Mercurial > octave
annotate doc/interpreter/var.txi @ 31706:597f3ee61a48 stable
update Octave Project Developers copyright for the new year
| author | John W. Eaton <jwe@octave.org> |
|---|---|
| date | Fri, 06 Jan 2023 13:11:27 -0500 |
| parents | 796f54d4ddbf |
| children | 2e484f9f1f18 |
| rev | line source |
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update Octave Project Developers copyright for the new year
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1 @c Copyright (C) 1996-2023 The Octave Project Developers |
| 7018 | 2 @c |
| 3 @c This file is part of Octave. | |
| 4 @c | |
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5 @c Octave is free software: you can redistribute it and/or modify it |
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6 @c under the terms of the GNU General Public License as published by |
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7 @c the Free Software Foundation, either version 3 of the License, or |
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8 @c (at your option) any later version. |
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9 @c |
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10 @c Octave is distributed in the hope that it will be useful, but |
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11 @c WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 @c MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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13 @c GNU General Public License 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 | |
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17 @c <https://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 automatically created variable with a special meaning. The |
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55 @code{ans} variable always contains the result of the last computation, |
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56 where the output wasn't assigned to any variable. The code @code{a = |
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57 cos (pi)} will assign the value -1 to the variable @code{a}, but will |
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58 not change the value of @code{ans}. However, the code @code{cos (pi)} |
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59 will set the value of @code{ans} to -1. |
| 3294 | 60 |
| 61 Variables in Octave do not have fixed types, so it is possible to first | |
| 62 store a numeric value in a variable and then to later use the same name | |
| 63 to hold a string value in the same program. Variables may not be used | |
| 64 before they have been given a value. Doing so results in an error. | |
| 65 | |
| 8567 | 66 @cindex @code{ans} |
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67 @DOCSTRING(ans) |
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68 |
| 6550 | 69 @DOCSTRING(isvarname) |
| 70 | |
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71 @DOCSTRING(matlab.lang.makeValidName) |
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72 |
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73 @DOCSTRING(matlab.lang.makeUniqueStrings) |
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74 |
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75 @DOCSTRING(namelengthmax) |
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76 |
| 3294 | 77 @menu |
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78 * Global Variables:: |
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79 * Persistent Variables:: |
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80 * Status of Variables:: |
| 3294 | 81 @end menu |
| 82 | |
| 4167 | 83 @node Global Variables |
| 3294 | 84 @section Global Variables |
| 85 @cindex global variables | |
| 86 @cindex @code{global} statement | |
| 87 @cindex variables, global | |
| 88 | |
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89 A @dfn{global} variable is one that may be accessed anywhere within Octave. |
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90 This is in contrast to a local variable which can only be accessed outside |
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91 of its current context if it is passed explicitly, such as by including it as a |
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92 parameter when calling a function |
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93 (@code{fcn (@var{local_var1}, @var{local_var2})}). |
| 3294 | 94 |
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95 A variable is declared global by using a @code{global} declaration statement. |
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96 The following statements are all global declarations. |
| 3294 | 97 |
| 98 @example | |
| 99 @group | |
| 100 global a | |
| 4504 | 101 global a b |
| 102 global c = 2 | |
| 103 global d = 3 e f = 5 | |
| 3294 | 104 @end group |
| 105 @end example | |
| 106 | |
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107 Note that the @code{global} qualifier extends only to the next end-of-statement |
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108 indicator which could be a comma (@samp{,}), semicolon (@samp{;}), or newline |
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109 (@samp{'\n'}). For example, the following code declares one global variable, |
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110 @var{a}, and one local variable @var{b} to which the value 1 is assigned. |
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111 |
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112 @example |
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113 global a, b = 1 |
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114 @end example |
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115 |
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116 A global variable may only be initialized once in a @code{global} statement. |
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117 For example, after executing the following code |
| 4504 | 118 |
| 119 @example | |
| 120 @group | |
| 121 global gvar = 1 | |
| 122 global gvar = 2 | |
| 123 @end group | |
| 124 @end example | |
| 125 | |
| 126 @noindent | |
| 6077 | 127 the value of the global variable @code{gvar} is 1, not 2. Issuing a |
| 6623 | 128 @samp{clear gvar} command does not change the above behavior, but |
| 6077 | 129 @samp{clear all} does. |
| 4504 | 130 |
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131 It is necessary declare a variable as global within a function body in order to |
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132 access the one universal variable. For example, |
| 3294 | 133 |
| 134 @example | |
| 135 @group | |
| 136 global x | |
| 137 function f () | |
| 138 x = 1; | |
| 139 endfunction | |
| 140 f () | |
| 141 @end group | |
| 142 @end example | |
| 143 | |
| 144 @noindent | |
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145 does @emph{not} set the value of the global variable @code{x} to 1. Instead, |
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146 a local variable, with name @code{x}, is created and assigned the value of 1. |
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147 In order to change the value of the global variable @code{x}, you must also |
| 3294 | 148 declare it to be global within the function body, like this |
| 149 | |
| 150 @example | |
| 151 @group | |
| 152 function f () | |
| 153 global x; | |
| 154 x = 1; | |
| 155 endfunction | |
| 156 @end group | |
| 157 @end example | |
| 158 | |
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159 Passing a global variable in a function parameter list will make a local copy |
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160 and @emph{not} modify the global value. For example, given the function |
| 3294 | 161 |
| 162 @example | |
| 163 @group | |
| 164 function f (x) | |
| 165 x = 0 | |
| 166 endfunction | |
| 167 @end group | |
| 168 @end example | |
| 169 | |
| 170 @noindent | |
| 171 and the definition of @code{x} as a global variable at the top level, | |
| 172 | |
| 173 @example | |
| 174 global x = 13 | |
| 175 @end example | |
| 176 | |
| 177 @noindent | |
| 178 the expression | |
| 179 | |
| 180 @example | |
| 181 f (x) | |
| 182 @end example | |
| 183 | |
| 184 @noindent | |
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185 will display the value of @code{x} from inside the function as 0, but the value |
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186 of @code{x} at the top level remains unchanged, because the function works with |
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187 a @emph{copy} of its argument. |
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188 |
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189 Programming Note: While global variables occasionally are the right solution to |
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190 a coding problem, modern best practice discourages their use. Code which |
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191 relies on global variables may behave unpredictably between different users |
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192 and can be difficult to debug. This is because global variables can introduce |
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193 systemic changes so that localizing a bug to a particular function, or to a |
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194 particular loop within a function, becomes difficult. |
| 3294 | 195 |
| 4029 | 196 @DOCSTRING(isglobal) |
| 3294 | 197 |
| 4686 | 198 @node Persistent Variables |
| 199 @section Persistent Variables | |
| 200 @cindex persistent variables | |
| 201 @cindex @code{persistent} statement | |
| 202 @cindex variables, persistent | |
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203 @anchor{XREFpersistent} |
| 4686 | 204 |
| 205 A variable that has been declared @dfn{persistent} within a function | |
| 206 will retain its contents in memory between subsequent calls to the | |
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207 same function. The difference between persistent variables and global |
| 4686 | 208 variables is that persistent variables are local in scope to a |
| 209 particular function and are not visible elsewhere. | |
| 210 | |
| 6899 | 211 The following example uses a persistent variable to create a function |
| 212 that prints the number of times it has been called. | |
| 213 | |
| 214 @example | |
| 215 @group | |
| 216 function count_calls () | |
| 217 persistent calls = 0; | |
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218 printf ("'count_calls' has been called %d times\n", |
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219 ++calls); |
| 6899 | 220 endfunction |
| 221 | |
| 222 for i = 1:3 | |
| 223 count_calls (); | |
| 224 endfor | |
| 225 | |
| 226 @print{} 'count_calls' has been called 1 times | |
| 227 @print{} 'count_calls' has been called 2 times | |
| 228 @print{} 'count_calls' has been called 3 times | |
| 229 @end group | |
| 230 @end example | |
| 231 | |
| 232 As the example shows, a variable may be declared persistent using a | |
| 233 @code{persistent} declaration statement. The following statements are | |
| 234 all persistent declarations. | |
| 4686 | 235 |
| 236 @example | |
| 237 @group | |
| 238 persistent a | |
| 239 persistent a b | |
| 240 persistent c = 2 | |
| 241 persistent d = 3 e f = 5 | |
| 242 @end group | |
| 243 @end example | |
| 244 | |
| 245 The behavior of persistent variables is equivalent to the behavior of | |
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246 static variables in C@. |
| 6899 | 247 |
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248 One restriction for persistent variables is, that neither input nor |
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249 output arguments of a function can be persistent: |
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250 |
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251 @example |
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252 @group |
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253 function y = foo () |
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254 persistent y = 0; # Not allowed! |
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255 endfunction |
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256 |
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257 foo () |
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258 @print{} error: can't make function parameter y persistent |
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259 @end group |
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260 @end example |
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261 |
| 6899 | 262 Like global variables, a persistent variable may only be initialized once. |
| 6896 | 263 For example, after executing the following code |
| 4686 | 264 |
| 265 @example | |
| 266 @group | |
| 267 persistent pvar = 1 | |
| 268 persistent pvar = 2 | |
| 269 @end group | |
| 270 @end example | |
| 271 | |
| 272 @noindent | |
| 6896 | 273 the value of the persistent variable @code{pvar} is 1, not 2. |
| 4686 | 274 |
| 6899 | 275 If a persistent variable is declared but not initialized to a specific |
| 276 value, it will contain an empty matrix. So, it is also possible to | |
| 277 initialize a persistent variable by checking whether it is empty, as the | |
| 278 following example illustrates. | |
| 279 | |
| 280 @example | |
| 281 @group | |
| 282 function count_calls () | |
| 283 persistent calls; | |
| 284 if (isempty (calls)) | |
| 285 calls = 0; | |
| 286 endif | |
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287 printf ("'count_calls' has been called %d times\n", |
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288 ++calls); |
| 6899 | 289 endfunction |
| 290 @end group | |
| 291 @end example | |
| 292 | |
| 293 @noindent | |
| 294 This implementation behaves in exactly the same way as the previous | |
| 295 implementation of @code{count_calls}. | |
| 296 | |
| 297 The value of a persistent variable is kept in memory until it is | |
| 298 explicitly cleared. Assuming that the implementation of @code{count_calls} | |
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299 is saved on disk, we get the following behavior. |
| 6899 | 300 |
| 301 @example | |
| 302 for i = 1:2 | |
| 303 count_calls (); | |
| 304 endfor | |
| 305 @print{} 'count_calls' has been called 1 times | |
| 306 @print{} 'count_calls' has been called 2 times | |
| 307 | |
| 308 clear | |
| 309 for i = 1:2 | |
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310 count_calls (); |
| 6899 | 311 endfor |
| 312 @print{} 'count_calls' has been called 3 times | |
| 313 @print{} 'count_calls' has been called 4 times | |
| 314 | |
| 315 clear all | |
| 316 for i = 1:2 | |
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317 count_calls (); |
| 6899 | 318 endfor |
| 319 @print{} 'count_calls' has been called 1 times | |
| 320 @print{} 'count_calls' has been called 2 times | |
| 321 | |
| 322 clear count_calls | |
| 323 for i = 1:2 | |
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324 count_calls (); |
| 6899 | 325 endfor |
| 326 @print{} 'count_calls' has been called 1 times | |
| 327 @print{} 'count_calls' has been called 2 times | |
| 328 @end example | |
| 329 | |
| 330 @noindent | |
| 331 That is, the persistent variable is only removed from memory when the | |
| 332 function containing the variable is removed. Note that if the function | |
| 333 definition is typed directly into the Octave prompt, the persistent | |
| 334 variable will be cleared by a simple @code{clear} command as the entire | |
| 335 function definition will be removed from memory. If you do not want | |
| 336 a persistent variable to be removed from memory even if the function is | |
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337 cleared, you should use the @code{mlock} function |
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338 (@pxref{Function Locking}). |
| 6899 | 339 |
| 4167 | 340 @node Status of Variables |
| 3294 | 341 @section Status of Variables |
| 342 | |
| 6623 | 343 When creating simple one-shot programs it can be very convenient to |
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344 see which variables are available at the prompt. The function @code{who} |
| 6623 | 345 and its siblings @code{whos} and @code{whos_line_format} will show |
| 346 different information about what is in memory, as the following shows. | |
| 347 | |
| 348 @example | |
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349 @group |
| 6623 | 350 str = "A random string"; |
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351 who |
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352 @print{} Variables in the current scope: |
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353 @print{} |
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354 @print{} ans str |
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355 @end group |
| 6623 | 356 @end example |
| 3294 | 357 |
| 3361 | 358 @DOCSTRING(who) |
| 3294 | 359 |
| 4913 | 360 @DOCSTRING(whos) |
| 361 | |
| 362 @DOCSTRING(whos_line_format) | |
| 363 | |
| 6623 | 364 Instead of displaying which variables are in memory, it is possible |
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365 to determine if a given variable is available. That way it is possible |
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366 to alter the behavior of a program depending on the existence of a |
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367 variable. The following example illustrates this. |
| 6623 | 368 |
| 369 @example | |
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370 @group |
| 6623 | 371 if (! exist ("meaning", "var")) |
| 372 disp ("The program has no 'meaning'"); | |
| 373 endif | |
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374 @end group |
| 6623 | 375 @end example |
| 376 | |
| 3361 | 377 @DOCSTRING(exist) |
| 3294 | 378 |
| 6623 | 379 Usually Octave will manage the memory, but sometimes it can be practical |
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380 to remove variables from memory manually. This is usually needed when |
| 6623 | 381 working with large variables that fill a substantial part of the memory. |
| 382 On a computer that uses the IEEE floating point format, the following | |
| 383 program allocates a matrix that requires around 128 MB memory. | |
| 384 | |
| 385 @example | |
| 386 large_matrix = zeros (4000, 4000); | |
| 387 @end example | |
| 388 | |
| 389 @noindent | |
| 390 Since having this variable in memory might slow down other computations, | |
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391 it can be necessary to remove it manually from memory. The @code{clear} |
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392 or @code{clearvars} functions do this. |
| 6623 | 393 |
| 394 @DOCSTRING(clear) | |
| 395 | |
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396 @DOCSTRING(clearvars) |
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397 |
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398 @DOCSTRING(pack) |
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399 |
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400 Information about a function or variable such as its location in the |
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401 file system can also be acquired from within Octave. This is usually |
| 6623 | 402 only useful during development of programs, and not within a program. |
| 403 | |
| 3361 | 404 @DOCSTRING(type) |
| 3294 | 405 |
| 3361 | 406 @DOCSTRING(which) |
| 3294 | 407 |
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408 @DOCSTRING(what) |