Mercurial > octave-antonio
annotate doc/interpreter/var.txi @ 7984:bbaa5d7d0143
Some documentation updates
| author | David Bateman <dbateman@free.fr> |
|---|---|
| date | Mon, 28 Jul 2008 15:47:40 +0200 |
| parents | 745a8299c2b5 |
| children | 83c59e3f3106 |
| rev | line source |
|---|---|
| 7018 | 1 @c Copyright (C) 1996, 1997, 1999, 2000, 2002, 2003, 2004, 2005, |
| 2 @c 2006, 2007 John W. Eaton | |
| 3 @c | |
| 4 @c This file is part of Octave. | |
| 5 @c | |
| 6 @c Octave is free software; you can redistribute it and/or modify it | |
| 7 @c under the terms of the GNU General Public License as published by the | |
| 8 @c Free Software Foundation; either version 3 of the License, or (at | |
| 9 @c your option) any later version. | |
| 10 @c | |
| 11 @c Octave is distributed in the hope that it will be useful, but WITHOUT | |
| 12 @c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
| 13 @c FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
| 14 @c for more details. | |
| 15 @c | |
| 16 @c You should have received a copy of the GNU General Public License | |
| 17 @c along with Octave; see the file COPYING. If not, see | |
| 18 @c <http://www.gnu.org/licenses/>. | |
| 3294 | 19 |
| 4167 | 20 @node Variables |
| 3294 | 21 @chapter Variables |
| 22 @cindex variables, user-defined | |
| 23 @cindex user-defined variables | |
| 24 | |
| 25 Variables let you give names to values and refer to them later. You have | |
| 26 already seen variables in many of the examples. The name of a variable | |
| 27 must be a sequence of letters, digits and underscores, but it may not begin | |
| 28 with a digit. Octave does not enforce a limit on the length of variable | |
| 29 names, but it is seldom useful to have variables with names longer than | |
| 30 about 30 characters. The following are all valid variable names | |
| 31 | |
| 32 @cindex job hunting | |
| 33 @cindex getting a good job | |
| 34 @cindex flying high and fast | |
| 35 @example | |
| 36 @group | |
| 37 x | |
| 38 x15 | |
| 39 __foo_bar_baz__ | |
| 40 fucnrdthsucngtagdjb | |
| 41 @end group | |
| 42 @end example | |
| 43 | |
| 44 @noindent | |
| 45 However, names like @code{__foo_bar_baz__} that begin and end with two | |
| 46 underscores are understood to be reserved for internal use by Octave. | |
| 47 You should not use them in code you write, except to access Octave's | |
| 48 documented internal variables and built-in symbolic constants. | |
| 49 | |
| 50 Case is significant in variable names. The symbols @code{a} and | |
| 51 @code{A} are distinct variables. | |
| 52 | |
| 53 A variable name is a valid expression by itself. It represents the | |
| 54 variable's current value. Variables are given new values with | |
| 55 @dfn{assignment operators} and @dfn{increment operators}. | |
| 56 @xref{Assignment Ops, ,Assignment Expressions}. | |
| 57 | |
| 58 A number of variables have special built-in meanings. For example, | |
| 59 @code{ans} holds the current working directory, and @code{pi} names the | |
| 60 ratio of the circumference of a circle to its diameter. @xref{Summary of | |
| 61 Built-in Variables}, for a list of all the predefined variables. Some | |
| 62 of these built-in symbols are constants and may not be changed. Others | |
| 63 can be used and assigned just like all other variables, but their values | |
| 64 are also used or changed automatically by Octave. | |
| 65 | |
| 66 Variables in Octave do not have fixed types, so it is possible to first | |
| 67 store a numeric value in a variable and then to later use the same name | |
| 68 to hold a string value in the same program. Variables may not be used | |
| 69 before they have been given a value. Doing so results in an error. | |
| 70 | |
| 6550 | 71 @DOCSTRING(isvarname) |
| 72 | |
|
7984
bbaa5d7d0143
Some documentation updates
David Bateman <dbateman@free.fr>
parents:
7336
diff
changeset
|
73 @DOCSTRING(genvarname) |
|
bbaa5d7d0143
Some documentation updates
David Bateman <dbateman@free.fr>
parents:
7336
diff
changeset
|
74 |
|
bbaa5d7d0143
Some documentation updates
David Bateman <dbateman@free.fr>
parents:
7336
diff
changeset
|
75 @DOCSTRING(namelengthmax) |
|
bbaa5d7d0143
Some documentation updates
David Bateman <dbateman@free.fr>
parents:
7336
diff
changeset
|
76 |
| 3294 | 77 @menu |
| 78 * Global Variables:: | |
| 4686 | 79 * Persistent Variables:: |
| 3294 | 80 * Status of Variables:: |
| 81 * Summary of Built-in Variables:: | |
| 82 * Defaults from the Environment:: | |
| 83 @end menu | |
| 84 | |
| 4167 | 85 @node Global Variables |
| 3294 | 86 @section Global Variables |
| 87 @cindex global variables | |
| 88 @cindex @code{global} statement | |
| 89 @cindex variables, global | |
| 90 | |
| 91 A variable that has been declared @dfn{global} may be accessed from | |
| 92 within a function body without having to pass it as a formal parameter. | |
| 93 | |
| 94 A variable may be declared global using a @code{global} declaration | |
| 95 statement. The following statements are all global declarations. | |
| 96 | |
| 97 @example | |
| 98 @group | |
| 99 global a | |
| 4504 | 100 global a b |
| 101 global c = 2 | |
| 102 global d = 3 e f = 5 | |
| 3294 | 103 @end group |
| 104 @end example | |
| 105 | |
| 4504 | 106 A global variable may only be initialized once in a @code{global} |
| 107 statement. For example, after executing the following code | |
| 108 | |
| 109 @example | |
| 110 @group | |
| 111 global gvar = 1 | |
| 112 global gvar = 2 | |
| 113 @end group | |
| 114 @end example | |
| 115 | |
| 116 @noindent | |
| 6077 | 117 the value of the global variable @code{gvar} is 1, not 2. Issuing a |
| 6623 | 118 @samp{clear gvar} command does not change the above behavior, but |
| 6077 | 119 @samp{clear all} does. |
| 4504 | 120 |
| 3294 | 121 It is necessary declare a variable as global within a function body in |
| 122 order to access it. For example, | |
| 123 | |
| 124 @example | |
| 125 @group | |
| 126 global x | |
| 127 function f () | |
| 128 x = 1; | |
| 129 endfunction | |
| 130 f () | |
| 131 @end group | |
| 132 @end example | |
| 133 | |
| 134 @noindent | |
| 135 does @emph{not} set the value of the global variable @code{x} to 1. In | |
| 136 order to change the value of the global variable @code{x}, you must also | |
| 137 declare it to be global within the function body, like this | |
| 138 | |
| 139 @example | |
| 140 @group | |
| 141 function f () | |
| 142 global x; | |
| 143 x = 1; | |
| 144 endfunction | |
| 145 @end group | |
| 146 @end example | |
| 147 | |
| 148 Passing a global variable in a function parameter list will | |
| 149 make a local copy and not modify the global value. For example, given | |
| 150 the function | |
| 151 | |
| 152 @example | |
| 153 @group | |
| 154 function f (x) | |
| 155 x = 0 | |
| 156 endfunction | |
| 157 @end group | |
| 158 @end example | |
| 159 | |
| 160 @noindent | |
| 161 and the definition of @code{x} as a global variable at the top level, | |
| 162 | |
| 163 @example | |
| 164 global x = 13 | |
| 165 @end example | |
| 166 | |
| 167 @noindent | |
| 168 the expression | |
| 169 | |
| 170 @example | |
| 171 f (x) | |
| 172 @end example | |
| 173 | |
| 174 @noindent | |
| 175 will display the value of @code{x} from inside the function as 0, | |
| 176 but the value of @code{x} at the top level remains unchanged, because | |
| 177 the function works with a @emph{copy} of its argument. | |
| 178 | |
| 4029 | 179 @DOCSTRING(isglobal) |
| 3294 | 180 |
| 4686 | 181 @node Persistent Variables |
| 182 @section Persistent Variables | |
| 183 @cindex persistent variables | |
| 184 @cindex @code{persistent} statement | |
| 185 @cindex variables, persistent | |
| 186 | |
| 187 A variable that has been declared @dfn{persistent} within a function | |
| 188 will retain its contents in memory between subsequent calls to the | |
| 189 same function. The difference between persistent variables and global | |
| 190 variables is that persistent variables are local in scope to a | |
| 191 particular function and are not visible elsewhere. | |
| 192 | |
| 6899 | 193 The following example uses a persistent variable to create a function |
| 194 that prints the number of times it has been called. | |
| 195 | |
| 196 @example | |
| 197 @group | |
| 198 function count_calls () | |
| 199 persistent calls = 0; | |
| 7031 | 200 printf ("'count_calls' has been called %d times\n", |
| 201 ++calls); | |
| 6899 | 202 endfunction |
| 203 | |
| 204 for i = 1:3 | |
| 205 count_calls (); | |
| 206 endfor | |
| 207 | |
| 208 @print{} 'count_calls' has been called 1 times | |
| 209 @print{} 'count_calls' has been called 2 times | |
| 210 @print{} 'count_calls' has been called 3 times | |
| 211 @end group | |
| 212 @end example | |
| 213 | |
| 214 As the example shows, a variable may be declared persistent using a | |
| 215 @code{persistent} declaration statement. The following statements are | |
| 216 all persistent declarations. | |
| 4686 | 217 |
| 218 @example | |
| 219 @group | |
| 220 persistent a | |
| 221 persistent a b | |
| 222 persistent c = 2 | |
| 223 persistent d = 3 e f = 5 | |
| 224 @end group | |
| 225 @end example | |
| 226 | |
| 227 The behavior of persistent variables is equivalent to the behavior of | |
| 228 static variables in C. The command @code{static} in octave is also | |
| 6899 | 229 recognized and is equivalent to @code{persistent}. |
| 230 | |
| 231 Like global variables, a persistent variable may only be initialized once. | |
| 6896 | 232 For example, after executing the following code |
| 4686 | 233 |
| 234 @example | |
| 235 @group | |
| 236 persistent pvar = 1 | |
| 237 persistent pvar = 2 | |
| 238 @end group | |
| 239 @end example | |
| 240 | |
| 241 @noindent | |
| 6896 | 242 the value of the persistent variable @code{pvar} is 1, not 2. |
| 4686 | 243 |
| 6899 | 244 If a persistent variable is declared but not initialized to a specific |
| 245 value, it will contain an empty matrix. So, it is also possible to | |
| 246 initialize a persistent variable by checking whether it is empty, as the | |
| 247 following example illustrates. | |
| 248 | |
| 249 @example | |
| 250 @group | |
| 251 function count_calls () | |
| 252 persistent calls; | |
| 253 if (isempty (calls)) | |
| 254 calls = 0; | |
| 255 endif | |
| 7031 | 256 printf ("'count_calls' has been called %d times\n", |
| 257 ++calls); | |
| 6899 | 258 endfunction |
| 259 @end group | |
| 260 @end example | |
| 261 | |
| 262 @noindent | |
| 263 This implementation behaves in exactly the same way as the previous | |
| 264 implementation of @code{count_calls}. | |
| 265 | |
| 266 The value of a persistent variable is kept in memory until it is | |
| 267 explicitly cleared. Assuming that the implementation of @code{count_calls} | |
| 268 is saved on disc, we get the following behaviour. | |
| 269 | |
| 270 @example | |
| 271 @group | |
| 272 for i = 1:2 | |
| 273 count_calls (); | |
| 274 endfor | |
| 275 @print{} 'count_calls' has been called 1 times | |
| 276 @print{} 'count_calls' has been called 2 times | |
| 277 | |
| 278 clear | |
| 279 for i = 1:2 | |
| 280 count_calls(); | |
| 281 endfor | |
| 282 @print{} 'count_calls' has been called 3 times | |
| 283 @print{} 'count_calls' has been called 4 times | |
| 284 | |
| 285 clear all | |
| 286 for i = 1:2 | |
| 287 count_calls(); | |
| 288 endfor | |
| 289 @print{} 'count_calls' has been called 1 times | |
| 290 @print{} 'count_calls' has been called 2 times | |
| 291 | |
| 292 clear count_calls | |
| 293 for i = 1:2 | |
| 294 count_calls(); | |
| 295 endfor | |
| 296 @print{} 'count_calls' has been called 1 times | |
| 297 @print{} 'count_calls' has been called 2 times | |
| 298 @end group | |
| 299 @end example | |
| 300 | |
| 301 @noindent | |
| 302 That is, the persistent variable is only removed from memory when the | |
| 303 function containing the variable is removed. Note that if the function | |
| 304 definition is typed directly into the Octave prompt, the persistent | |
| 305 variable will be cleared by a simple @code{clear} command as the entire | |
| 306 function definition will be removed from memory. If you do not want | |
| 307 a persistent variable to be removed from memory even if the function is | |
| 308 cleared, you should use the @code{mlock} function as described in | |
| 309 @xref{Function Locking}. | |
| 310 | |
| 4167 | 311 @node Status of Variables |
| 3294 | 312 @section Status of Variables |
| 313 | |
| 6623 | 314 When creating simple one-shot programs it can be very convenient to |
| 315 see which variables are available at the prompt. The function @code{who} | |
| 316 and its siblings @code{whos} and @code{whos_line_format} will show | |
| 317 different information about what is in memory, as the following shows. | |
| 318 | |
| 319 @example | |
| 320 str = "A random string"; | |
| 321 who -variables | |
| 322 @print{} *** local user variables: | |
| 323 @print{} | |
| 324 @print{} __nargin__ str | |
| 325 @end example | |
| 3294 | 326 |
| 3361 | 327 @DOCSTRING(who) |
| 3294 | 328 |
| 4913 | 329 @DOCSTRING(whos) |
| 330 | |
| 331 @DOCSTRING(whos_line_format) | |
| 332 | |
| 6623 | 333 Instead of displaying which variables are in memory, it is possible |
| 334 to determine if a given variable is available. That way it is possible | |
| 335 to alter the behaviour of a program depending on the existence of a | |
| 336 variable. The following example illustrates this. | |
| 337 | |
| 338 @example | |
| 339 if (! exist ("meaning", "var")) | |
| 340 disp ("The program has no 'meaning'"); | |
| 341 endif | |
| 342 @end example | |
| 343 | |
| 3361 | 344 @DOCSTRING(exist) |
| 3294 | 345 |
| 6623 | 346 Usually Octave will manage the memory, but sometimes it can be practical |
| 347 to remove variables from memory manually. This is usually needed when | |
| 348 working with large variables that fill a substantial part of the memory. | |
| 349 On a computer that uses the IEEE floating point format, the following | |
| 350 program allocates a matrix that requires around 128 MB memory. | |
| 351 | |
| 352 @example | |
| 353 large_matrix = zeros (4000, 4000); | |
| 354 @end example | |
| 355 | |
| 356 @noindent | |
| 357 Since having this variable in memory might slow down other computations, | |
| 358 it can be necessary to remove it manually from memory. The @code{clear} | |
| 359 function allows this. | |
| 360 | |
| 361 @DOCSTRING(clear) | |
| 362 | |
| 363 Information about a function or variable such as it's location in the | |
| 364 file system can also be acquired from within Octave. This is usually | |
| 365 only useful during development of programs, and not within a program. | |
| 366 | |
| 3361 | 367 @DOCSTRING(document) |
| 3294 | 368 |
| 3361 | 369 @DOCSTRING(type) |
| 3294 | 370 |
| 3361 | 371 @DOCSTRING(which) |
| 3294 | 372 |
| 4167 | 373 @node Summary of Built-in Variables |
| 3294 | 374 @section Summary of Built-in Variables |
| 375 | |
| 376 Here is a summary of all of Octave's built-in variables along with | |
| 377 cross references to additional information and their default values. In | |
| 378 the following table @var{octave-home} stands for the root directory | |
| 379 where all of Octave is installed (the default is @file{@value{OCTAVEHOME}}, | |
| 380 @var{version} stands for the Octave version number (for example, | |
| 381 @value{VERSION}) and @var{arch} stands for the type of system for which | |
| 5942 | 382 Octave was compiled (for example, @code{x86_64-unknown-linux-gnu}). |
| 3294 | 383 |
| 384 @vtable @code | |
| 385 @item EDITOR | |
| 386 @xref{Commands For History}. | |
| 387 | |
| 388 Default value: @code{"emacs"}. | |
| 389 | |
| 390 @item EXEC_PATH | |
| 391 @xref{Controlling Subprocesses}. | |
| 392 | |
| 393 Default value: @code{":$PATH"}. | |
| 394 | |
| 395 @item OCTAVE_HOME | |
| 396 | |
| 397 Default value: @code{"@value{OCTAVEHOME}"}. | |
| 398 | |
| 399 @item PAGER | |
| 400 @xref{Input and Output}. | |
| 401 | |
| 402 Default value: @code{"less", or "more"}. | |
| 403 | |
| 404 @item PS1 | |
| 405 @xref{Customizing the Prompt}. | |
| 406 | |
| 407 Default value: @code{"\s:\#> "}. | |
| 408 | |
| 409 @item PS2 | |
| 410 @xref{Customizing the Prompt}. | |
| 411 | |
| 412 Default value: @code{"> "}. | |
| 413 | |
| 414 @item PS4 | |
| 415 @xref{Customizing the Prompt}. | |
| 416 | |
| 417 Default value: @code{"+ "}. | |
| 418 | |
| 419 @item beep_on_error | |
| 6667 | 420 @xref{Errors and Warnings}. |
| 3294 | 421 |
| 422 Default value: 0. | |
| 423 | |
| 424 @item completion_append_char | |
| 425 @xref{Commands For Completion}. | |
| 426 | |
| 427 Default value: @code{" "}. | |
| 428 | |
| 5287 | 429 @item default_save_options |
| 3294 | 430 @xref{Simple File I/O}. |
| 431 | |
| 432 Default value: @code{"ascii"}. | |
| 433 | |
| 434 @item crash_dumps_octave_core | |
| 435 @xref{Simple File I/O}. | |
| 436 | |
| 437 Default value: 1. | |
| 438 | |
| 439 @item fixed_point_format | |
| 440 @xref{Matrices}. | |
| 441 | |
| 442 Default value: 0. | |
| 443 | |
| 444 @item gnuplot_binary | |
| 445 @xref{Three-Dimensional Plotting}. | |
| 446 | |
| 447 Default value: @code{"gnuplot"}. | |
| 448 | |
| 449 @item history_file | |
| 450 @xref{Commands For History}. | |
| 451 | |
| 452 Default value: @code{"~/.octave_hist"}. | |
| 453 | |
| 454 @item history_size | |
| 455 @xref{Commands For History}. | |
| 456 | |
| 457 Default value: 1024. | |
| 458 | |
| 459 @item ignore_function_time_stamp | |
| 460 @xref{Function Files}. | |
| 461 | |
| 462 Default value: @code{"system"}. | |
| 463 | |
| 464 @item max_recursion_depth | |
| 465 @xref{Recursion}. | |
| 466 | |
| 467 Default value: 256. | |
| 468 | |
| 469 @item output_max_field_width | |
| 470 @xref{Matrices}. | |
| 471 | |
| 472 Default value: 10. | |
| 473 | |
| 474 @item output_precision | |
| 475 @xref{Matrices}. | |
| 476 | |
| 477 Default value: 5. | |
| 478 | |
| 479 @item page_screen_output | |
| 480 @xref{Input and Output}. | |
| 481 | |
| 482 Default value: 1. | |
| 483 | |
| 484 @item print_empty_dimensions | |
| 485 @xref{Empty Matrices}. | |
| 486 | |
| 487 Default value: 1. | |
| 488 | |
| 489 @item return_last_computed_value | |
| 490 @xref{Returning From a Function}. | |
| 491 | |
| 492 Default value: 0. | |
| 493 | |
| 494 @item save_precision | |
| 495 @xref{Simple File I/O}. | |
| 496 | |
| 497 Default value: 17. | |
| 498 | |
| 499 @item saving_history | |
| 500 @xref{Commands For History}. | |
| 501 | |
| 502 Default value: 1. | |
| 503 | |
| 4449 | 504 @item sighup_dumps_octave_core |
| 505 @xref{Simple File I/O}. | |
| 506 | |
| 507 Default value: 1. | |
| 508 | |
| 509 @item sigterm_dumps_octave_core | |
| 510 @xref{Simple File I/O}. | |
| 511 | |
| 512 Default value: 1. | |
| 513 | |
| 3294 | 514 @item silent_functions |
| 515 @xref{Defining Functions}. | |
| 516 | |
| 517 Default value: 0. | |
| 518 | |
| 519 @item split_long_rows | |
| 520 @xref{Matrices}. | |
| 521 | |
| 522 Default value: 1. | |
| 523 | |
| 524 @item struct_levels_to_print | |
| 525 @xref{Data Structures}. | |
| 526 | |
| 527 Default value: 2. | |
| 528 | |
| 529 @item suppress_verbose_help_message | |
| 530 @xref{Getting Help}. | |
| 531 | |
| 532 Default value: 1. | |
| 533 @end vtable | |
| 534 | |
| 535 | |
| 4167 | 536 @node Defaults from the Environment |
| 3294 | 537 @section Defaults from the Environment |
| 538 | |
| 539 Octave uses the values of the following environment variables to set the | |
| 6477 | 540 default values for the corresponding built-in or internal variables. |
| 541 In addition, the values from the environment may be overridden by | |
| 542 command-line arguments. @xref{Command Line Options}. | |
| 3294 | 543 |
| 544 @vtable @code | |
| 545 @item EDITOR | |
| 546 @xref{Commands For History}. | |
| 547 | |
| 548 Built-in variable: @code{EDITOR}. | |
| 549 | |
| 550 @item OCTAVE_EXEC_PATH | |
| 551 @xref{Controlling Subprocesses}. | |
| 552 | |
| 553 Built-in variable: @code{EXEC_PATH}. | |
| 554 Command-line argument: @code{--exec-path}. | |
| 555 | |
| 556 @item OCTAVE_PATH | |
| 557 @xref{Function Files}. | |
| 558 | |
| 6477 | 559 Internal variable changed by function @code{path}. |
| 3294 | 560 Command-line argument: @code{--path}. |
| 561 | |
| 562 @item OCTAVE_INFO_FILE | |
| 563 @xref{Getting Help}. | |
| 564 | |
| 6477 | 565 Internal variable changed by function @code{info_file}. |
| 3294 | 566 Command-line argument: @code{--info-file}. |
| 567 | |
| 568 @item OCTAVE_INFO_PROGRAM | |
| 569 @xref{Getting Help}. | |
| 570 | |
| 6477 | 571 Internal variable changed by function @code{info_program}. |
| 3294 | 572 Command-line argument: @code{--info-program}. |
| 573 | |
| 574 @item OCTAVE_HISTSIZE | |
| 575 @xref{Commands For History}. | |
| 576 | |
| 577 Built-in variable: @code{history_size}. | |
| 578 | |
| 579 @item OCTAVE_HISTFILE | |
| 580 @xref{Commands For History}. | |
| 581 | |
| 582 Built-in variable: @code{history_file}. | |
| 583 @end vtable |