Mercurial > octave-nkf
annotate doc/interpreter/var.txi @ 19628:fe689210525c gui-release
maint: Periodic merge of stable to gui-release.
| author | John W. Eaton <jwe@octave.org> |
|---|---|
| date | Tue, 20 Jan 2015 10:05:42 -0500 |
| parents | e0775b4f41dd 446c46af4b42 |
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1 @c Copyright (C) 1996-2013 John W. Eaton |
| 7018 | 2 @c |
| 3 @c This file is part of Octave. | |
| 4 @c | |
| 5 @c Octave is free software; you can redistribute it and/or modify it | |
| 6 @c under the terms of the GNU General Public License as published by the | |
| 7 @c Free Software Foundation; either version 3 of the License, or (at | |
| 8 @c your option) any later version. | |
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9 @c |
| 7018 | 10 @c Octave is distributed in the hope that it will be useful, but WITHOUT |
| 11 @c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
| 12 @c FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
| 13 @c for more details. | |
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14 @c |
| 7018 | 15 @c You should have received a copy of the GNU General Public License |
| 16 @c along with Octave; see the file COPYING. If not, see | |
| 17 @c <http://www.gnu.org/licenses/>. | |
| 3294 | 18 |
| 4167 | 19 @node Variables |
| 3294 | 20 @chapter Variables |
| 21 @cindex variables, user-defined | |
| 22 @cindex user-defined variables | |
| 23 | |
| 24 Variables let you give names to values and refer to them later. You have | |
| 25 already seen variables in many of the examples. The name of a variable | |
| 26 must be a sequence of letters, digits and underscores, but it may not begin | |
| 27 with a digit. Octave does not enforce a limit on the length of variable | |
| 28 names, but it is seldom useful to have variables with names longer than | |
| 29 about 30 characters. The following are all valid variable names | |
| 30 | |
| 31 @example | |
| 32 @group | |
| 33 x | |
| 34 x15 | |
| 35 __foo_bar_baz__ | |
| 36 fucnrdthsucngtagdjb | |
| 37 @end group | |
| 38 @end example | |
| 39 | |
| 40 @noindent | |
| 41 However, names like @code{__foo_bar_baz__} that begin and end with two | |
| 42 underscores are understood to be reserved for internal use by Octave. | |
| 43 You should not use them in code you write, except to access Octave's | |
| 44 documented internal variables and built-in symbolic constants. | |
| 45 | |
| 46 Case is significant in variable names. The symbols @code{a} and | |
| 47 @code{A} are distinct variables. | |
| 48 | |
| 49 A variable name is a valid expression by itself. It represents the | |
| 50 variable's current value. Variables are given new values with | |
| 51 @dfn{assignment operators} and @dfn{increment operators}. | |
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52 @xref{Assignment Ops,,Assignment Expressions}. |
| 3294 | 53 |
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54 There is one built-in variable with a special meaning. The @code{ans} variable |
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55 always contains the result of the last computation, where the output wasn't |
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56 assigned to any variable. The code @code{a = cos (pi)} will assign the value -1 |
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57 to the variable @code{a}, but will not change the value of @code{ans}. However, |
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58 the code @code{cos (pi)} will set the value of @code{ans} to -1. |
| 3294 | 59 |
| 60 Variables in Octave do not have fixed types, so it is possible to first | |
| 61 store a numeric value in a variable and then to later use the same name | |
| 62 to hold a string value in the same program. Variables may not be used | |
| 63 before they have been given a value. Doing so results in an error. | |
| 64 | |
| 8567 | 65 @cindex @code{ans} |
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66 @DOCSTRING(ans) |
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67 |
| 6550 | 68 @DOCSTRING(isvarname) |
| 69 | |
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70 @DOCSTRING(genvarname) |
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71 |
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72 @DOCSTRING(namelengthmax) |
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73 |
| 3294 | 74 @menu |
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75 * Global Variables:: |
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76 * Persistent Variables:: |
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77 * Status of Variables:: |
| 3294 | 78 @end menu |
| 79 | |
| 4167 | 80 @node Global Variables |
| 3294 | 81 @section Global Variables |
| 82 @cindex global variables | |
| 83 @cindex @code{global} statement | |
| 84 @cindex variables, global | |
| 85 | |
| 86 A variable that has been declared @dfn{global} may be accessed from | |
| 87 within a function body without having to pass it as a formal parameter. | |
| 88 | |
| 89 A variable may be declared global using a @code{global} declaration | |
| 90 statement. The following statements are all global declarations. | |
| 91 | |
| 92 @example | |
| 93 @group | |
| 94 global a | |
| 4504 | 95 global a b |
| 96 global c = 2 | |
| 97 global d = 3 e f = 5 | |
| 3294 | 98 @end group |
| 99 @end example | |
| 100 | |
| 4504 | 101 A global variable may only be initialized once in a @code{global} |
| 102 statement. For example, after executing the following code | |
| 103 | |
| 104 @example | |
| 105 @group | |
| 106 global gvar = 1 | |
| 107 global gvar = 2 | |
| 108 @end group | |
| 109 @end example | |
| 110 | |
| 111 @noindent | |
| 6077 | 112 the value of the global variable @code{gvar} is 1, not 2. Issuing a |
| 6623 | 113 @samp{clear gvar} command does not change the above behavior, but |
| 6077 | 114 @samp{clear all} does. |
| 4504 | 115 |
| 3294 | 116 It is necessary declare a variable as global within a function body in |
| 117 order to access it. For example, | |
| 118 | |
| 119 @example | |
| 120 @group | |
| 121 global x | |
| 122 function f () | |
| 123 x = 1; | |
| 124 endfunction | |
| 125 f () | |
| 126 @end group | |
| 127 @end example | |
| 128 | |
| 129 @noindent | |
| 130 does @emph{not} set the value of the global variable @code{x} to 1. In | |
| 131 order to change the value of the global variable @code{x}, you must also | |
| 132 declare it to be global within the function body, like this | |
| 133 | |
| 134 @example | |
| 135 @group | |
| 136 function f () | |
| 137 global x; | |
| 138 x = 1; | |
| 139 endfunction | |
| 140 @end group | |
| 141 @end example | |
| 142 | |
| 143 Passing a global variable in a function parameter list will | |
| 144 make a local copy and not modify the global value. For example, given | |
| 145 the function | |
| 146 | |
| 147 @example | |
| 148 @group | |
| 149 function f (x) | |
| 150 x = 0 | |
| 151 endfunction | |
| 152 @end group | |
| 153 @end example | |
| 154 | |
| 155 @noindent | |
| 156 and the definition of @code{x} as a global variable at the top level, | |
| 157 | |
| 158 @example | |
| 159 global x = 13 | |
| 160 @end example | |
| 161 | |
| 162 @noindent | |
| 163 the expression | |
| 164 | |
| 165 @example | |
| 166 f (x) | |
| 167 @end example | |
| 168 | |
| 169 @noindent | |
| 170 will display the value of @code{x} from inside the function as 0, | |
| 171 but the value of @code{x} at the top level remains unchanged, because | |
| 172 the function works with a @emph{copy} of its argument. | |
| 173 | |
| 4029 | 174 @DOCSTRING(isglobal) |
| 3294 | 175 |
| 4686 | 176 @node Persistent Variables |
| 177 @section Persistent Variables | |
| 178 @cindex persistent variables | |
| 179 @cindex @code{persistent} statement | |
| 180 @cindex variables, persistent | |
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181 @anchor{XREFpersistent} |
| 4686 | 182 |
| 183 A variable that has been declared @dfn{persistent} within a function | |
| 184 will retain its contents in memory between subsequent calls to the | |
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185 same function. The difference between persistent variables and global |
| 4686 | 186 variables is that persistent variables are local in scope to a |
| 187 particular function and are not visible elsewhere. | |
| 188 | |
| 6899 | 189 The following example uses a persistent variable to create a function |
| 190 that prints the number of times it has been called. | |
| 191 | |
| 192 @example | |
| 193 @group | |
| 194 function count_calls () | |
| 195 persistent calls = 0; | |
| 7031 | 196 printf ("'count_calls' has been called %d times\n", |
| 197 ++calls); | |
| 6899 | 198 endfunction |
| 199 | |
| 200 for i = 1:3 | |
| 201 count_calls (); | |
| 202 endfor | |
| 203 | |
| 204 @print{} 'count_calls' has been called 1 times | |
| 205 @print{} 'count_calls' has been called 2 times | |
| 206 @print{} 'count_calls' has been called 3 times | |
| 207 @end group | |
| 208 @end example | |
| 209 | |
| 210 As the example shows, a variable may be declared persistent using a | |
| 211 @code{persistent} declaration statement. The following statements are | |
| 212 all persistent declarations. | |
| 4686 | 213 |
| 214 @example | |
| 215 @group | |
| 216 persistent a | |
| 217 persistent a b | |
| 218 persistent c = 2 | |
| 219 persistent d = 3 e f = 5 | |
| 220 @end group | |
| 221 @end example | |
| 222 | |
| 223 The behavior of persistent variables is equivalent to the behavior of | |
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224 static variables in C@. The command @code{static} in Octave is also |
| 6899 | 225 recognized and is equivalent to @code{persistent}. |
| 226 | |
| 227 Like global variables, a persistent variable may only be initialized once. | |
| 6896 | 228 For example, after executing the following code |
| 4686 | 229 |
| 230 @example | |
| 231 @group | |
| 232 persistent pvar = 1 | |
| 233 persistent pvar = 2 | |
| 234 @end group | |
| 235 @end example | |
| 236 | |
| 237 @noindent | |
| 6896 | 238 the value of the persistent variable @code{pvar} is 1, not 2. |
| 4686 | 239 |
| 6899 | 240 If a persistent variable is declared but not initialized to a specific |
| 241 value, it will contain an empty matrix. So, it is also possible to | |
| 242 initialize a persistent variable by checking whether it is empty, as the | |
| 243 following example illustrates. | |
| 244 | |
| 245 @example | |
| 246 @group | |
| 247 function count_calls () | |
| 248 persistent calls; | |
| 249 if (isempty (calls)) | |
| 250 calls = 0; | |
| 251 endif | |
| 7031 | 252 printf ("'count_calls' has been called %d times\n", |
| 253 ++calls); | |
| 6899 | 254 endfunction |
| 255 @end group | |
| 256 @end example | |
| 257 | |
| 258 @noindent | |
| 259 This implementation behaves in exactly the same way as the previous | |
| 260 implementation of @code{count_calls}. | |
| 261 | |
| 262 The value of a persistent variable is kept in memory until it is | |
| 263 explicitly cleared. Assuming that the implementation of @code{count_calls} | |
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264 is saved on disk, we get the following behavior. |
| 6899 | 265 |
| 266 @example | |
| 267 for i = 1:2 | |
| 268 count_calls (); | |
| 269 endfor | |
| 270 @print{} 'count_calls' has been called 1 times | |
| 271 @print{} 'count_calls' has been called 2 times | |
| 272 | |
| 273 clear | |
| 274 for i = 1:2 | |
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275 count_calls (); |
| 6899 | 276 endfor |
| 277 @print{} 'count_calls' has been called 3 times | |
| 278 @print{} 'count_calls' has been called 4 times | |
| 279 | |
| 280 clear all | |
| 281 for i = 1:2 | |
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282 count_calls (); |
| 6899 | 283 endfor |
| 284 @print{} 'count_calls' has been called 1 times | |
| 285 @print{} 'count_calls' has been called 2 times | |
| 286 | |
| 287 clear count_calls | |
| 288 for i = 1:2 | |
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289 count_calls (); |
| 6899 | 290 endfor |
| 291 @print{} 'count_calls' has been called 1 times | |
| 292 @print{} 'count_calls' has been called 2 times | |
| 293 @end example | |
| 294 | |
| 295 @noindent | |
| 296 That is, the persistent variable is only removed from memory when the | |
| 297 function containing the variable is removed. Note that if the function | |
| 298 definition is typed directly into the Octave prompt, the persistent | |
| 299 variable will be cleared by a simple @code{clear} command as the entire | |
| 300 function definition will be removed from memory. If you do not want | |
| 301 a persistent variable to be removed from memory even if the function is | |
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302 cleared, you should use the @code{mlock} function |
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303 (@pxref{Function Locking}). |
| 6899 | 304 |
| 4167 | 305 @node Status of Variables |
| 3294 | 306 @section Status of Variables |
| 307 | |
| 6623 | 308 When creating simple one-shot programs it can be very convenient to |
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309 see which variables are available at the prompt. The function @code{who} |
| 6623 | 310 and its siblings @code{whos} and @code{whos_line_format} will show |
| 311 different information about what is in memory, as the following shows. | |
| 312 | |
| 313 @example | |
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314 @group |
| 6623 | 315 str = "A random string"; |
| 316 who -variables | |
| 317 @print{} *** local user variables: | |
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318 @print{} |
| 6623 | 319 @print{} __nargin__ str |
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320 @end group |
| 6623 | 321 @end example |
| 3294 | 322 |
| 3361 | 323 @DOCSTRING(who) |
| 3294 | 324 |
| 4913 | 325 @DOCSTRING(whos) |
| 326 | |
| 327 @DOCSTRING(whos_line_format) | |
| 328 | |
| 6623 | 329 Instead of displaying which variables are in memory, it is possible |
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330 to determine if a given variable is available. That way it is possible |
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331 to alter the behavior of a program depending on the existence of a |
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332 variable. The following example illustrates this. |
| 6623 | 333 |
| 334 @example | |
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335 @group |
| 6623 | 336 if (! exist ("meaning", "var")) |
| 337 disp ("The program has no 'meaning'"); | |
| 338 endif | |
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339 @end group |
| 6623 | 340 @end example |
| 341 | |
| 3361 | 342 @DOCSTRING(exist) |
| 3294 | 343 |
| 6623 | 344 Usually Octave will manage the memory, but sometimes it can be practical |
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345 to remove variables from memory manually. This is usually needed when |
| 6623 | 346 working with large variables that fill a substantial part of the memory. |
| 347 On a computer that uses the IEEE floating point format, the following | |
| 348 program allocates a matrix that requires around 128 MB memory. | |
| 349 | |
| 350 @example | |
| 351 large_matrix = zeros (4000, 4000); | |
| 352 @end example | |
| 353 | |
| 354 @noindent | |
| 355 Since having this variable in memory might slow down other computations, | |
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356 it can be necessary to remove it manually from memory. The @code{clear} |
| 6623 | 357 function allows this. |
| 358 | |
| 359 @DOCSTRING(clear) | |
| 360 | |
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361 @DOCSTRING(pack) |
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362 |
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363 Information about a function or variable such as its location in the |
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364 file system can also be acquired from within Octave. This is usually |
| 6623 | 365 only useful during development of programs, and not within a program. |
| 366 | |
| 3361 | 367 @DOCSTRING(type) |
| 3294 | 368 |
| 3361 | 369 @DOCSTRING(which) |
| 3294 | 370 |
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371 @DOCSTRING(what) |