Mercurial > octave-nkf
annotate doc/interpreter/var.txi @ 8566:da95767511f5
Remove descriptions of built-in variables from manual
| author | sh@sh-laptop |
|---|---|
| date | Thu, 22 Jan 2009 11:06:56 -0500 |
| parents | 68aa5abfd136 |
| children | 674d00f5e072 |
| rev | line source |
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| 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 | |
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58 There is one built-in variable with a special meaning. The @code{ans} variable |
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59 always contains the result of the last computation, where the output wasn't |
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60 assigned to any variable. The code @code{a = cos (pi)} will assign the value -1 |
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61 to the variable @code{a}, but will not change the value of @code{ans}. However, |
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62 the code @code{cos (pi)} will set the value of @code{ans} to -1. |
| 3294 | 63 |
| 64 Variables in Octave do not have fixed types, so it is possible to first | |
| 65 store a numeric value in a variable and then to later use the same name | |
| 66 to hold a string value in the same program. Variables may not be used | |
| 67 before they have been given a value. Doing so results in an error. | |
| 68 | |
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69 @DOCSTRING(ans) |
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70 |
| 6550 | 71 @DOCSTRING(isvarname) |
| 72 | |
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73 @DOCSTRING(genvarname) |
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74 |
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75 @DOCSTRING(namelengthmax) |
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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 | |
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186 @anchor{doc-persistent} |
| 4686 | 187 |
| 188 A variable that has been declared @dfn{persistent} within a function | |
| 189 will retain its contents in memory between subsequent calls to the | |
| 190 same function. The difference between persistent variables and global | |
| 191 variables is that persistent variables are local in scope to a | |
| 192 particular function and are not visible elsewhere. | |
| 193 | |
| 6899 | 194 The following example uses a persistent variable to create a function |
| 195 that prints the number of times it has been called. | |
| 196 | |
| 197 @example | |
| 198 @group | |
| 199 function count_calls () | |
| 200 persistent calls = 0; | |
| 7031 | 201 printf ("'count_calls' has been called %d times\n", |
| 202 ++calls); | |
| 6899 | 203 endfunction |
| 204 | |
| 205 for i = 1:3 | |
| 206 count_calls (); | |
| 207 endfor | |
| 208 | |
| 209 @print{} 'count_calls' has been called 1 times | |
| 210 @print{} 'count_calls' has been called 2 times | |
| 211 @print{} 'count_calls' has been called 3 times | |
| 212 @end group | |
| 213 @end example | |
| 214 | |
| 215 As the example shows, a variable may be declared persistent using a | |
| 216 @code{persistent} declaration statement. The following statements are | |
| 217 all persistent declarations. | |
| 4686 | 218 |
| 219 @example | |
| 220 @group | |
| 221 persistent a | |
| 222 persistent a b | |
| 223 persistent c = 2 | |
| 224 persistent d = 3 e f = 5 | |
| 225 @end group | |
| 226 @end example | |
| 227 | |
| 228 The behavior of persistent variables is equivalent to the behavior of | |
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229 static variables in C. The command @code{static} in Octave is also |
| 6899 | 230 recognized and is equivalent to @code{persistent}. |
| 231 | |
| 232 Like global variables, a persistent variable may only be initialized once. | |
| 6896 | 233 For example, after executing the following code |
| 4686 | 234 |
| 235 @example | |
| 236 @group | |
| 237 persistent pvar = 1 | |
| 238 persistent pvar = 2 | |
| 239 @end group | |
| 240 @end example | |
| 241 | |
| 242 @noindent | |
| 6896 | 243 the value of the persistent variable @code{pvar} is 1, not 2. |
| 4686 | 244 |
| 6899 | 245 If a persistent variable is declared but not initialized to a specific |
| 246 value, it will contain an empty matrix. So, it is also possible to | |
| 247 initialize a persistent variable by checking whether it is empty, as the | |
| 248 following example illustrates. | |
| 249 | |
| 250 @example | |
| 251 @group | |
| 252 function count_calls () | |
| 253 persistent calls; | |
| 254 if (isempty (calls)) | |
| 255 calls = 0; | |
| 256 endif | |
| 7031 | 257 printf ("'count_calls' has been called %d times\n", |
| 258 ++calls); | |
| 6899 | 259 endfunction |
| 260 @end group | |
| 261 @end example | |
| 262 | |
| 263 @noindent | |
| 264 This implementation behaves in exactly the same way as the previous | |
| 265 implementation of @code{count_calls}. | |
| 266 | |
| 267 The value of a persistent variable is kept in memory until it is | |
| 268 explicitly cleared. Assuming that the implementation of @code{count_calls} | |
| 269 is saved on disc, we get the following behaviour. | |
| 270 | |
| 271 @example | |
| 272 @group | |
| 273 for i = 1:2 | |
| 274 count_calls (); | |
| 275 endfor | |
| 276 @print{} 'count_calls' has been called 1 times | |
| 277 @print{} 'count_calls' has been called 2 times | |
| 278 | |
| 279 clear | |
| 280 for i = 1:2 | |
| 281 count_calls(); | |
| 282 endfor | |
| 283 @print{} 'count_calls' has been called 3 times | |
| 284 @print{} 'count_calls' has been called 4 times | |
| 285 | |
| 286 clear all | |
| 287 for i = 1:2 | |
| 288 count_calls(); | |
| 289 endfor | |
| 290 @print{} 'count_calls' has been called 1 times | |
| 291 @print{} 'count_calls' has been called 2 times | |
| 292 | |
| 293 clear count_calls | |
| 294 for i = 1:2 | |
| 295 count_calls(); | |
| 296 endfor | |
| 297 @print{} 'count_calls' has been called 1 times | |
| 298 @print{} 'count_calls' has been called 2 times | |
| 299 @end group | |
| 300 @end example | |
| 301 | |
| 302 @noindent | |
| 303 That is, the persistent variable is only removed from memory when the | |
| 304 function containing the variable is removed. Note that if the function | |
| 305 definition is typed directly into the Octave prompt, the persistent | |
| 306 variable will be cleared by a simple @code{clear} command as the entire | |
| 307 function definition will be removed from memory. If you do not want | |
| 308 a persistent variable to be removed from memory even if the function is | |
| 309 cleared, you should use the @code{mlock} function as described in | |
| 310 @xref{Function Locking}. | |
| 311 | |
| 4167 | 312 @node Status of Variables |
| 3294 | 313 @section Status of Variables |
| 314 | |
| 6623 | 315 When creating simple one-shot programs it can be very convenient to |
| 316 see which variables are available at the prompt. The function @code{who} | |
| 317 and its siblings @code{whos} and @code{whos_line_format} will show | |
| 318 different information about what is in memory, as the following shows. | |
| 319 | |
| 320 @example | |
| 321 str = "A random string"; | |
| 322 who -variables | |
| 323 @print{} *** local user variables: | |
| 324 @print{} | |
| 325 @print{} __nargin__ str | |
| 326 @end example | |
| 3294 | 327 |
| 3361 | 328 @DOCSTRING(who) |
| 3294 | 329 |
| 4913 | 330 @DOCSTRING(whos) |
| 331 | |
| 332 @DOCSTRING(whos_line_format) | |
| 333 | |
| 6623 | 334 Instead of displaying which variables are in memory, it is possible |
| 335 to determine if a given variable is available. That way it is possible | |
| 336 to alter the behaviour of a program depending on the existence of a | |
| 337 variable. The following example illustrates this. | |
| 338 | |
| 339 @example | |
| 340 if (! exist ("meaning", "var")) | |
| 341 disp ("The program has no 'meaning'"); | |
| 342 endif | |
| 343 @end example | |
| 344 | |
| 3361 | 345 @DOCSTRING(exist) |
| 3294 | 346 |
| 6623 | 347 Usually Octave will manage the memory, but sometimes it can be practical |
| 348 to remove variables from memory manually. This is usually needed when | |
| 349 working with large variables that fill a substantial part of the memory. | |
| 350 On a computer that uses the IEEE floating point format, the following | |
| 351 program allocates a matrix that requires around 128 MB memory. | |
| 352 | |
| 353 @example | |
| 354 large_matrix = zeros (4000, 4000); | |
| 355 @end example | |
| 356 | |
| 357 @noindent | |
| 358 Since having this variable in memory might slow down other computations, | |
| 359 it can be necessary to remove it manually from memory. The @code{clear} | |
| 360 function allows this. | |
| 361 | |
| 362 @DOCSTRING(clear) | |
| 363 | |
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364 Information about a function or variable such as its location in the |
| 6623 | 365 file system can also be acquired from within Octave. This is usually |
| 366 only useful during development of programs, and not within a program. | |
| 367 | |
| 3361 | 368 @DOCSTRING(type) |
| 3294 | 369 |
| 3361 | 370 @DOCSTRING(which) |
| 3294 | 371 |