Mercurial > jwe > octave
annotate doc/interpreter/stmt.txi @ 21122:dba88797f69f
doc: Periodic grammarcheck of documentation.
| author | Rik <rik@octave.org> |
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| date | Wed, 20 Jan 2016 21:14:47 -0800 |
| parents | fdffc955b4ed |
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| rev | line source |
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1 @c Copyright (C) 1996-2015 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 Statements |
| 3294 | 20 @chapter Statements |
| 21 @cindex statements | |
| 22 | |
| 23 Statements may be a simple constant expression or a complicated list of | |
| 24 nested loops and conditional statements. | |
| 25 | |
| 26 @dfn{Control statements} such as @code{if}, @code{while}, and so on | |
| 27 control the flow of execution in Octave programs. All the control | |
| 28 statements start with special keywords such as @code{if} and | |
| 29 @code{while}, to distinguish them from simple expressions. | |
| 30 Many control statements contain other statements; for example, the | |
| 31 @code{if} statement contains another statement which may or may not be | |
| 32 executed. | |
| 33 | |
| 34 @cindex @code{end} statement | |
| 35 Each control statement has a corresponding @dfn{end} statement that | |
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36 marks the end of the control statement. For example, the |
| 3294 | 37 keyword @code{endif} marks the end of an @code{if} statement, and |
| 38 @code{endwhile} marks the end of a @code{while} statement. You can use | |
| 39 the keyword @code{end} anywhere a more specific end keyword is expected, | |
| 40 but using the more specific keywords is preferred because if you use | |
| 41 them, Octave is able to provide better diagnostics for mismatched or | |
| 42 missing end tokens. | |
| 43 | |
| 44 The list of statements contained between keywords like @code{if} or | |
| 45 @code{while} and the corresponding end statement is called the | |
| 46 @dfn{body} of a control statement. | |
| 47 | |
| 48 @menu | |
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49 * The if Statement:: |
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50 * The switch Statement:: |
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51 * The while Statement:: |
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52 * The do-until Statement:: |
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53 * The for Statement:: |
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54 * The break Statement:: |
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55 * The continue Statement:: |
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56 * The unwind_protect Statement:: |
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57 * The try Statement:: |
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58 * Continuation Lines:: |
| 3294 | 59 @end menu |
| 60 | |
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61 @node The if Statement |
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62 @section The if Statement |
| 3294 | 63 @cindex @code{if} statement |
| 64 @cindex @code{else} statement | |
| 65 @cindex @code{elseif} statement | |
| 66 @cindex @code{endif} statement | |
| 67 | |
| 68 The @code{if} statement is Octave's decision-making statement. There | |
| 69 are three basic forms of an @code{if} statement. In its simplest form, | |
| 70 it looks like this: | |
| 71 | |
| 72 @example | |
| 73 @group | |
| 74 if (@var{condition}) | |
| 75 @var{then-body} | |
| 76 endif | |
| 77 @end group | |
| 78 @end example | |
| 79 | |
| 80 @noindent | |
| 81 @var{condition} is an expression that controls what the rest of the | |
| 82 statement will do. The @var{then-body} is executed only if | |
| 83 @var{condition} is true. | |
| 84 | |
| 85 The condition in an @code{if} statement is considered true if its value | |
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86 is nonzero, and false if its value is zero. If the value of the |
| 3294 | 87 conditional expression in an @code{if} statement is a vector or a |
| 6637 | 88 matrix, it is considered true only if it is non-empty and @emph{all} |
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89 of the elements are nonzero. The conceptually equivalent code when |
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90 @var{condition} is a matrix is shown below. |
| 3294 | 91 |
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92 @example |
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93 if (@var{matrix}) @equiv{} if (all (@var{matrix}(:))) |
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94 @end example |
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95 |
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96 @noindent |
| 3294 | 97 The second form of an if statement looks like this: |
| 98 | |
| 99 @example | |
| 100 @group | |
| 101 if (@var{condition}) | |
| 102 @var{then-body} | |
| 103 else | |
| 104 @var{else-body} | |
| 105 endif | |
| 106 @end group | |
| 107 @end example | |
| 108 | |
| 109 @noindent | |
| 110 If @var{condition} is true, @var{then-body} is executed; otherwise, | |
| 111 @var{else-body} is executed. | |
| 112 | |
| 113 Here is an example: | |
| 114 | |
| 115 @example | |
| 116 @group | |
| 117 if (rem (x, 2) == 0) | |
| 118 printf ("x is even\n"); | |
| 119 else | |
| 120 printf ("x is odd\n"); | |
| 121 endif | |
| 122 @end group | |
| 123 @end example | |
| 124 | |
| 125 In this example, if the expression @code{rem (x, 2) == 0} is true (that | |
| 126 is, the value of @code{x} is divisible by 2), then the first | |
| 127 @code{printf} statement is evaluated, otherwise the second @code{printf} | |
| 128 statement is evaluated. | |
| 129 | |
| 130 The third and most general form of the @code{if} statement allows | |
| 131 multiple decisions to be combined in a single statement. It looks like | |
| 132 this: | |
| 133 | |
| 134 @example | |
| 135 @group | |
| 136 if (@var{condition}) | |
| 137 @var{then-body} | |
| 138 elseif (@var{condition}) | |
| 139 @var{elseif-body} | |
| 140 else | |
| 141 @var{else-body} | |
| 142 endif | |
| 143 @end group | |
| 144 @end example | |
| 145 | |
| 146 @noindent | |
| 147 Any number of @code{elseif} clauses may appear. Each condition is | |
| 148 tested in turn, and if one is found to be true, its corresponding | |
| 149 @var{body} is executed. If none of the conditions are true and the | |
| 150 @code{else} clause is present, its body is executed. Only one | |
| 151 @code{else} clause may appear, and it must be the last part of the | |
| 152 statement. | |
| 153 | |
| 154 In the following example, if the first condition is true (that is, the | |
| 155 value of @code{x} is divisible by 2), then the first @code{printf} | |
| 156 statement is executed. If it is false, then the second condition is | |
| 157 tested, and if it is true (that is, the value of @code{x} is divisible | |
| 158 by 3), then the second @code{printf} statement is executed. Otherwise, | |
| 159 the third @code{printf} statement is performed. | |
| 160 | |
| 161 @example | |
| 162 @group | |
| 163 if (rem (x, 2) == 0) | |
| 164 printf ("x is even\n"); | |
| 165 elseif (rem (x, 3) == 0) | |
| 166 printf ("x is odd and divisible by 3\n"); | |
| 167 else | |
| 168 printf ("x is odd\n"); | |
| 169 endif | |
| 170 @end group | |
| 171 @end example | |
| 172 | |
| 173 Note that the @code{elseif} keyword must not be spelled @code{else if}, | |
| 174 as is allowed in Fortran. If it is, the space between the @code{else} | |
| 175 and @code{if} will tell Octave to treat this as a new @code{if} | |
| 176 statement within another @code{if} statement's @code{else} clause. For | |
| 177 example, if you write | |
| 178 | |
| 179 @example | |
| 180 @group | |
| 181 if (@var{c1}) | |
| 182 @var{body-1} | |
| 183 else if (@var{c2}) | |
| 184 @var{body-2} | |
| 185 endif | |
| 186 @end group | |
| 187 @end example | |
| 188 | |
| 189 @noindent | |
| 190 Octave will expect additional input to complete the first @code{if} | |
| 191 statement. If you are using Octave interactively, it will continue to | |
| 192 prompt you for additional input. If Octave is reading this input from a | |
| 193 file, it may complain about missing or mismatched @code{end} statements, | |
| 194 or, if you have not used the more specific @code{end} statements | |
| 195 (@code{endif}, @code{endfor}, etc.), it may simply produce incorrect | |
| 196 results, without producing any warning messages. | |
| 197 | |
| 198 It is much easier to see the error if we rewrite the statements above | |
| 199 like this, | |
| 200 | |
| 201 @example | |
| 202 @group | |
| 203 if (@var{c1}) | |
| 204 @var{body-1} | |
| 205 else | |
| 206 if (@var{c2}) | |
| 207 @var{body-2} | |
| 208 endif | |
| 209 @end group | |
| 210 @end example | |
| 211 | |
| 212 @noindent | |
| 213 using the indentation to show how Octave groups the statements. | |
| 214 @xref{Functions and Scripts}. | |
| 215 | |
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216 @node The switch Statement |
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217 @section The switch Statement |
| 3294 | 218 @cindex @code{switch} statement |
| 219 @cindex @code{case} statement | |
| 220 @cindex @code{otherwise} statement | |
| 221 @cindex @code{endswitch} statement | |
| 222 | |
| 6530 | 223 It is very common to take different actions depending on the value of |
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224 one variable. This is possible using the @code{if} statement in the |
| 6530 | 225 following way |
| 226 | |
| 227 @example | |
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228 @group |
| 6530 | 229 if (X == 1) |
| 230 do_something (); | |
| 231 elseif (X == 2) | |
| 232 do_something_else (); | |
| 233 else | |
| 234 do_something_completely_different (); | |
| 235 endif | |
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236 @end group |
| 6530 | 237 @end example |
| 238 | |
| 239 @noindent | |
| 240 This kind of code can however be very cumbersome to both write and | |
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241 maintain. To overcome this problem Octave supports the @code{switch} |
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242 statement. Using this statement, the above example becomes |
| 6530 | 243 |
| 244 @example | |
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245 @group |
| 6530 | 246 switch (X) |
| 247 case 1 | |
| 248 do_something (); | |
| 249 case 2 | |
| 250 do_something_else (); | |
| 251 otherwise | |
| 252 do_something_completely_different (); | |
| 253 endswitch | |
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254 @end group |
| 6530 | 255 @end example |
| 256 | |
| 257 @noindent | |
| 258 This code makes the repetitive structure of the problem more explicit, | |
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259 making the code easier to read, and hence maintain. Also, if the |
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260 variable @code{X} should change its name, only one line would need |
| 6530 | 261 changing compared to one line per case when @code{if} statements are |
| 262 used. | |
| 3294 | 263 |
| 264 The general form of the @code{switch} statement is | |
| 265 | |
| 266 @example | |
| 267 @group | |
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268 switch (@var{expression}) |
| 3294 | 269 case @var{label} |
| 270 @var{command_list} | |
| 271 case @var{label} | |
| 272 @var{command_list} | |
| 273 @dots{} | |
| 274 | |
| 275 otherwise | |
| 276 @var{command_list} | |
| 277 endswitch | |
| 278 @end group | |
| 279 @end example | |
| 280 | |
| 6530 | 281 @noindent |
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282 where @var{label} can be any expression. However, duplicate |
| 6530 | 283 @var{label} values are not detected, and only the @var{command_list} |
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284 corresponding to the first match will be executed. For the |
| 6530 | 285 @code{switch} statement to be meaningful at least one |
| 286 @code{case @var{label} @var{command_list}} clause must be present, | |
| 287 while the @code{otherwise @var{command_list}} clause is optional. | |
| 3294 | 288 |
| 6637 | 289 If @var{label} is a cell array the corresponding @var{command_list} |
| 290 is executed if @emph{any} of the elements of the cell array match | |
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291 @var{expression}. As an example, the following program will print |
| 6637 | 292 @samp{Variable is either 6 or 7}. |
| 293 | |
| 294 @example | |
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295 @group |
| 6637 | 296 A = 7; |
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297 switch (A) |
| 6637 | 298 case @{ 6, 7 @} |
| 299 printf ("variable is either 6 or 7\n"); | |
| 300 otherwise | |
| 301 printf ("variable is neither 6 nor 7\n"); | |
| 302 endswitch | |
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303 @end group |
| 6637 | 304 @end example |
| 305 | |
| 3294 | 306 As with all other specific @code{end} keywords, @code{endswitch} may be |
| 307 replaced by @code{end}, but you can get better diagnostics if you use | |
| 308 the specific forms. | |
| 309 | |
| 6530 | 310 @c Strings can be matched |
| 311 | |
| 312 One advantage of using the @code{switch} statement compared to using | |
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313 @code{if} statements is that the @var{label}s can be strings. If an |
| 6530 | 314 @code{if} statement is used it is @emph{not} possible to write |
| 315 | |
| 316 @example | |
| 317 if (X == "a string") # This is NOT valid | |
| 318 @end example | |
| 319 | |
| 320 @noindent | |
| 321 since a character-to-character comparison between @code{X} and the | |
| 322 string will be made instead of evaluating if the strings are equal. | |
| 323 This special-case is handled by the @code{switch} statement, and it | |
| 324 is possible to write programs that look like this | |
| 325 | |
| 326 @example | |
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327 @group |
| 6530 | 328 switch (X) |
| 329 case "a string" | |
| 330 do_something | |
| 331 @dots{} | |
| 332 endswitch | |
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333 @end group |
| 6530 | 334 @end example |
| 335 | |
| 6535 | 336 @menu |
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337 * Notes for the C Programmer:: |
| 6535 | 338 @end menu |
| 339 | |
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340 @node Notes for the C Programmer |
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341 @subsection Notes for the C Programmer |
| 6530 | 342 |
| 6637 | 343 The @code{switch} statement is also available in the widely used C |
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344 programming language. There are, however, some differences |
| 6530 | 345 between the statement in Octave and C |
| 346 | |
| 347 @itemize @bullet | |
| 3294 | 348 @item |
| 349 Cases are exclusive, so they don't `fall through' as do the cases | |
| 6637 | 350 in the @code{switch} statement of the C language. |
| 3294 | 351 |
| 352 @item | |
| 353 The @var{command_list} elements are not optional. Making the list | |
| 354 optional would have meant requiring a separator between the label and | |
| 355 the command list. Otherwise, things like | |
| 356 | |
| 357 @example | |
| 358 @group | |
| 359 switch (foo) | |
| 360 case (1) -2 | |
| 361 @dots{} | |
| 362 @end group | |
| 363 @end example | |
| 364 | |
| 365 @noindent | |
| 366 would produce surprising results, as would | |
| 367 | |
| 368 @example | |
| 369 @group | |
| 370 switch (foo) | |
| 371 case (1) | |
| 372 case (2) | |
| 373 doit (); | |
| 374 @dots{} | |
| 375 @end group | |
| 376 @end example | |
| 377 | |
| 378 @noindent | |
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379 particularly for C programmers. If @code{doit()} should be executed if |
| 6637 | 380 @var{foo} is either @code{1} or @code{2}, the above code should be |
| 381 written with a cell array like this | |
| 382 | |
| 383 @example | |
| 384 @group | |
| 385 switch (foo) | |
| 386 case @{ 1, 2 @} | |
| 387 doit (); | |
| 388 @dots{} | |
| 389 @end group | |
| 390 @end example | |
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391 |
| 3294 | 392 @end itemize |
| 393 | |
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394 @node The while Statement |
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395 @section The while Statement |
| 3294 | 396 @cindex @code{while} statement |
| 397 @cindex @code{endwhile} statement | |
| 398 @cindex loop | |
| 399 @cindex body of a loop | |
| 400 | |
| 401 In programming, a @dfn{loop} means a part of a program that is (or at least can | |
| 402 be) executed two or more times in succession. | |
| 403 | |
| 404 The @code{while} statement is the simplest looping statement in Octave. | |
| 405 It repeatedly executes a statement as long as a condition is true. As | |
| 406 with the condition in an @code{if} statement, the condition in a | |
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407 @code{while} statement is considered true if its value is nonzero, and |
| 3294 | 408 false if its value is zero. If the value of the conditional expression |
| 409 in a @code{while} statement is a vector or a matrix, it is considered | |
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410 true only if it is non-empty and @emph{all} of the elements are nonzero. |
| 3294 | 411 |
| 412 Octave's @code{while} statement looks like this: | |
| 413 | |
| 414 @example | |
| 415 @group | |
| 416 while (@var{condition}) | |
| 417 @var{body} | |
| 418 endwhile | |
| 419 @end group | |
| 420 @end example | |
| 421 | |
| 422 @noindent | |
| 423 Here @var{body} is a statement or list of statements that we call the | |
| 424 @dfn{body} of the loop, and @var{condition} is an expression that | |
| 425 controls how long the loop keeps running. | |
| 426 | |
| 427 The first thing the @code{while} statement does is test @var{condition}. | |
| 428 If @var{condition} is true, it executes the statement @var{body}. After | |
| 429 @var{body} has been executed, @var{condition} is tested again, and if it | |
| 430 is still true, @var{body} is executed again. This process repeats until | |
| 431 @var{condition} is no longer true. If @var{condition} is initially | |
| 432 false, the body of the loop is never executed. | |
| 433 | |
| 434 This example creates a variable @code{fib} that contains the first ten | |
| 435 elements of the Fibonacci sequence. | |
| 436 | |
| 437 @example | |
| 438 @group | |
| 439 fib = ones (1, 10); | |
| 440 i = 3; | |
| 441 while (i <= 10) | |
| 442 fib (i) = fib (i-1) + fib (i-2); | |
| 443 i++; | |
| 444 endwhile | |
| 445 @end group | |
| 446 @end example | |
| 447 | |
| 448 @noindent | |
| 449 Here the body of the loop contains two statements. | |
| 450 | |
| 451 The loop works like this: first, the value of @code{i} is set to 3. | |
| 452 Then, the @code{while} tests whether @code{i} is less than or equal to | |
| 453 10. This is the case when @code{i} equals 3, so the value of the | |
| 454 @code{i}-th element of @code{fib} is set to the sum of the previous two | |
| 455 values in the sequence. Then the @code{i++} increments the value of | |
| 456 @code{i} and the loop repeats. The loop terminates when @code{i} | |
| 457 reaches 11. | |
| 458 | |
| 459 A newline is not required between the condition and the | |
| 460 body; but using one makes the program clearer unless the body is very | |
| 461 simple. | |
| 462 | |
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463 @node The do-until Statement |
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464 @section The do-until Statement |
| 3489 | 465 @cindex @code{do-until} statement |
| 466 | |
| 467 The @code{do-until} statement is similar to the @code{while} statement, | |
| 468 except that it repeatedly executes a statement until a condition becomes | |
| 469 true, and the test of the condition is at the end of the loop, so the | |
| 470 body of the loop is always executed at least once. As with the | |
| 471 condition in an @code{if} statement, the condition in a @code{do-until} | |
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472 statement is considered true if its value is nonzero, and false if its |
| 3489 | 473 value is zero. If the value of the conditional expression in a |
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474 @code{do-until} statement is a vector or a matrix, it is considered |
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475 true only if it is non-empty and @emph{all} of the elements are nonzero. |
| 3489 | 476 |
| 477 Octave's @code{do-until} statement looks like this: | |
| 478 | |
| 479 @example | |
| 480 @group | |
| 481 do | |
| 482 @var{body} | |
| 483 until (@var{condition}) | |
| 484 @end group | |
| 485 @end example | |
| 486 | |
| 487 @noindent | |
| 488 Here @var{body} is a statement or list of statements that we call the | |
| 489 @dfn{body} of the loop, and @var{condition} is an expression that | |
| 490 controls how long the loop keeps running. | |
| 491 | |
| 492 This example creates a variable @code{fib} that contains the first ten | |
| 493 elements of the Fibonacci sequence. | |
| 494 | |
| 495 @example | |
| 496 @group | |
| 497 fib = ones (1, 10); | |
| 498 i = 2; | |
| 499 do | |
| 500 i++; | |
| 501 fib (i) = fib (i-1) + fib (i-2); | |
| 502 until (i == 10) | |
| 503 @end group | |
| 504 @end example | |
| 505 | |
| 506 A newline is not required between the @code{do} keyword and the | |
| 507 body; but using one makes the program clearer unless the body is very | |
| 508 simple. | |
| 509 | |
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510 @node The for Statement |
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511 @section The for Statement |
| 3294 | 512 @cindex @code{for} statement |
| 513 @cindex @code{endfor} statement | |
| 514 | |
| 515 The @code{for} statement makes it more convenient to count iterations of a | |
| 516 loop. The general form of the @code{for} statement looks like this: | |
| 517 | |
| 518 @example | |
| 519 @group | |
| 520 for @var{var} = @var{expression} | |
| 521 @var{body} | |
| 522 endfor | |
| 523 @end group | |
| 524 @end example | |
| 525 | |
| 526 @noindent | |
| 527 where @var{body} stands for any statement or list of statements, | |
| 528 @var{expression} is any valid expression, and @var{var} may take several | |
| 529 forms. Usually it is a simple variable name or an indexed variable. If | |
| 530 the value of @var{expression} is a structure, @var{var} may also be a | |
| 6637 | 531 vector with two elements. @xref{Looping Over Structure Elements}, below. |
| 3294 | 532 |
| 533 The assignment expression in the @code{for} statement works a bit | |
| 534 differently than Octave's normal assignment statement. Instead of | |
| 535 assigning the complete result of the expression, it assigns each column | |
| 536 of the expression to @var{var} in turn. If @var{expression} is a range, | |
| 537 a row vector, or a scalar, the value of @var{var} will be a scalar each | |
| 538 time the loop body is executed. If @var{var} is a column vector or a | |
| 539 matrix, @var{var} will be a column vector each time the loop body is | |
| 540 executed. | |
| 541 | |
| 542 The following example shows another way to create a vector containing | |
| 543 the first ten elements of the Fibonacci sequence, this time using the | |
| 544 @code{for} statement: | |
| 545 | |
| 546 @example | |
| 547 @group | |
| 548 fib = ones (1, 10); | |
| 549 for i = 3:10 | |
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550 fib(i) = fib(i-1) + fib(i-2); |
| 3294 | 551 endfor |
| 552 @end group | |
| 553 @end example | |
| 554 | |
| 555 @noindent | |
| 556 This code works by first evaluating the expression @code{3:10}, to | |
| 557 produce a range of values from 3 to 10 inclusive. Then the variable | |
| 558 @code{i} is assigned the first element of the range and the body of the | |
| 559 loop is executed once. When the end of the loop body is reached, the | |
| 560 next value in the range is assigned to the variable @code{i}, and the | |
| 561 loop body is executed again. This process continues until there are no | |
| 562 more elements to assign. | |
| 563 | |
| 6587 | 564 Within Octave is it also possible to iterate over matrices or cell arrays |
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565 using the @code{for} statement. For example consider |
| 6587 | 566 |
| 567 @example | |
| 568 @group | |
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569 disp ("Loop over a matrix") |
| 6587 | 570 for i = [1,3;2,4] |
| 571 i | |
| 572 endfor | |
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573 disp ("Loop over a cell array") |
| 6587 | 574 for i = @{1,"two";"three",4@} |
| 575 i | |
| 576 endfor | |
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577 @end group |
| 6587 | 578 @end example |
| 579 | |
| 580 @noindent | |
| 581 In this case the variable @code{i} takes on the value of the columns of | |
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582 the matrix or cell matrix. So the first loop iterates twice, producing |
| 7001 | 583 two column vectors @code{[1;2]}, followed by @code{[3;4]}, and likewise |
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584 for the loop over the cell array. This can be extended to loops over |
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585 multi-dimensional arrays. For example: |
| 6587 | 586 |
| 587 @example | |
| 588 @group | |
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589 a = [1,3;2,4]; c = cat (3, a, 2*a); |
| 6587 | 590 for i = c |
| 591 i | |
| 592 endfor | |
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593 @end group |
| 6587 | 594 @end example |
| 595 | |
| 596 @noindent | |
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597 In the above case, the multi-dimensional matrix @var{c} is reshaped to a |
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598 two-dimensional matrix as @code{reshape (c, rows (c), prod (size (c)(2:end)))} |
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599 and then the same behavior as a loop over a two-dimensional matrix is produced. |
| 6587 | 600 |
| 3294 | 601 Although it is possible to rewrite all @code{for} loops as @code{while} |
| 602 loops, the Octave language has both statements because often a | |
| 603 @code{for} loop is both less work to type and more natural to think of. | |
| 604 Counting the number of iterations is very common in loops and it can be | |
| 605 easier to think of this counting as part of looping rather than as | |
| 606 something to do inside the loop. | |
| 607 | |
| 608 @menu | |
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609 * Looping Over Structure Elements:: |
| 3294 | 610 @end menu |
| 611 | |
| 4167 | 612 @node Looping Over Structure Elements |
| 3294 | 613 @subsection Looping Over Structure Elements |
| 614 @cindex structure elements, looping over | |
| 615 @cindex looping over structure elements | |
| 616 | |
| 617 A special form of the @code{for} statement allows you to loop over all | |
| 618 the elements of a structure: | |
| 619 | |
| 620 @example | |
| 621 @group | |
| 622 for [ @var{val}, @var{key} ] = @var{expression} | |
| 623 @var{body} | |
| 624 endfor | |
| 625 @end group | |
| 626 @end example | |
| 627 | |
| 628 @noindent | |
| 629 In this form of the @code{for} statement, the value of @var{expression} | |
| 630 must be a structure. If it is, @var{key} and @var{val} are set to the | |
| 631 name of the element and the corresponding value in turn, until there are | |
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632 no more elements. For example: |
| 3294 | 633 |
| 634 @example | |
| 635 @group | |
| 636 x.a = 1 | |
| 637 x.b = [1, 2; 3, 4] | |
| 638 x.c = "string" | |
| 639 for [val, key] = x | |
| 640 key | |
| 641 val | |
| 642 endfor | |
| 643 | |
| 644 @print{} key = a | |
| 645 @print{} val = 1 | |
| 646 @print{} key = b | |
| 647 @print{} val = | |
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648 @print{} |
| 3294 | 649 @print{} 1 2 |
| 650 @print{} 3 4 | |
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651 @print{} |
| 3294 | 652 @print{} key = c |
| 653 @print{} val = string | |
| 654 @end group | |
| 655 @end example | |
| 656 | |
| 657 The elements are not accessed in any particular order. If you need to | |
| 658 cycle through the list in a particular way, you will have to use the | |
| 6637 | 659 function @code{fieldnames} and sort the list yourself. |
| 3294 | 660 |
| 661 The @var{key} variable may also be omitted. If it is, the brackets are | |
| 662 also optional. This is useful for cycling through the values of all the | |
| 663 structure elements when the names of the elements do not need to be | |
| 664 known. | |
| 665 | |
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666 @node The break Statement |
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667 @section The break Statement |
| 3294 | 668 @cindex @code{break} statement |
| 669 | |
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670 The @code{break} statement jumps out of the innermost @code{while}, |
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671 @code{do-until}, or @code{for} loop that encloses it. The @code{break} |
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672 statement may only be used within the body of a loop. The following |
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673 example finds the smallest divisor of a given integer, and also |
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674 identifies prime numbers: |
| 3294 | 675 |
| 676 @example | |
| 677 @group | |
| 678 num = 103; | |
| 679 div = 2; | |
| 680 while (div*div <= num) | |
| 681 if (rem (num, div) == 0) | |
| 682 break; | |
| 683 endif | |
| 684 div++; | |
| 685 endwhile | |
| 686 if (rem (num, div) == 0) | |
| 687 printf ("Smallest divisor of %d is %d\n", num, div) | |
| 688 else | |
| 689 printf ("%d is prime\n", num); | |
| 690 endif | |
| 691 @end group | |
| 692 @end example | |
| 693 | |
| 694 When the remainder is zero in the first @code{while} statement, Octave | |
| 695 immediately @dfn{breaks out} of the loop. This means that Octave | |
| 696 proceeds immediately to the statement following the loop and continues | |
| 697 processing. (This is very different from the @code{exit} statement | |
| 698 which stops the entire Octave program.) | |
| 699 | |
| 700 Here is another program equivalent to the previous one. It illustrates | |
| 701 how the @var{condition} of a @code{while} statement could just as well | |
| 702 be replaced with a @code{break} inside an @code{if}: | |
| 703 | |
| 704 @example | |
| 705 @group | |
| 706 num = 103; | |
| 707 div = 2; | |
| 708 while (1) | |
| 709 if (rem (num, div) == 0) | |
| 710 printf ("Smallest divisor of %d is %d\n", num, div); | |
| 711 break; | |
| 712 endif | |
| 713 div++; | |
| 714 if (div*div > num) | |
| 715 printf ("%d is prime\n", num); | |
| 716 break; | |
| 717 endif | |
| 718 endwhile | |
| 719 @end group | |
| 720 @end example | |
| 721 | |
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722 @node The continue Statement |
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723 @section The continue Statement |
| 3294 | 724 @cindex @code{continue} statement |
| 725 | |
| 726 The @code{continue} statement, like @code{break}, is used only inside | |
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727 @code{while}, @code{do-until}, or @code{for} loops. It skips over the |
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728 rest of the loop body, causing the next cycle around the loop to begin |
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729 immediately. Contrast this with @code{break}, which jumps out of the |
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730 loop altogether. |
| 3294 | 731 Here is an example: |
| 732 | |
| 733 @example | |
| 734 @group | |
| 735 # print elements of a vector of random | |
| 736 # integers that are even. | |
| 737 | |
| 738 # first, create a row vector of 10 random | |
| 739 # integers with values between 0 and 100: | |
| 740 | |
| 741 vec = round (rand (1, 10) * 100); | |
| 742 | |
| 743 # print what we're interested in: | |
| 744 | |
| 745 for x = vec | |
| 746 if (rem (x, 2) != 0) | |
| 747 continue; | |
| 748 endif | |
| 749 printf ("%d\n", x); | |
| 750 endfor | |
| 751 @end group | |
| 752 @end example | |
| 753 | |
| 754 If one of the elements of @var{vec} is an odd number, this example skips | |
| 755 the print statement for that element, and continues back to the first | |
| 756 statement in the loop. | |
| 757 | |
| 758 This is not a practical example of the @code{continue} statement, but it | |
| 759 should give you a clear understanding of how it works. Normally, one | |
| 760 would probably write the loop like this: | |
| 761 | |
| 762 @example | |
| 763 @group | |
| 764 for x = vec | |
| 765 if (rem (x, 2) == 0) | |
| 766 printf ("%d\n", x); | |
| 767 endif | |
| 768 endfor | |
| 769 @end group | |
| 770 @end example | |
| 771 | |
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772 @node The unwind_protect Statement |
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773 @section The unwind_protect Statement |
| 3294 | 774 @cindex @code{unwind_protect} statement |
| 775 @cindex @code{unwind_protect_cleanup} | |
| 776 @cindex @code{end_unwind_protect} | |
| 777 | |
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778 Octave supports a limited form of exception handling modeled after the |
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779 unwind-protect form of Lisp. |
| 3294 | 780 |
| 781 The general form of an @code{unwind_protect} block looks like this: | |
| 782 | |
| 783 @example | |
| 784 @group | |
| 785 unwind_protect | |
| 786 @var{body} | |
| 787 unwind_protect_cleanup | |
| 788 @var{cleanup} | |
| 789 end_unwind_protect | |
| 790 @end group | |
| 791 @end example | |
| 792 | |
| 793 @noindent | |
| 6637 | 794 where @var{body} and @var{cleanup} are both optional and may contain any |
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795 Octave expressions or commands. The statements in @var{cleanup} are |
| 3294 | 796 guaranteed to be executed regardless of how control exits @var{body}. |
| 797 | |
| 798 This is useful to protect temporary changes to global variables from | |
| 799 possible errors. For example, the following code will always restore | |
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800 the original value of the global variable @code{frobnosticate} |
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801 even if an error occurs in the first part of the @code{unwind_protect} |
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802 block. |
| 3294 | 803 |
| 804 @example | |
| 805 @group | |
| 6501 | 806 save_frobnosticate = frobnosticate; |
| 3294 | 807 unwind_protect |
| 6501 | 808 frobnosticate = true; |
| 809 @dots{} | |
| 3294 | 810 unwind_protect_cleanup |
| 6501 | 811 frobnosticate = save_frobnosticate; |
| 3294 | 812 end_unwind_protect |
| 813 @end group | |
| 814 @end example | |
| 815 | |
| 6637 | 816 @noindent |
| 6501 | 817 Without @code{unwind_protect}, the value of @var{frobnosticate} |
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818 would not be restored if an error occurs while evaluating the first part |
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819 of the @code{unwind_protect} block because evaluation would stop at the |
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820 point of the error and the statement to restore the value would not be |
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821 executed. |
| 3294 | 822 |
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823 In addition to unwind_protect, Octave supports another form of |
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824 exception handling, the @code{try} block. |
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825 |
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826 |
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827 @node The try Statement |
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828 @section The try Statement |
| 3294 | 829 @cindex @code{try} statement |
| 830 @cindex @code{catch} | |
| 831 @cindex @code{end_try_catch} | |
| 832 | |
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833 The original form of a @code{try} block looks like this: |
| 3294 | 834 |
| 835 @example | |
| 836 @group | |
| 837 try | |
| 838 @var{body} | |
| 839 catch | |
| 840 @var{cleanup} | |
| 841 end_try_catch | |
| 842 @end group | |
| 843 @end example | |
| 844 | |
| 6637 | 845 @noindent |
| 846 where @var{body} and @var{cleanup} are both optional and may contain any | |
| 3294 | 847 Octave expressions or commands. The statements in @var{cleanup} are |
| 848 only executed if an error occurs in @var{body}. | |
| 849 | |
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850 No warnings or error messages are printed while @var{body} is executing. |
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851 If an error does occur during the execution of @var{body}, @var{cleanup} |
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852 can use the functions @code{lasterr} or @code{lasterror} to access the |
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853 text of the message that would have been printed, as well as its |
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854 identifier. The alternative form, |
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855 |
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856 @example |
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857 @group |
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858 try |
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859 @var{body} |
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860 catch @var{err} |
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861 @var{cleanup} |
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862 end_try_catch |
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863 @end group |
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864 @end example |
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865 |
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866 @noindent |
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867 will automatically store the output of @code{lasterror} in the structure |
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868 @var{err}. @xref{Errors and Warnings}, for more information about the |
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869 @code{lasterr} and @code{lasterror} functions. |
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870 |
| 3294 | 871 |
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872 @node Continuation Lines |
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873 @section Continuation Lines |
| 3294 | 874 @cindex continuation lines |
| 875 @cindex @code{...} continuation marker | |
| 876 @cindex @code{\} continuation marker | |
| 877 | |
| 878 In the Octave language, most statements end with a newline character and | |
| 879 you must tell Octave to ignore the newline character in order to | |
| 880 continue a statement from one line to the next. Lines that end with the | |
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881 characters @code{...} are joined with the following line before they are |
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882 divided into tokens by Octave's parser. For example, the lines |
| 3294 | 883 |
| 884 @example | |
| 885 @group | |
| 886 x = long_variable_name ... | |
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887 + longer_variable_name ... |
| 3294 | 888 - 42 |
| 889 @end group | |
| 890 @end example | |
| 891 | |
| 892 @noindent | |
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893 form a single statement. |
| 3294 | 894 |
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895 Any text between the continuation marker and the newline character is |
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896 ignored. For example, the statement |
| 3294 | 897 |
| 898 @example | |
| 899 @group | |
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900 x = long_variable_name ... # comment one |
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901 + longer_variable_name ...comment two |
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902 - 42 # last comment |
| 3294 | 903 @end group |
| 904 @end example | |
| 905 | |
| 906 @noindent | |
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907 is equivalent to the one shown above. |
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908 |
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909 Inside double-quoted string constants, the character @code{\} has to be |
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910 used as continuation marker. The @code{\} must appear at the end of the |
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911 line just before the newline character: |
| 3294 | 912 |
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913 @example |
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914 @group |
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915 s = "This text starts in the first line \ |
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916 and is continued in the second line." |
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917 @end group |
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918 @end example |
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919 |
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920 @noindent |
| 3294 | 921 Input that occurs inside parentheses can be continued to the next line |
| 922 without having to use a continuation marker. For example, it is | |
| 923 possible to write statements like | |
| 924 | |
| 925 @example | |
| 926 @group | |
| 927 if (fine_dining_destination == on_a_boat | |
| 928 || fine_dining_destination == on_a_train) | |
| 4541 | 929 seuss (i, will, not, eat, them, sam, i, am, i, |
| 3294 | 930 will, not, eat, green, eggs, and, ham); |
| 931 endif | |
| 932 @end group | |
| 933 @end example | |
| 934 | |
| 935 @noindent | |
| 936 without having to add to the clutter with continuation markers. |