @example |
| 231 | @group |
| 232 | (car '(a b c)) |
| 233 | @result{} a |
| 234 | @end group |
| 235 | @group |
| 236 | (car '()) |
| 237 | @result{} nil |
| 238 | @end group |
| 239 | @end example |
| 240 | @end defun |
| 241 | |
| 242 | @defun cdr cons-cell |
| 243 | This function returns the value pointed to by the second pointer of |
| 244 | the cons cell @var{cons-cell}. Expressed another way, this function |
| 245 | returns the @sc{cdr} of @var{cons-cell}. |
| 246 | |
| 247 | As a special case, if @var{cons-cell} is @code{nil}, then @code{cdr} |
| 248 | is defined to return @code{nil}; therefore, any list is a valid argument |
| 249 | for @code{cdr}. An error is signaled if the argument is not a cons cell |
| 250 | or @code{nil}. |
| 251 | |
| 252 | @example |
| 253 | @group |
| 254 | (cdr '(a b c)) |
| 255 | @result{} (b c) |
| 256 | @end group |
| 257 | @group |
| 258 | (cdr '()) |
| 259 | @result{} nil |
| 260 | @end group |
| 261 | @end example |
| 262 | @end defun |
| 263 | |
| 264 | @defun car-safe object |
| 265 | This function lets you take the @sc{car} of a cons cell while avoiding |
| 266 | errors for other data types. It returns the @sc{car} of @var{object} if |
| 267 | @var{object} is a cons cell, @code{nil} otherwise. This is in contrast |
| 268 | to @code{car}, which signals an error if @var{object} is not a list. |
| 269 | |
| 270 | @example |
| 271 | @group |
| 272 | (car-safe @var{object}) |
| 273 | @equiv{} |
| 274 | (let ((x @var{object})) |
| 275 | (if (consp x) |
| 276 | (car x) |
| 277 | nil)) |
| 278 | @end group |
| 279 | @end example |
| 280 | @end defun |
| 281 | |
| 282 | @defun cdr-safe object |
| 283 | This function lets you take the @sc{cdr} of a cons cell while |
| 284 | avoiding errors for other data types. It returns the @sc{cdr} of |
| 285 | @var{object} if @var{object} is a cons cell, @code{nil} otherwise. |
| 286 | This is in contrast to @code{cdr}, which signals an error if |
| 287 | @var{object} is not a list. |
| 288 | |
| 289 | @example |
| 290 | @group |
| 291 | (cdr-safe @var{object}) |
| 292 | @equiv{} |
| 293 | (let ((x @var{object})) |
| 294 | (if (consp x) |
| 295 | (cdr x) |
| 296 | nil)) |
| 297 | @end group |
| 298 | @end example |
| 299 | @end defun |
| 300 | |
| 301 | @defun nth n list |
| 302 | This function returns the @var{n}th element of @var{list}. Elements |
| 303 | are numbered starting with zero, so the @sc{car} of @var{list} is |
| 304 | element number zero. If the length of @var{list} is @var{n} or less, |
| 305 | the value is @code{nil}. |
| 306 | |
| 307 | If @var{n} is negative, @code{nth} returns the first element of |
| 308 | @var{list}. |
| 309 | |
| 310 | @example |
| 311 | @group |
| 312 | (nth 2 '(1 2 3 4)) |
| 313 | @result{} 3 |
| 314 | @end group |
| 315 | @group |
| 316 | (nth 10 '(1 2 3 4)) |
| 317 | @result{} nil |
| 318 | @end group |
| 319 | @group |
| 320 | (nth -3 '(1 2 3 4)) |
| 321 | @result{} 1 |
| 322 | |
| 323 | (nth n x) @equiv{} (car (nthcdr n x)) |
| 324 | @end group |
| 325 | @end example |
| 326 | @end defun |
| 327 | |
| 328 | @defun nthcdr n list |
| 329 | This function returns the @var{n}th @sc{cdr} of @var{list}. In other |
| 330 | words, it removes the first @var{n} links of @var{list} and returns |
| 331 | what follows. |
| 332 | |
| 333 | If @var{n} is zero or negative, @code{nthcdr} returns all of |
| 334 | @var{list}. If the length of @var{list} is @var{n} or less, |
| 335 | @code{nthcdr} returns @code{nil}. |
| 336 | |
| 337 | @example |
| 338 | @group |
| 339 | (nthcdr 1 '(1 2 3 4)) |
| 340 | @result{} (2 3 4) |
| 341 | @end group |
| 342 | @group |
| 343 | (nthcdr 10 '(1 2 3 4)) |
| 344 | @result{} nil |
| 345 | @end group |
| 346 | @group |
| 347 | (nthcdr -3 '(1 2 3 4)) |
| 348 | @result{} (1 2 3 4) |
| 349 | @end group |
| 350 | @end example |
| 351 | @end defun |
| 352 | |
| 353 | @node Building Lists |
| 354 | @comment node-name, next, previous, up |
| 355 | @section Building Cons Cells and Lists |
| 356 | @cindex cons cells |
| 357 | @cindex building lists |
| 358 | |
| 359 | Many functions build lists, as lists reside at the very heart of Lisp. |
| 360 | @code{cons} is the fundamental list-building function; however, it is |
| 361 | interesting to note that @code{list} is used more times in the source |
| 362 | code for Emacs than @code{cons}. |
| 363 | |
| 364 | @defun cons object1 object2 |
| 365 | This function is the fundamental function used to build new list |
| 366 | structure. It creates a new cons cell, making @var{object1} the |
| 367 | @sc{car}, and @var{object2} the @sc{cdr}. It then returns the new cons |
| 368 | cell. The arguments @var{object1} and @var{object2} may be any Lisp |
| 369 | objects, but most often @var{object2} is a list. |
| 370 | |
| 371 | @example |
| 372 | @group |
| 373 | (cons 1 '(2)) |
| 374 | @result{} (1 2) |
| 375 | @end group |
| 376 | @group |
| 377 | (cons 1 '()) |
| 378 | @result{} (1) |
| 379 | @end group |
| 380 | @group |
| 381 | (cons 1 2) |
| 382 | @result{} (1 . 2) |
| 383 | @end group |
| 384 | @end example |
| 385 | |
| 386 | @cindex consing |
| 387 | @code{cons} is often used to add a single element to the front of a |
| 388 | list. This is called @dfn{consing the element onto the list}. For |
| 389 | example: |
| 390 | |
| 391 | @example |
| 392 | (setq list (cons newelt list)) |
| 393 | @end example |
| 394 | |
| 395 | Note that there is no conflict between the variable named @code{list} |
| 396 | used in this example and the function named @code{list} described below; |
| 397 | any symbol can serve both purposes. |
| 398 | @end defun |
| 399 | |
| 400 | @defun list &rest objects |
| 401 | This function creates a list with @var{objects} as its elements. The |
| 402 | resulting list is always @code{nil}-terminated. If no @var{objects} |
| 403 | are given, the empty list is returned. |
| 404 | |
| 405 | @example |
| 406 | @group |
| 407 | (list 1 2 3 4 5) |
| 408 | @result{} (1 2 3 4 5) |
| 409 | @end group |
| 410 | @group |
| 411 | (list 1 2 '(3 4 5) 'foo) |
| 412 | @result{} (1 2 (3 4 5) foo) |
| 413 | @end group |
| 414 | @group |
| 415 | (list) |
| 416 | @result{} nil |
| 417 | @end group |
| 418 | @end example |
| 419 | @end defun |
| 420 | |
| 421 | @defun make-list length object |
| 422 | This function creates a list of length @var{length}, in which all the |
| 423 | elements have the identical value @var{object}. Compare |
| 424 | @code{make-list} with @code{make-string} (@pxref{Creating Strings}). |
| 425 | |
| 426 | @example |
| 427 | @group |
| 428 | (make-list 3 'pigs) |
| 429 | @result{} (pigs pigs pigs) |
| 430 | @end group |
| 431 | @group |
| 432 | (make-list 0 'pigs) |
| 433 | @result{} nil |
| 434 | @end group |
| 435 | @end example |
| 436 | @end defun |
| 437 | |
| 438 | @defun append &rest sequences |
| 439 | @cindex copying lists |
| 440 | This function returns a list containing all the elements of |
| 441 | @var{sequences}. The @var{sequences} may be lists, vectors, or strings, |
| 442 | but the last one should be a list. All arguments except the last one |
| 443 | are copied, so none of them are altered. |
| 444 | |
| 445 | More generally, the final argument to @code{append} may be any Lisp |
| 446 | object. The final argument is not copied or converted; it becomes the |
| 447 | @sc{cdr} of the last cons cell in the new list. If the final argument |
| 448 | is itself a list, then its elements become in effect elements of the |
| 449 | result list. If the final element is not a list, the result is a |
| 450 | ``dotted list'' since its final @sc{cdr} is not @code{nil} as required |
| 451 | in a true list. |
| 452 | |
| 453 | See @code{nconc} in @ref{Rearrangement}, for a way to join lists with no |
| 454 | copying. |
| 455 | |
| 456 | Here is an example of using @code{append}: |
| 457 | |
| 458 | @example |
| 459 | @group |
| 460 | (setq trees '(pine oak)) |
| 461 | @result{} (pine oak) |
| 462 | (setq more-trees (append '(maple birch) trees)) |
| 463 | @result{} (maple birch pine oak) |
| 464 | @end group |
| 465 | |
| 466 | @group |
| 467 | trees |
| 468 | @result{} (pine oak) |
| 469 | more-trees |
| 470 | @result{} (maple birch pine oak) |
| 471 | @end group |
| 472 | @group |
| 473 | (eq trees (cdr (cdr more-trees))) |
| 474 | @result{} t |
| 475 | @end group |
| 476 | @end example |
| 477 | |
| 478 | You can see how @code{append} works by looking at a box diagram. The |
| 479 | variable @code{trees} is set to the list @code{(pine oak)} and then the |
| 480 | variable @code{more-trees} is set to the list @code{(maple birch pine |
| 481 | oak)}. However, the variable @code{trees} continues to refer to the |
| 482 | original list: |
| 483 | |
| 484 | @smallexample |
| 485 | @group |
| 486 | more-trees trees |
| 487 | | | |
| 488 | | ___ ___ ___ ___ -> ___ ___ ___ ___ |
| 489 | --> |___|___|--> |___|___|--> |___|___|--> |___|___|--> nil |
| 490 | | | | | |
| 491 | | | | | |
| 492 | --> maple -->birch --> pine --> oak |
| 493 | @end group |
| 494 | @end smallexample |
| 495 | |
| 496 | An empty sequence contributes nothing to the value returned by |
| 497 | @code{append}. As a consequence of this, a final @code{nil} argument |
| 498 | forces a copy of the previous argument. |
| 499 | |
| 500 | @example |
| 501 | @group |
| 502 | trees |
| 503 | @result{} (pine oak) |
| 504 | @end group |
| 505 | @group |
| 506 | (setq wood (append trees ())) |
| 507 | @result{} (pine oak) |
| 508 | @end group |
| 509 | @group |
| 510 | wood |
| 511 | @result{} (pine oak) |
| 512 | @end group |
| 513 | @group |
| 514 | (eq wood trees) |
| 515 | @result{} nil |
| 516 | @end group |
| 517 | @end example |
| 518 | |
| 519 | @noindent |
| 520 | This once was the usual way to copy a list, before the function |
| 521 | @code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}. |
| 522 | |
| 523 | With the help of @code{apply}, we can append all the lists in a list of |
| 524 | lists: |
| 525 | |
| 526 | @example |
| 527 | @group |
| 528 | (apply 'append '((a b c) nil (x y z) nil)) |
| 529 | @result{} (a b c x y z) |
| 530 | @end group |
| 531 | @end example |
| 532 | |
| 533 | If no @var{sequences} are given, @code{nil} is returned: |
| 534 | |
| 535 | @example |
| 536 | @group |
| 537 | (append) |
| 538 | @result{} nil |
| 539 | @end group |
| 540 | @end example |
| 541 | |
| 542 | Here are some examples where the final argument is not a list: |
| 543 | |
| 544 | @example |
| 545 | (append '(x y) 'z) |
| 546 | @result{} (x y . z) |
| 547 | (append '(x y) [z]) |
| 548 | @result{} (x y . [z]) |
| 549 | @end example |
| 550 | |
| 551 | @noindent |
| 552 | The second example shows that when the final argument is a sequence but |
| 553 | not a list, the sequence's elements do not become elements of the |
| 554 | resulting list. Instead, the sequence becomes the final @sc{cdr}, like |
| 555 | any other non-list final argument. |
| 556 | |
| 557 | The @code{append} function also allows integers as arguments. It |
| 558 | converts them to strings of digits, making up the decimal print |
| 559 | representation of the integer, and then uses the strings instead of the |
| 560 | original integers. @strong{Don't use this feature; we plan to eliminate |
| 561 | it. If you already use this feature, change your programs now!} The |
| 562 | proper way to convert an integer to a decimal number in this way is with |
| 563 | @code{format} (@pxref{Formatting Strings}) or @code{number-to-string} |
| 564 | (@pxref{String Conversion}). |
| 565 | @end defun |
| 566 | |
| 567 | @defun reverse list |
| 568 | This function creates a new list whose elements are the elements of |
| 569 | @var{list}, but in reverse order. The original argument @var{list} is |
| 570 | @emph{not} altered. |
| 571 | |
| 572 | @example |
| 573 | @group |
| 574 | (setq x '(1 2 3 4)) |
| 575 | @result{} (1 2 3 4) |
| 576 | @end group |
| 577 | @group |
| 578 | (reverse x) |
| 579 | @result{} (4 3 2 1) |
| 580 | x |
| 581 | @result{} (1 2 3 4) |
| 582 | @end group |
| 583 | @end example |
| 584 | @end defun |
| 585 | |
| 586 | @node Modifying Lists |
| 587 | @section Modifying Existing List Structure |
| 588 | |
| 589 | You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the |
| 590 | primitives @code{setcar} and @code{setcdr}. |
| 591 | |
| 592 | @cindex CL note---@code{rplaca} vrs @code{setcar} |
| 593 | @quotation |
| 594 | @findex rplaca |
| 595 | @findex rplacd |
| 596 | @b{Common Lisp note:} Common Lisp uses functions @code{rplaca} and |
| 597 | @code{rplacd} to alter list structure; they change structure the same |
| 598 | way as @code{setcar} and @code{setcdr}, but the Common Lisp functions |
| 599 | return the cons cell while @code{setcar} and @code{setcdr} return the |
| 600 | new @sc{car} or @sc{cdr}. |
| 601 | @end quotation |
| 602 | |
| 603 | @menu |
| 604 | * Setcar:: Replacing an element in a list. |
| 605 | * Setcdr:: Replacing part of the list backbone. |
| 606 | This can be used to remove or add elements. |
| 607 | * Rearrangement:: Reordering the elements in a list; combining lists. |
| 608 | @end menu |
| 609 | |
| 610 | @node Setcar |
| 611 | @subsection Altering List Elements with @code{setcar} |
| 612 | |
| 613 | Changing the @sc{car} of a cons cell is done with @code{setcar}. When |
| 614 | used on a list, @code{setcar} replaces one element of a list with a |
| 615 | different element. |
| 616 | |
| 617 | @defun setcar cons object |
| 618 | This function stores @var{object} as the new @sc{car} of @var{cons}, |
| 619 | replacing its previous @sc{car}. It returns the value @var{object}. |
| 620 | For example: |
| 621 | |
| 622 | @example |
| 623 | @group |
| 624 | (setq x '(1 2)) |
| 625 | @result{} (1 2) |
| 626 | @end group |
| 627 | @group |
| 628 | (setcar x 4) |
| 629 | @result{} 4 |
| 630 | @end group |
| 631 | @group |
| 632 | x |
| 633 | @result{} (4 2) |
| 634 | @end group |
| 635 | @end example |
| 636 | @end defun |
| 637 | |
| 638 | When a cons cell is part of the shared structure of several lists, |
| 639 | storing a new @sc{car} into the cons changes one element of each of |
| 640 | these lists. Here is an example: |
| 641 | |
| 642 | @example |
| 643 | @group |
| 644 | ;; @r{Create two lists that are partly shared.} |
| 645 | (setq x1 '(a b c)) |
| 646 | @result{} (a b c) |
| 647 | (setq x2 (cons 'z (cdr x1))) |
| 648 | @result{} (z b c) |
| 649 | @end group |
| 650 | |
| 651 | @group |
| 652 | ;; @r{Replace the @sc{car} of a shared link.} |
| 653 | (setcar (cdr x1) 'foo) |
| 654 | @result{} foo |
| 655 | x1 ; @r{Both lists are changed.} |
| 656 | @result{} (a foo c) |
| 657 | x2 |
| 658 | @result{} (z foo c) |
| 659 | @end group |
| 660 | |
| 661 | @group |
| 662 | ;; @r{Replace the @sc{car} of a link that is not shared.} |
| 663 | (setcar x1 'baz) |
| 664 | @result{} baz |
| 665 | x1 ; @r{Only one list is changed.} |
| 666 | @result{} (baz foo c) |
| 667 | x2 |
| 668 | @result{} (z foo c) |
| 669 | @end group |
| 670 | @end example |
| 671 | |
| 672 | Here is a graphical depiction of the shared structure of the two lists |
| 673 | in the variables @code{x1} and @code{x2}, showing why replacing @code{b} |
| 674 | changes them both: |
| 675 | |
| 676 | @example |
| 677 | @group |
| 678 | ___ ___ ___ ___ ___ ___ |
| 679 | x1---> |___|___|----> |___|___|--> |___|___|--> nil |
| 680 | | --> | | |
| 681 | | | | | |
| 682 | --> a | --> b --> c |
| 683 | | |
| 684 | ___ ___ | |
| 685 | x2--> |___|___|-- |
| 686 | | |
| 687 | | |
| 688 | --> z |
| 689 | @end group |
| 690 | @end example |
| 691 | |
| 692 | Here is an alternative form of box diagram, showing the same relationship: |
| 693 | |
| 694 | @example |
| 695 | @group |
| 696 | x1: |
| 697 | -------------- -------------- -------------- |
| 698 | | car | cdr | | car | cdr | | car | cdr | |
| 699 | | a | o------->| b | o------->| c | nil | |
| 700 | | | | -->| | | | | | |
| 701 | -------------- | -------------- -------------- |
| 702 | | |
| 703 | x2: | |
| 704 | -------------- | |
| 705 | | car | cdr | | |
| 706 | | z | o---- |
| 707 | | | | |
| 708 | -------------- |
| 709 | @end group |
| 710 | @end example |
| 711 | |
| 712 | @node Setcdr |
| 713 | @subsection Altering the CDR of a List |
| 714 | |
| 715 | The lowest-level primitive for modifying a @sc{cdr} is @code{setcdr}: |
| 716 | |
| 717 | @defun setcdr cons object |
| 718 | This function stores @var{object} as the new @sc{cdr} of @var{cons}, |
| 719 | replacing its previous @sc{cdr}. It returns the value @var{object}. |
| 720 | @end defun |
| 721 | |
| 722 | Here is an example of replacing the @sc{cdr} of a list with a |
| 723 | different list. All but the first element of the list are removed in |
| 724 | favor of a different sequence of elements. The first element is |
| 725 | unchanged, because it resides in the @sc{car} of the list, and is not |
| 726 | reached via the @sc{cdr}. |
| 727 | |
| 728 | @example |
| 729 | @group |
| 730 | (setq x '(1 2 3)) |
| 731 | @result{} (1 2 3) |
| 732 | @end group |
| 733 | @group |
| 734 | (setcdr x '(4)) |
| 735 | @result{} (4) |
| 736 | @end group |
| 737 | @group |
| 738 | x |
| 739 | @result{} (1 4) |
| 740 | @end group |
| 741 | @end example |
| 742 | |
| 743 | You can delete elements from the middle of a list by altering the |
| 744 | @sc{cdr}s of the cons cells in the list. For example, here we delete |
| 745 | the second element, @code{b}, from the list @code{(a b c)}, by changing |
| 746 | the @sc{cdr} of the first cell: |
| 747 | |
| 748 | @example |
| 749 | @group |
| 750 | (setq x1 '(a b c)) |
| 751 | @result{} (a b c) |
| 752 | (setcdr x1 (cdr (cdr x1))) |
| 753 | @result{} (c) |
| 754 | x1 |
| 755 | @result{} (a c) |
| 756 | @end group |
| 757 | @end example |
| 758 | |
| 759 | @need 4000 |
| 760 | Here is the result in box notation: |
| 761 | |
| 762 | @example |
| 763 | @group |
| 764 | -------------------- |
| 765 | | | |
| 766 | -------------- | -------------- | -------------- |
| 767 | | car | cdr | | | car | cdr | -->| car | cdr | |
| 768 | | a | o----- | b | o-------->| c | nil | |
| 769 | | | | | | | | | | |
| 770 | -------------- -------------- -------------- |
| 771 | @end group |
| 772 | @end example |
| 773 | |
| 774 | @noindent |
| 775 | The second cons cell, which previously held the element @code{b}, still |
| 776 | exists and its @sc{car} is still @code{b}, but it no longer forms part |
| 777 | of this list. |
| 778 | |
| 779 | It is equally easy to insert a new element by changing @sc{cdr}s: |
| 780 | |
| 781 | @example |
| 782 | @group |
| 783 | (setq x1 '(a b c)) |
| 784 | @result{} (a b c) |
| 785 | (setcdr x1 (cons 'd (cdr x1))) |
| 786 | @result{} (d b c) |
| 787 | x1 |
| 788 | @result{} (a d b c) |
| 789 | @end group |
| 790 | @end example |
| 791 | |
| 792 | Here is this result in box notation: |
| 793 | |
| 794 | @smallexample |
| 795 | @group |
| 796 | -------------- ------------- ------------- |
| 797 | | car | cdr | | car | cdr | | car | cdr | |
| 798 | | a | o | -->| b | o------->| c | nil | |
| 799 | | | | | | | | | | | | |
| 800 | --------- | -- | ------------- ------------- |
| 801 | | | |
| 802 | ----- -------- |
| 803 | | | |
| 804 | | --------------- | |
| 805 | | | car | cdr | | |
| 806 | -->| d | o------ |
| 807 | | | | |
| 808 | --------------- |
| 809 | @end group |
| 810 | @end smallexample |
| 811 | |
| 812 | @node Rearrangement |
| 813 | @subsection Functions that Rearrange Lists |
| 814 | @cindex rearrangement of lists |
| 815 | @cindex modification of lists |
| 816 | |
| 817 | Here are some functions that rearrange lists ``destructively'' by |
| 818 | modifying the @sc{cdr}s of their component cons cells. We call these |
| 819 | functions ``destructive'' because they chew up the original lists passed |
| 820 | to them as arguments, to produce a new list that is the returned value. |
| 821 | |
| 822 | @ifinfo |
| 823 | See @code{delq}, in @ref{Sets And Lists}, for another function |
| 824 | that modifies cons cells. |
| 825 | @end ifinfo |
| 826 | @iftex |
| 827 | The function @code{delq} in the following section is another example |
| 828 | of destructive list manipulation. |
| 829 | @end iftex |
| 830 | |
| 831 | @defun nconc &rest lists |
| 832 | @cindex concatenating lists |
| 833 | @cindex joining lists |
| 834 | This function returns a list containing all the elements of @var{lists}. |
| 835 | Unlike @code{append} (@pxref{Building Lists}), the @var{lists} are |
| 836 | @emph{not} copied. Instead, the last @sc{cdr} of each of the |
| 837 | @var{lists} is changed to refer to the following list. The last of the |
| 838 | @var{lists} is not altered. For example: |
| 839 | |
| 840 | @example |
| 841 | @group |
| 842 | (setq x '(1 2 3)) |
| 843 | @result{} (1 2 3) |
| 844 | @end group |
| 845 | @group |
| 846 | (nconc x '(4 5)) |
| 847 | @result{} (1 2 3 4 5) |
| 848 | @end group |
| 849 | @group |
| 850 | x |
| 851 | @result{} (1 2 3 4 5) |
| 852 | @end group |
| 853 | @end example |
| 854 | |
| 855 | Since the last argument of @code{nconc} is not itself modified, it is |
| 856 | reasonable to use a constant list, such as @code{'(4 5)}, as in the |
| 857 | above example. For the same reason, the last argument need not be a |
| 858 | list: |
| 859 | |
| 860 | @example |
| 861 | @group |
| 862 | (setq x '(1 2 3)) |
| 863 | @result{} (1 2 3) |
| 864 | @end group |
| 865 | @group |
| 866 | (nconc x 'z) |
| 867 | @result{} (1 2 3 . z) |
| 868 | @end group |
| 869 | @group |
| 870 | x |
| 871 | @result{} (1 2 3 . z) |
| 872 | @end group |
| 873 | @end example |
| 874 | |
| 875 | A common pitfall is to use a quoted constant list as a non-last |
| 876 | argument to @code{nconc}. If you do this, your program will change |
| 877 | each time you run it! Here is what happens: |
| 878 | |
| 879 | @smallexample |
| 880 | @group |
| 881 | (defun add-foo (x) ; @r{We want this function to add} |
| 882 | (nconc '(foo) x)) ; @r{@code{foo} to the front of its arg.} |
| 883 | @end group |
| 884 | |
| 885 | @group |
| 886 | (symbol-function 'add-foo) |
| 887 | @result{} (lambda (x) (nconc (quote (foo)) x)) |
| 888 | @end group |
| 889 | |
| 890 | @group |
| 891 | (setq xx (add-foo '(1 2))) ; @r{It seems to work.} |
| 892 | @result{} (foo 1 2) |
| 893 | @end group |
| 894 | @group |
| 895 | (setq xy (add-foo '(3 4))) ; @r{What happened?} |
| 896 | @result{} (foo 1 2 3 4) |
| 897 | @end group |
| 898 | @group |
| 899 | (eq xx xy) |
| 900 | @result{} t |
| 901 | @end group |
| 902 | |
| 903 | @group |
| 904 | (symbol-function 'add-foo) |
| 905 | @result{} (lambda (x) (nconc (quote (foo 1 2 3 4) x))) |
| 906 | @end group |
| 907 | @end smallexample |
| 908 | @end defun |
| 909 | |
| 910 | @defun nreverse list |
| 911 | @cindex reversing a list |
| 912 | This function reverses the order of the elements of @var{list}. |
| 913 | Unlike @code{reverse}, @code{nreverse} alters its argument by reversing |
| 914 | the @sc{cdr}s in the cons cells forming the list. The cons cell that |
| 915 | used to be the last one in @var{list} becomes the first cell of the |
| 916 | value. |
| 917 | |
| 918 | For example: |
| 919 | |
| 920 | @example |
| 921 | @group |
| 922 | (setq x '(1 2 3 4)) |
| 923 | @result{} (1 2 3 4) |
| 924 | @end group |
| 925 | @group |
| 926 | x |
| 927 | @result{} (1 2 3 4) |
| 928 | (nreverse x) |
| 929 | @result{} (4 3 2 1) |
| 930 | @end group |
| 931 | @group |
| 932 | ;; @r{The cell that was first is now last.} |
| 933 | x |
| 934 | @result{} (1) |
| 935 | @end group |
| 936 | @end example |
| 937 | |
| 938 | To avoid confusion, we usually store the result of @code{nreverse} |
| 939 | back in the same variable which held the original list: |
| 940 | |
| 941 | @example |
| 942 | (setq x (nreverse x)) |
| 943 | @end example |
| 944 | |
| 945 | Here is the @code{nreverse} of our favorite example, @code{(a b c)}, |
| 946 | presented graphically: |
| 947 | |
| 948 | @smallexample |
| 949 | @group |
| 950 | @r{Original list head:} @r{Reversed list:} |
| 951 | ------------- ------------- ------------ |
| 952 | | car | cdr | | car | cdr | | car | cdr | |
| 953 | | a | nil |<-- | b | o |<-- | c | o | |
| 954 | | | | | | | | | | | | | | |
| 955 | ------------- | --------- | - | -------- | - |
| 956 | | | | | |
| 957 | ------------- ------------ |
| 958 | @end group |
| 959 | @end smallexample |
| 960 | @end defun |
| 961 | |
| 962 | @defun sort list predicate |
| 963 | @cindex stable sort |
| 964 | @cindex sorting lists |
| 965 | This function sorts @var{list} stably, though destructively, and |
| 966 | returns the sorted list. It compares elements using @var{predicate}. A |
| 967 | stable sort is one in which elements with equal sort keys maintain their |
| 968 | relative order before and after the sort. Stability is important when |
| 969 | successive sorts are used to order elements according to different |
| 970 | criteria. |
| 971 | |
| 972 | The argument @var{predicate} must be a function that accepts two |
| 973 | arguments. It is called with two elements of @var{list}. To get an |
| 974 | increasing order sort, the @var{predicate} should return @code{t} if the |
| 975 | first element is ``less than'' the second, or @code{nil} if not. |
| 976 | |
| 977 | The destructive aspect of @code{sort} is that it rearranges the cons |
| 978 | cells forming @var{list} by changing @sc{cdr}s. A nondestructive sort |
| 979 | function would create new cons cells to store the elements in their |
| 980 | sorted order. If you wish to make a sorted copy without destroying the |
| 981 | original, copy it first with @code{copy-sequence} and then sort. |
| 982 | |
| 983 | Sorting does not change the @sc{car}s of the cons cells in @var{list}; |
| 984 | the cons cell that originally contained the element @code{a} in |
| 985 | @var{list} still has @code{a} in its @sc{car} after sorting, but it now |
| 986 | appears in a different position in the list due to the change of |
| 987 | @sc{cdr}s. For example: |
| 988 | |
| 989 | @example |
| 990 | @group |
| 991 | (setq nums '(1 3 2 6 5 4 0)) |
| 992 | @result{} (1 3 2 6 5 4 0) |
| 993 | @end group |
| 994 | @group |
| 995 | (sort nums '<) |
| 996 | @result{} (0 1 2 3 4 5 6) |
| 997 | @end group |
| 998 | @group |
| 999 | nums |
| 1000 | @result{} (1 2 3 4 5 6) |
| 1001 | @end group |
| 1002 | @end example |
| 1003 | |
| 1004 | @noindent |
| 1005 | Note that the list in @code{nums} no longer contains 0; this is the same |
| 1006 | cons cell that it was before, but it is no longer the first one in the |
| 1007 | list. Don't assume a variable that formerly held the argument now holds |
| 1008 | the entire sorted list! Instead, save the result of @code{sort} and use |
| 1009 | that. Most often we store the result back into the variable that held |
| 1010 | the original list: |
| 1011 | |
| 1012 | @example |
| 1013 | (setq nums (sort nums '<)) |
| 1014 | @end example |
| 1015 | |
| 1016 | @xref{Sorting}, for more functions that perform sorting. |
| 1017 | See @code{documentation} in @ref{Accessing Documentation}, for a |
| 1018 | useful example of @code{sort}. |
| 1019 | @end defun |
| 1020 | |
| 1021 | @node Sets And Lists |
| 1022 | @section Using Lists as Sets |
| 1023 | @cindex lists as sets |
| 1024 | @cindex sets |
| 1025 | |
| 1026 | A list can represent an unordered mathematical set---simply consider a |
| 1027 | value an element of a set if it appears in the list, and ignore the |
| 1028 | order of the list. To form the union of two sets, use @code{append} (as |
| 1029 | long as you don't mind having duplicate elements). Other useful |
| 1030 | functions for sets include @code{memq} and @code{delq}, and their |
| 1031 | @code{equal} versions, @code{member} and @code{delete}. |
| 1032 | |
| 1033 | @cindex CL note---lack @code{union}, @code{intersection} |
| 1034 | @quotation |
| 1035 | @b{Common Lisp note:} Common Lisp has functions @code{union} (which |
| 1036 | avoids duplicate elements) and @code{intersection} for set operations, |
| 1037 | but GNU Emacs Lisp does not have them. You can write them in Lisp if |
| 1038 | you wish. |
| 1039 | @end quotation |
| 1040 | |
| 1041 | @defun memq object list |
| 1042 | @cindex membership in a list |
| 1043 | This function tests to see whether @var{object} is a member of |
| 1044 | @var{list}. If it is, @code{memq} returns a list starting with the |
| 1045 | first occurrence of @var{object}. Otherwise, it returns @code{nil}. |
| 1046 | The letter @samp{q} in @code{memq} says that it uses @code{eq} to |
| 1047 | compare @var{object} against the elements of the list. For example: |
| 1048 | |
| 1049 | @example |
| 1050 | @group |
| 1051 | (memq 'b '(a b c b a)) |
| 1052 | @result{} (b c b a) |
| 1053 | @end group |
| 1054 | @group |
| 1055 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} |
| 1056 | @result{} nil |
| 1057 | @end group |
| 1058 | @end example |
| 1059 | @end defun |
| 1060 | |
| 1061 | @defun delq object list |
| 1062 | @cindex deletion of elements |
| 1063 | This function destructively removes all elements @code{eq} to |
| 1064 | @var{object} from @var{list}. The letter @samp{q} in @code{delq} says |
| 1065 | that it uses @code{eq} to compare @var{object} against the elements of |
| 1066 | the list, like @code{memq}. |
| 1067 | @end defun |
| 1068 | |
| 1069 | When @code{delq} deletes elements from the front of the list, it does so |
| 1070 | simply by advancing down the list and returning a sublist that starts |
| 1071 | after those elements: |
| 1072 | |
| 1073 | @example |
| 1074 | @group |
| 1075 | (delq 'a '(a b c)) @equiv{} (cdr '(a b c)) |
| 1076 | @end group |
| 1077 | @end example |
| 1078 | |
| 1079 | When an element to be deleted appears in the middle of the list, |
| 1080 | removing it involves changing the @sc{cdr}s (@pxref{Setcdr}). |
| 1081 | |
| 1082 | @example |
| 1083 | @group |
| 1084 | (setq sample-list '(a b c (4))) |
| 1085 | @result{} (a b c (4)) |
| 1086 | @end group |
| 1087 | @group |
| 1088 | (delq 'a sample-list) |
| 1089 | @result{} (b c (4)) |
| 1090 | @end group |
| 1091 | @group |
| 1092 | sample-list |
| 1093 | @result{} (a b c (4)) |
| 1094 | @end group |
| 1095 | @group |
| 1096 | (delq 'c sample-list) |
| 1097 | @result{} (a b (4)) |
| 1098 | @end group |
| 1099 | @group |
| 1100 | sample-list |
| 1101 | @result{} (a b (4)) |
| 1102 | @end group |
| 1103 | @end example |
| 1104 | |
| 1105 | Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to |
| 1106 | splice out the third element, but @code{(delq 'a sample-list)} does not |
| 1107 | splice anything---it just returns a shorter list. Don't assume that a |
| 1108 | variable which formerly held the argument @var{list} now has fewer |
| 1109 | elements, or that it still holds the original list! Instead, save the |
| 1110 | result of @code{delq} and use that. Most often we store the result back |
| 1111 | into the variable that held the original list: |
| 1112 | |
| 1113 | @example |
| 1114 | (setq flowers (delq 'rose flowers)) |
| 1115 | @end example |
| 1116 | |
| 1117 | In the following example, the @code{(4)} that @code{delq} attempts to match |
| 1118 | and the @code{(4)} in the @code{sample-list} are not @code{eq}: |
| 1119 | |
| 1120 | @example |
| 1121 | @group |
| 1122 | (delq '(4) sample-list) |
| 1123 | @result{} (a c (4)) |
| 1124 | @end group |
| 1125 | @end example |
| 1126 | |
| 1127 | The following two functions are like @code{memq} and @code{delq} but use |
| 1128 | @code{equal} rather than @code{eq} to compare elements. They are new in |
| 1129 | Emacs 19. |
| 1130 | |
| 1131 | @defun member object list |
| 1132 | The function @code{member} tests to see whether @var{object} is a member |
| 1133 | of @var{list}, comparing members with @var{object} using @code{equal}. |
| 1134 | If @var{object} is a member, @code{member} returns a list starting with |
| 1135 | its first occurrence in @var{list}. Otherwise, it returns @code{nil}. |
| 1136 | |
| 1137 | Compare this with @code{memq}: |
| 1138 | |
| 1139 | @example |
| 1140 | @group |
| 1141 | (member '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are @code{equal}.} |
| 1142 | @result{} ((2)) |
| 1143 | @end group |
| 1144 | @group |
| 1145 | (memq '(2) '((1) (2))) ; @r{@code{(2)} and @code{(2)} are not @code{eq}.} |
| 1146 | @result{} nil |
| 1147 | @end group |
| 1148 | @group |
| 1149 | ;; @r{Two strings with the same contents are @code{equal}.} |
| 1150 | (member "foo" '("foo" "bar")) |
| 1151 | @result{} ("foo" "bar") |
| 1152 | @end group |
| 1153 | @end example |
| 1154 | @end defun |
| 1155 | |
| 1156 | @defun delete object list |
| 1157 | This function destructively removes all elements @code{equal} to |
| 1158 | @var{object} from @var{list}. It is to @code{delq} as @code{member} is |
| 1159 | to @code{memq}: it uses @code{equal} to compare elements with |
| 1160 | @var{object}, like @code{member}; when it finds an element that matches, |
| 1161 | it removes the element just as @code{delq} would. For example: |
| 1162 | |
| 1163 | @example |
| 1164 | @group |
| 1165 | (delete '(2) '((2) (1) (2))) |
| 1166 | @result{} ((1)) |
| 1167 | @end group |
| 1168 | @end example |
| 1169 | @end defun |
| 1170 | |
| 1171 | @quotation |
| 1172 | @b{Common Lisp note:} The functions @code{member} and @code{delete} in |
| 1173 | GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common |
| 1174 | Lisp versions do not use @code{equal} to compare elements. |
| 1175 | @end quotation |
| 1176 | |
| 1177 | See also the function @code{add-to-list}, in @ref{Setting Variables}, |
| 1178 | for another way to add an element to a list stored in a variable. |
| 1179 | |
| 1180 | @node Association Lists |
| 1181 | @section Association Lists |
| 1182 | @cindex association list |
| 1183 | @cindex alist |
| 1184 | |
| 1185 | An @dfn{association list}, or @dfn{alist} for short, records a mapping |
| 1186 | from keys to values. It is a list of cons cells called |
| 1187 | @dfn{associations}: the @sc{car} of each cell is the @dfn{key}, and the |
| 1188 | @sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key'' |
| 1189 | is not related to the term ``key sequence''; it means a value used to |
| 1190 | look up an item in a table. In this case, the table is the alist, and |
| 1191 | the alist associations are the items.} |
| 1192 | |
| 1193 | Here is an example of an alist. The key @code{pine} is associated with |
| 1194 | the value @code{cones}; the key @code{oak} is associated with |
| 1195 | @code{acorns}; and the key @code{maple} is associated with @code{seeds}. |
| 1196 | |
| 1197 | @example |
| 1198 | @group |
| 1199 | '((pine . cones) |
| 1200 | (oak . acorns) |
| 1201 | (maple . seeds)) |
| 1202 | @end group |
| 1203 | @end example |
| 1204 | |
| 1205 | The associated values in an alist may be any Lisp objects; so may the |
| 1206 | keys. For example, in the following alist, the symbol @code{a} is |
| 1207 | associated with the number @code{1}, and the string @code{"b"} is |
| 1208 | associated with the @emph{list} @code{(2 3)}, which is the @sc{cdr} of |
| 1209 | the alist element: |
| 1210 | |
| 1211 | @example |
| 1212 | ((a . 1) ("b" 2 3)) |
| 1213 | @end example |
| 1214 | |
| 1215 | Sometimes it is better to design an alist to store the associated |
| 1216 | value in the @sc{car} of the @sc{cdr} of the element. Here is an |
| 1217 | example: |
| 1218 | |
| 1219 | @example |
| 1220 | '((rose red) (lily white) (buttercup yellow)) |
| 1221 | @end example |
| 1222 | |
| 1223 | @noindent |
| 1224 | Here we regard @code{red} as the value associated with @code{rose}. One |
| 1225 | advantage of this method is that you can store other related |
| 1226 | information---even a list of other items---in the @sc{cdr} of the |
| 1227 | @sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see |
| 1228 | below) to find the element containing a given value. When neither of |
| 1229 | these considerations is important, the choice is a matter of taste, as |
| 1230 | long as you are consistent about it for any given alist. |
| 1231 | |
| 1232 | Note that the same alist shown above could be regarded as having the |
| 1233 | associated value in the @sc{cdr} of the element; the value associated |
| 1234 | with @code{rose} would be the list @code{(red)}. |
| 1235 | |
| 1236 | Association lists are often used to record information that you might |
| 1237 | otherwise keep on a stack, since new associations may be added easily to |
| 1238 | the front of the list. When searching an association list for an |
| 1239 | association with a given key, the first one found is returned, if there |
| 1240 | is more than one. |
| 1241 | |
| 1242 | In Emacs Lisp, it is @emph{not} an error if an element of an |
| 1243 | association list is not a cons cell. The alist search functions simply |
| 1244 | ignore such elements. Many other versions of Lisp signal errors in such |
| 1245 | cases. |
| 1246 | |
| 1247 | Note that property lists are similar to association lists in several |
| 1248 | respects. A property list behaves like an association list in which |
| 1249 | each key can occur only once. @xref{Property Lists}, for a comparison |
| 1250 | of property lists and association lists. |
| 1251 | |
| 1252 | @defun assoc key alist |
| 1253 | This function returns the first association for @var{key} in |
| 1254 | @var{alist}. It compares @var{key} against the alist elements using |
| 1255 | @code{equal} (@pxref{Equality Predicates}). It returns @code{nil} if no |
| 1256 | association in @var{alist} has a @sc{car} @code{equal} to @var{key}. |
| 1257 | For example: |
| 1258 | |
| 1259 | @smallexample |
| 1260 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) |
| 1261 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) |
| 1262 | (assoc 'oak trees) |
| 1263 | @result{} (oak . acorns) |
| 1264 | (cdr (assoc 'oak trees)) |
| 1265 | @result{} acorns |
| 1266 | (assoc 'birch trees) |
| 1267 | @result{} nil |
| 1268 | @end smallexample |
| 1269 | |
| 1270 | Here is another example, in which the keys and values are not symbols: |
| 1271 | |
| 1272 | @smallexample |
| 1273 | (setq needles-per-cluster |
| 1274 | '((2 "Austrian Pine" "Red Pine") |
| 1275 | (3 "Pitch Pine") |
| 1276 | (5 "White Pine"))) |
| 1277 | |
| 1278 | (cdr (assoc 3 needles-per-cluster)) |
| 1279 | @result{} ("Pitch Pine") |
| 1280 | (cdr (assoc 2 needles-per-cluster)) |
| 1281 | @result{} ("Austrian Pine" "Red Pine") |
| 1282 | @end smallexample |
| 1283 | @end defun |
| 1284 | |
| 1285 | @defun rassoc value alist |
| 1286 | This function returns the first association with value @var{value} in |
| 1287 | @var{alist}. It returns @code{nil} if no association in @var{alist} has |
| 1288 | a @sc{cdr} @code{equal} to @var{value}. |
| 1289 | |
| 1290 | @code{rassoc} is like @code{assoc} except that it compares the @sc{cdr} of |
| 1291 | each @var{alist} association instead of the @sc{car}. You can think of |
| 1292 | this as ``reverse @code{assoc}'', finding the key for a given value. |
| 1293 | @end defun |
| 1294 | |
| 1295 | @defun assq key alist |
| 1296 | This function is like @code{assoc} in that it returns the first |
| 1297 | association for @var{key} in @var{alist}, but it makes the comparison |
| 1298 | using @code{eq} instead of @code{equal}. @code{assq} returns @code{nil} |
| 1299 | if no association in @var{alist} has a @sc{car} @code{eq} to @var{key}. |
| 1300 | This function is used more often than @code{assoc}, since @code{eq} is |
| 1301 | faster than @code{equal} and most alists use symbols as keys. |
| 1302 | @xref{Equality Predicates}. |
| 1303 | |
| 1304 | @smallexample |
| 1305 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) |
| 1306 | @result{} ((pine . cones) (oak . acorns) (maple . seeds)) |
| 1307 | (assq 'pine trees) |
| 1308 | @result{} (pine . cones) |
| 1309 | @end smallexample |
| 1310 | |
| 1311 | On the other hand, @code{assq} is not usually useful in alists where the |
| 1312 | keys may not be symbols: |
| 1313 | |
| 1314 | @smallexample |
| 1315 | (setq leaves |
| 1316 | '(("simple leaves" . oak) |
| 1317 | ("compound leaves" . horsechestnut))) |
| 1318 | |
| 1319 | (assq "simple leaves" leaves) |
| 1320 | @result{} nil |
| 1321 | (assoc "simple leaves" leaves) |
| 1322 | @result{} ("simple leaves" . oak) |
| 1323 | @end smallexample |
| 1324 | @end defun |
| 1325 | |
| 1326 | @defun rassq value alist |
| 1327 | This function returns the first association with value @var{value} in |
| 1328 | @var{alist}. It returns @code{nil} if no association in @var{alist} has |
| 1329 | a @sc{cdr} @code{eq} to @var{value}. |
| 1330 | |
| 1331 | @code{rassq} is like @code{assq} except that it compares the @sc{cdr} of |
| 1332 | each @var{alist} association instead of the @sc{car}. You can think of |
| 1333 | this as ``reverse @code{assq}'', finding the key for a given value. |
| 1334 | |
| 1335 | For example: |
| 1336 | |
| 1337 | @smallexample |
| 1338 | (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) |
| 1339 | |
| 1340 | (rassq 'acorns trees) |
| 1341 | @result{} (oak . acorns) |
| 1342 | (rassq 'spores trees) |
| 1343 | @result{} nil |
| 1344 | @end smallexample |
| 1345 | |
| 1346 | Note that @code{rassq} cannot search for a value stored in the @sc{car} |
| 1347 | of the @sc{cdr} of an element: |
| 1348 | |
| 1349 | @smallexample |
| 1350 | (setq colors '((rose red) (lily white) (buttercup yellow))) |
| 1351 | |
| 1352 | (rassq 'white colors) |
| 1353 | @result{} nil |
| 1354 | @end smallexample |
| 1355 | |
| 1356 | In this case, the @sc{cdr} of the association @code{(lily white)} is not |
| 1357 | the symbol @code{white}, but rather the list @code{(white)}. This |
| 1358 | becomes clearer if the association is written in dotted pair notation: |
| 1359 | |
| 1360 | @smallexample |
| 1361 | (lily white) @equiv{} (lily . (white)) |
| 1362 | @end smallexample |
| 1363 | @end defun |
| 1364 | |
| 1365 | @defun copy-alist alist |
| 1366 | @cindex copying alists |
| 1367 | This function returns a two-level deep copy of @var{alist}: it creates a |
| 1368 | new copy of each association, so that you can alter the associations of |
| 1369 | the new alist without changing the old one. |
| 1370 | |
| 1371 | @smallexample |
| 1372 | @group |
| 1373 | (setq needles-per-cluster |
| 1374 | '((2 . ("Austrian Pine" "Red Pine")) |
| 1375 | (3 . ("Pitch Pine")) |
| 1376 | @end group |
| 1377 | (5 . ("White Pine")))) |
| 1378 | @result{} |
| 1379 | ((2 "Austrian Pine" "Red Pine") |
| 1380 | (3 "Pitch Pine") |
| 1381 | (5 "White Pine")) |
| 1382 | |
| 1383 | (setq copy (copy-alist needles-per-cluster)) |
| 1384 | @result{} |
| 1385 | ((2 "Austrian Pine" "Red Pine") |
| 1386 | (3 "Pitch Pine") |
| 1387 | (5 "White Pine")) |
| 1388 | |
| 1389 | (eq needles-per-cluster copy) |
| 1390 | @result{} nil |
| 1391 | (equal needles-per-cluster copy) |
| 1392 | @result{} t |
| 1393 | (eq (car needles-per-cluster) (car copy)) |
| 1394 | @result{} nil |
| 1395 | (cdr (car (cdr needles-per-cluster))) |
| 1396 | @result{} ("Pitch Pine") |
| 1397 | @group |
| 1398 | (eq (cdr (car (cdr needles-per-cluster))) |
| 1399 | (cdr (car (cdr copy)))) |
| 1400 | @result{} t |
| 1401 | @end group |
| 1402 | @end smallexample |
| 1403 | |
| 1404 | This example shows how @code{copy-alist} makes it possible to change |
| 1405 | the associations of one copy without affecting the other: |
| 1406 | |
| 1407 | @smallexample |
| 1408 | @group |
| 1409 | (setcdr (assq 3 copy) '("Martian Vacuum Pine")) |
| 1410 | (cdr (assq 3 needles-per-cluster)) |
| 1411 | @result{} ("Pitch Pine") |
| 1412 | @end group |
| 1413 | @end smallexample |
| 1414 | @end defun |
| 1415 | |
| 1416 | |