Mercurial > octave-nkf
annotate doc/interpreter/stmt.txi @ 11255:d682cd6669ac
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| author | Rik <octave@nomad.inbox5.com> |
|---|---|
| date | Mon, 15 Nov 2010 17:57:43 -0800 |
| parents | 322f43e0e170 |
| children | fd0a3ac60b0e |
| rev | line source |
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| 8920 | 1 @c Copyright (C) 1996, 1997, 1999, 2000, 2002, 2003, 2004, 2007, 2008, 2009 |
| 2 @c John W. Eaton | |
| 7018 | 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 Statements |
| 3294 | 21 @chapter Statements |
| 22 @cindex statements | |
| 23 | |
| 24 Statements may be a simple constant expression or a complicated list of | |
| 25 nested loops and conditional statements. | |
| 26 | |
| 27 @dfn{Control statements} such as @code{if}, @code{while}, and so on | |
| 28 control the flow of execution in Octave programs. All the control | |
| 29 statements start with special keywords such as @code{if} and | |
| 30 @code{while}, to distinguish them from simple expressions. | |
| 31 Many control statements contain other statements; for example, the | |
| 32 @code{if} statement contains another statement which may or may not be | |
| 33 executed. | |
| 34 | |
| 35 @cindex @code{end} statement | |
| 36 Each control statement has a corresponding @dfn{end} statement that | |
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37 marks the end of the control statement. For example, the |
| 3294 | 38 keyword @code{endif} marks the end of an @code{if} statement, and |
| 39 @code{endwhile} marks the end of a @code{while} statement. You can use | |
| 40 the keyword @code{end} anywhere a more specific end keyword is expected, | |
| 41 but using the more specific keywords is preferred because if you use | |
| 42 them, Octave is able to provide better diagnostics for mismatched or | |
| 43 missing end tokens. | |
| 44 | |
| 45 The list of statements contained between keywords like @code{if} or | |
| 46 @code{while} and the corresponding end statement is called the | |
| 47 @dfn{body} of a control statement. | |
| 48 | |
| 49 @menu | |
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50 * The @code{if} Statement:: |
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51 * The @code{switch} Statement:: |
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52 * The @code{while} Statement:: |
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53 * The @code{do-until} Statement:: |
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54 * The @code{for} Statement:: |
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55 * The @code{break} Statement:: |
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56 * The @code{continue} Statement:: |
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57 * The @code{unwind_protect} Statement:: |
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58 * The @code{try} Statement:: |
| 3294 | 59 * Continuation Lines:: |
| 60 @end menu | |
| 61 | |
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62 @node The @code{if} Statement |
| 3294 | 63 @section The @code{if} Statement |
| 64 @cindex @code{if} statement | |
| 65 @cindex @code{else} statement | |
| 66 @cindex @code{elseif} statement | |
| 67 @cindex @code{endif} statement | |
| 68 | |
| 69 The @code{if} statement is Octave's decision-making statement. There | |
| 70 are three basic forms of an @code{if} statement. In its simplest form, | |
| 71 it looks like this: | |
| 72 | |
| 73 @example | |
| 74 @group | |
| 75 if (@var{condition}) | |
| 76 @var{then-body} | |
| 77 endif | |
| 78 @end group | |
| 79 @end example | |
| 80 | |
| 81 @noindent | |
| 82 @var{condition} is an expression that controls what the rest of the | |
| 83 statement will do. The @var{then-body} is executed only if | |
| 84 @var{condition} is true. | |
| 85 | |
| 86 The condition in an @code{if} statement is considered true if its value | |
| 87 is non-zero, and false if its value is zero. If the value of the | |
| 88 conditional expression in an @code{if} statement is a vector or a | |
| 6637 | 89 matrix, it is considered true only if it is non-empty and @emph{all} |
| 90 of the elements are non-zero. | |
| 3294 | 91 |
| 92 The second form of an if statement looks like this: | |
| 93 | |
| 94 @example | |
| 95 @group | |
| 96 if (@var{condition}) | |
| 97 @var{then-body} | |
| 98 else | |
| 99 @var{else-body} | |
| 100 endif | |
| 101 @end group | |
| 102 @end example | |
| 103 | |
| 104 @noindent | |
| 105 If @var{condition} is true, @var{then-body} is executed; otherwise, | |
| 106 @var{else-body} is executed. | |
| 107 | |
| 108 Here is an example: | |
| 109 | |
| 110 @example | |
| 111 @group | |
| 112 if (rem (x, 2) == 0) | |
| 113 printf ("x is even\n"); | |
| 114 else | |
| 115 printf ("x is odd\n"); | |
| 116 endif | |
| 117 @end group | |
| 118 @end example | |
| 119 | |
| 120 In this example, if the expression @code{rem (x, 2) == 0} is true (that | |
| 121 is, the value of @code{x} is divisible by 2), then the first | |
| 122 @code{printf} statement is evaluated, otherwise the second @code{printf} | |
| 123 statement is evaluated. | |
| 124 | |
| 125 The third and most general form of the @code{if} statement allows | |
| 126 multiple decisions to be combined in a single statement. It looks like | |
| 127 this: | |
| 128 | |
| 129 @example | |
| 130 @group | |
| 131 if (@var{condition}) | |
| 132 @var{then-body} | |
| 133 elseif (@var{condition}) | |
| 134 @var{elseif-body} | |
| 135 else | |
| 136 @var{else-body} | |
| 137 endif | |
| 138 @end group | |
| 139 @end example | |
| 140 | |
| 141 @noindent | |
| 142 Any number of @code{elseif} clauses may appear. Each condition is | |
| 143 tested in turn, and if one is found to be true, its corresponding | |
| 144 @var{body} is executed. If none of the conditions are true and the | |
| 145 @code{else} clause is present, its body is executed. Only one | |
| 146 @code{else} clause may appear, and it must be the last part of the | |
| 147 statement. | |
| 148 | |
| 149 In the following example, if the first condition is true (that is, the | |
| 150 value of @code{x} is divisible by 2), then the first @code{printf} | |
| 151 statement is executed. If it is false, then the second condition is | |
| 152 tested, and if it is true (that is, the value of @code{x} is divisible | |
| 153 by 3), then the second @code{printf} statement is executed. Otherwise, | |
| 154 the third @code{printf} statement is performed. | |
| 155 | |
| 156 @example | |
| 157 @group | |
| 158 if (rem (x, 2) == 0) | |
| 159 printf ("x is even\n"); | |
| 160 elseif (rem (x, 3) == 0) | |
| 161 printf ("x is odd and divisible by 3\n"); | |
| 162 else | |
| 163 printf ("x is odd\n"); | |
| 164 endif | |
| 165 @end group | |
| 166 @end example | |
| 167 | |
| 168 Note that the @code{elseif} keyword must not be spelled @code{else if}, | |
| 169 as is allowed in Fortran. If it is, the space between the @code{else} | |
| 170 and @code{if} will tell Octave to treat this as a new @code{if} | |
| 171 statement within another @code{if} statement's @code{else} clause. For | |
| 172 example, if you write | |
| 173 | |
| 174 @example | |
| 175 @group | |
| 176 if (@var{c1}) | |
| 177 @var{body-1} | |
| 178 else if (@var{c2}) | |
| 179 @var{body-2} | |
| 180 endif | |
| 181 @end group | |
| 182 @end example | |
| 183 | |
| 184 @noindent | |
| 185 Octave will expect additional input to complete the first @code{if} | |
| 186 statement. If you are using Octave interactively, it will continue to | |
| 187 prompt you for additional input. If Octave is reading this input from a | |
| 188 file, it may complain about missing or mismatched @code{end} statements, | |
| 189 or, if you have not used the more specific @code{end} statements | |
| 190 (@code{endif}, @code{endfor}, etc.), it may simply produce incorrect | |
| 191 results, without producing any warning messages. | |
| 192 | |
| 193 It is much easier to see the error if we rewrite the statements above | |
| 194 like this, | |
| 195 | |
| 196 @example | |
| 197 @group | |
| 198 if (@var{c1}) | |
| 199 @var{body-1} | |
| 200 else | |
| 201 if (@var{c2}) | |
| 202 @var{body-2} | |
| 203 endif | |
| 204 @end group | |
| 205 @end example | |
| 206 | |
| 207 @noindent | |
| 208 using the indentation to show how Octave groups the statements. | |
| 209 @xref{Functions and Scripts}. | |
| 210 | |
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211 @node The @code{switch} Statement |
| 3294 | 212 @section The @code{switch} Statement |
| 213 @cindex @code{switch} statement | |
| 214 @cindex @code{case} statement | |
| 215 @cindex @code{otherwise} statement | |
| 216 @cindex @code{endswitch} statement | |
| 217 | |
| 6530 | 218 It is very common to take different actions depending on the value of |
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219 one variable. This is possible using the @code{if} statement in the |
| 6530 | 220 following way |
| 221 | |
| 222 @example | |
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223 @group |
| 6530 | 224 if (X == 1) |
| 225 do_something (); | |
| 226 elseif (X == 2) | |
| 227 do_something_else (); | |
| 228 else | |
| 229 do_something_completely_different (); | |
| 230 endif | |
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231 @end group |
| 6530 | 232 @end example |
| 233 | |
| 234 @noindent | |
| 235 This kind of code can however be very cumbersome to both write and | |
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236 maintain. To overcome this problem Octave supports the @code{switch} |
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237 statement. Using this statement, the above example becomes |
| 6530 | 238 |
| 239 @example | |
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240 @group |
| 6530 | 241 switch (X) |
| 242 case 1 | |
| 243 do_something (); | |
| 244 case 2 | |
| 245 do_something_else (); | |
| 246 otherwise | |
| 247 do_something_completely_different (); | |
| 248 endswitch | |
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249 @end group |
| 6530 | 250 @end example |
| 251 | |
| 252 @noindent | |
| 253 This code makes the repetitive structure of the problem more explicit, | |
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254 making the code easier to read, and hence maintain. Also, if the |
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255 variable @code{X} should change its name, only one line would need |
| 6530 | 256 changing compared to one line per case when @code{if} statements are |
| 257 used. | |
| 3294 | 258 |
| 259 The general form of the @code{switch} statement is | |
| 260 | |
| 261 @example | |
| 262 @group | |
| 263 switch @var{expression} | |
| 264 case @var{label} | |
| 265 @var{command_list} | |
| 266 case @var{label} | |
| 267 @var{command_list} | |
| 268 @dots{} | |
| 269 | |
| 270 otherwise | |
| 271 @var{command_list} | |
| 272 endswitch | |
| 273 @end group | |
| 274 @end example | |
| 275 | |
| 6530 | 276 @noindent |
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277 where @var{label} can be any expression. However, duplicate |
| 6530 | 278 @var{label} values are not detected, and only the @var{command_list} |
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279 corresponding to the first match will be executed. For the |
| 6530 | 280 @code{switch} statement to be meaningful at least one |
| 281 @code{case @var{label} @var{command_list}} clause must be present, | |
| 282 while the @code{otherwise @var{command_list}} clause is optional. | |
| 3294 | 283 |
| 6637 | 284 If @var{label} is a cell array the corresponding @var{command_list} |
| 285 is executed if @emph{any} of the elements of the cell array match | |
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286 @var{expression}. As an example, the following program will print |
| 6637 | 287 @samp{Variable is either 6 or 7}. |
| 288 | |
| 289 @example | |
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290 @group |
| 6637 | 291 A = 7; |
| 292 switch A | |
| 293 case @{ 6, 7 @} | |
| 294 printf ("variable is either 6 or 7\n"); | |
| 295 otherwise | |
| 296 printf ("variable is neither 6 nor 7\n"); | |
| 297 endswitch | |
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298 @end group |
| 6637 | 299 @end example |
| 300 | |
| 3294 | 301 As with all other specific @code{end} keywords, @code{endswitch} may be |
| 302 replaced by @code{end}, but you can get better diagnostics if you use | |
| 303 the specific forms. | |
| 304 | |
| 6530 | 305 @c Strings can be matched |
| 306 | |
| 307 One advantage of using the @code{switch} statement compared to using | |
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308 @code{if} statements is that the @var{label}s can be strings. If an |
| 6530 | 309 @code{if} statement is used it is @emph{not} possible to write |
| 310 | |
| 311 @example | |
| 312 if (X == "a string") # This is NOT valid | |
| 313 @end example | |
| 314 | |
| 315 @noindent | |
| 316 since a character-to-character comparison between @code{X} and the | |
| 317 string will be made instead of evaluating if the strings are equal. | |
| 318 This special-case is handled by the @code{switch} statement, and it | |
| 319 is possible to write programs that look like this | |
| 320 | |
| 321 @example | |
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322 @group |
| 6530 | 323 switch (X) |
| 324 case "a string" | |
| 325 do_something | |
| 326 @dots{} | |
| 327 endswitch | |
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328 @end group |
| 6530 | 329 @end example |
| 330 | |
| 6535 | 331 @menu |
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332 * Notes for the C Programmer:: |
| 6535 | 333 @end menu |
| 334 | |
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335 @node Notes for the C Programmer |
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336 @subsection Notes for the C Programmer |
| 6530 | 337 |
| 6637 | 338 The @code{switch} statement is also available in the widely used C |
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339 programming language. There are, however, some differences |
| 6530 | 340 between the statement in Octave and C |
| 341 | |
| 342 @itemize @bullet | |
| 3294 | 343 @item |
| 344 Cases are exclusive, so they don't `fall through' as do the cases | |
| 6637 | 345 in the @code{switch} statement of the C language. |
| 3294 | 346 |
| 347 @item | |
| 348 The @var{command_list} elements are not optional. Making the list | |
| 349 optional would have meant requiring a separator between the label and | |
| 350 the command list. Otherwise, things like | |
| 351 | |
| 352 @example | |
| 353 @group | |
| 354 switch (foo) | |
| 355 case (1) -2 | |
| 356 @dots{} | |
| 357 @end group | |
| 358 @end example | |
| 359 | |
| 360 @noindent | |
| 361 would produce surprising results, as would | |
| 362 | |
| 363 @example | |
| 364 @group | |
| 365 switch (foo) | |
| 366 case (1) | |
| 367 case (2) | |
| 368 doit (); | |
| 369 @dots{} | |
| 370 @end group | |
| 371 @end example | |
| 372 | |
| 373 @noindent | |
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374 particularly for C programmers. If @code{doit()} should be executed if |
| 6637 | 375 @var{foo} is either @code{1} or @code{2}, the above code should be |
| 376 written with a cell array like this | |
| 377 | |
| 378 @example | |
| 379 @group | |
| 380 switch (foo) | |
| 381 case @{ 1, 2 @} | |
| 382 doit (); | |
| 383 @dots{} | |
| 384 @end group | |
| 385 @end example | |
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386 |
| 3294 | 387 @end itemize |
| 388 | |
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389 @node The @code{while} Statement |
| 3294 | 390 @section The @code{while} Statement |
| 391 @cindex @code{while} statement | |
| 392 @cindex @code{endwhile} statement | |
| 393 @cindex loop | |
| 394 @cindex body of a loop | |
| 395 | |
| 396 In programming, a @dfn{loop} means a part of a program that is (or at least can | |
| 397 be) executed two or more times in succession. | |
| 398 | |
| 399 The @code{while} statement is the simplest looping statement in Octave. | |
| 400 It repeatedly executes a statement as long as a condition is true. As | |
| 401 with the condition in an @code{if} statement, the condition in a | |
| 402 @code{while} statement is considered true if its value is non-zero, and | |
| 403 false if its value is zero. If the value of the conditional expression | |
| 404 in a @code{while} statement is a vector or a matrix, it is considered | |
| 6637 | 405 true only if it is non-empty and @emph{all} of the elements are non-zero. |
| 3294 | 406 |
| 407 Octave's @code{while} statement looks like this: | |
| 408 | |
| 409 @example | |
| 410 @group | |
| 411 while (@var{condition}) | |
| 412 @var{body} | |
| 413 endwhile | |
| 414 @end group | |
| 415 @end example | |
| 416 | |
| 417 @noindent | |
| 418 Here @var{body} is a statement or list of statements that we call the | |
| 419 @dfn{body} of the loop, and @var{condition} is an expression that | |
| 420 controls how long the loop keeps running. | |
| 421 | |
| 422 The first thing the @code{while} statement does is test @var{condition}. | |
| 423 If @var{condition} is true, it executes the statement @var{body}. After | |
| 424 @var{body} has been executed, @var{condition} is tested again, and if it | |
| 425 is still true, @var{body} is executed again. This process repeats until | |
| 426 @var{condition} is no longer true. If @var{condition} is initially | |
| 427 false, the body of the loop is never executed. | |
| 428 | |
| 429 This example creates a variable @code{fib} that contains the first ten | |
| 430 elements of the Fibonacci sequence. | |
| 431 | |
| 432 @example | |
| 433 @group | |
| 434 fib = ones (1, 10); | |
| 435 i = 3; | |
| 436 while (i <= 10) | |
| 437 fib (i) = fib (i-1) + fib (i-2); | |
| 438 i++; | |
| 439 endwhile | |
| 440 @end group | |
| 441 @end example | |
| 442 | |
| 443 @noindent | |
| 444 Here the body of the loop contains two statements. | |
| 445 | |
| 446 The loop works like this: first, the value of @code{i} is set to 3. | |
| 447 Then, the @code{while} tests whether @code{i} is less than or equal to | |
| 448 10. This is the case when @code{i} equals 3, so the value of the | |
| 449 @code{i}-th element of @code{fib} is set to the sum of the previous two | |
| 450 values in the sequence. Then the @code{i++} increments the value of | |
| 451 @code{i} and the loop repeats. The loop terminates when @code{i} | |
| 452 reaches 11. | |
| 453 | |
| 454 A newline is not required between the condition and the | |
| 455 body; but using one makes the program clearer unless the body is very | |
| 456 simple. | |
| 457 | |
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458 @node The @code{do-until} Statement |
| 3489 | 459 @section The @code{do-until} Statement |
| 460 @cindex @code{do-until} statement | |
| 461 | |
| 462 The @code{do-until} statement is similar to the @code{while} statement, | |
| 463 except that it repeatedly executes a statement until a condition becomes | |
| 464 true, and the test of the condition is at the end of the loop, so the | |
| 465 body of the loop is always executed at least once. As with the | |
| 466 condition in an @code{if} statement, the condition in a @code{do-until} | |
| 467 statement is considered true if its value is non-zero, and false if its | |
| 468 value is zero. If the value of the conditional expression in a | |
| 469 @code{do-until} statement is a vector or a matrix, it is considered | |
| 6637 | 470 true only if it is non-empty and @emph{all} of the elements are non-zero. |
| 3489 | 471 |
| 472 Octave's @code{do-until} statement looks like this: | |
| 473 | |
| 474 @example | |
| 475 @group | |
| 476 do | |
| 477 @var{body} | |
| 478 until (@var{condition}) | |
| 479 @end group | |
| 480 @end example | |
| 481 | |
| 482 @noindent | |
| 483 Here @var{body} is a statement or list of statements that we call the | |
| 484 @dfn{body} of the loop, and @var{condition} is an expression that | |
| 485 controls how long the loop keeps running. | |
| 486 | |
| 487 This example creates a variable @code{fib} that contains the first ten | |
| 488 elements of the Fibonacci sequence. | |
| 489 | |
| 490 @example | |
| 491 @group | |
| 492 fib = ones (1, 10); | |
| 493 i = 2; | |
| 494 do | |
| 495 i++; | |
| 496 fib (i) = fib (i-1) + fib (i-2); | |
| 497 until (i == 10) | |
| 498 @end group | |
| 499 @end example | |
| 500 | |
| 501 A newline is not required between the @code{do} keyword and the | |
| 502 body; but using one makes the program clearer unless the body is very | |
| 503 simple. | |
| 504 | |
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505 @node The @code{for} Statement |
| 3294 | 506 @section The @code{for} Statement |
| 507 @cindex @code{for} statement | |
| 508 @cindex @code{endfor} statement | |
| 509 | |
| 510 The @code{for} statement makes it more convenient to count iterations of a | |
| 511 loop. The general form of the @code{for} statement looks like this: | |
| 512 | |
| 513 @example | |
| 514 @group | |
| 515 for @var{var} = @var{expression} | |
| 516 @var{body} | |
| 517 endfor | |
| 518 @end group | |
| 519 @end example | |
| 520 | |
| 521 @noindent | |
| 522 where @var{body} stands for any statement or list of statements, | |
| 523 @var{expression} is any valid expression, and @var{var} may take several | |
| 524 forms. Usually it is a simple variable name or an indexed variable. If | |
| 525 the value of @var{expression} is a structure, @var{var} may also be a | |
| 6637 | 526 vector with two elements. @xref{Looping Over Structure Elements}, below. |
| 3294 | 527 |
| 528 The assignment expression in the @code{for} statement works a bit | |
| 529 differently than Octave's normal assignment statement. Instead of | |
| 530 assigning the complete result of the expression, it assigns each column | |
| 531 of the expression to @var{var} in turn. If @var{expression} is a range, | |
| 532 a row vector, or a scalar, the value of @var{var} will be a scalar each | |
| 533 time the loop body is executed. If @var{var} is a column vector or a | |
| 534 matrix, @var{var} will be a column vector each time the loop body is | |
| 535 executed. | |
| 536 | |
| 537 The following example shows another way to create a vector containing | |
| 538 the first ten elements of the Fibonacci sequence, this time using the | |
| 539 @code{for} statement: | |
| 540 | |
| 541 @example | |
| 542 @group | |
| 543 fib = ones (1, 10); | |
| 544 for i = 3:10 | |
| 545 fib (i) = fib (i-1) + fib (i-2); | |
| 546 endfor | |
| 547 @end group | |
| 548 @end example | |
| 549 | |
| 550 @noindent | |
| 551 This code works by first evaluating the expression @code{3:10}, to | |
| 552 produce a range of values from 3 to 10 inclusive. Then the variable | |
| 553 @code{i} is assigned the first element of the range and the body of the | |
| 554 loop is executed once. When the end of the loop body is reached, the | |
| 555 next value in the range is assigned to the variable @code{i}, and the | |
| 556 loop body is executed again. This process continues until there are no | |
| 557 more elements to assign. | |
| 558 | |
| 6587 | 559 Within Octave is it also possible to iterate over matrices or cell arrays |
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560 using the @code{for} statement. For example consider |
| 6587 | 561 |
| 562 @example | |
| 563 @group | |
| 564 disp("Loop over a matrix") | |
| 565 for i = [1,3;2,4] | |
| 566 i | |
| 567 endfor | |
| 568 disp("Loop over a cell array") | |
| 569 for i = @{1,"two";"three",4@} | |
| 570 i | |
| 571 endfor | |
| 572 @end group | |
| 573 @end example | |
| 574 | |
| 575 @noindent | |
| 576 In this case the variable @code{i} takes on the value of the columns of | |
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577 the matrix or cell matrix. So the first loop iterates twice, producing |
| 7001 | 578 two column vectors @code{[1;2]}, followed by @code{[3;4]}, and likewise |
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579 for the loop over the cell array. This can be extended to loops over |
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580 multi-dimensional arrays. For example: |
| 6587 | 581 |
| 582 @example | |
| 583 @group | |
| 584 a = [1,3;2,4]; b = cat(3, a, 2*a); | |
| 585 for i = c | |
| 586 i | |
| 587 endfor | |
| 588 @end group | |
| 589 @end example | |
| 590 | |
| 591 @noindent | |
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592 In the above case, the multi-dimensional matrix @var{c} is reshaped to a |
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593 two-dimensional matrix as @code{reshape (c, rows(c), |
| 6587 | 594 prod(size(c)(2:end)))} and then the same behavior as a loop over a two |
| 595 dimensional matrix is produced. | |
| 596 | |
| 3294 | 597 Although it is possible to rewrite all @code{for} loops as @code{while} |
| 598 loops, the Octave language has both statements because often a | |
| 599 @code{for} loop is both less work to type and more natural to think of. | |
| 600 Counting the number of iterations is very common in loops and it can be | |
| 601 easier to think of this counting as part of looping rather than as | |
| 602 something to do inside the loop. | |
| 603 | |
| 604 @menu | |
| 605 * Looping Over Structure Elements:: | |
| 606 @end menu | |
| 607 | |
| 4167 | 608 @node Looping Over Structure Elements |
| 3294 | 609 @subsection Looping Over Structure Elements |
| 610 @cindex structure elements, looping over | |
| 611 @cindex looping over structure elements | |
| 612 | |
| 613 A special form of the @code{for} statement allows you to loop over all | |
| 614 the elements of a structure: | |
| 615 | |
| 616 @example | |
| 617 @group | |
| 618 for [ @var{val}, @var{key} ] = @var{expression} | |
| 619 @var{body} | |
| 620 endfor | |
| 621 @end group | |
| 622 @end example | |
| 623 | |
| 624 @noindent | |
| 625 In this form of the @code{for} statement, the value of @var{expression} | |
| 626 must be a structure. If it is, @var{key} and @var{val} are set to the | |
| 627 name of the element and the corresponding value in turn, until there are | |
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628 no more elements. For example: |
| 3294 | 629 |
| 630 @example | |
| 631 @group | |
| 632 x.a = 1 | |
| 633 x.b = [1, 2; 3, 4] | |
| 634 x.c = "string" | |
| 635 for [val, key] = x | |
| 636 key | |
| 637 val | |
| 638 endfor | |
| 639 | |
| 640 @print{} key = a | |
| 641 @print{} val = 1 | |
| 642 @print{} key = b | |
| 643 @print{} val = | |
| 644 @print{} | |
| 645 @print{} 1 2 | |
| 646 @print{} 3 4 | |
| 647 @print{} | |
| 648 @print{} key = c | |
| 649 @print{} val = string | |
| 650 @end group | |
| 651 @end example | |
| 652 | |
| 653 The elements are not accessed in any particular order. If you need to | |
| 654 cycle through the list in a particular way, you will have to use the | |
| 6637 | 655 function @code{fieldnames} and sort the list yourself. |
| 3294 | 656 |
| 657 The @var{key} variable may also be omitted. If it is, the brackets are | |
| 658 also optional. This is useful for cycling through the values of all the | |
| 659 structure elements when the names of the elements do not need to be | |
| 660 known. | |
| 661 | |
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662 @node The @code{break} Statement |
| 3294 | 663 @section The @code{break} Statement |
| 664 @cindex @code{break} statement | |
| 665 | |
| 666 The @code{break} statement jumps out of the innermost @code{for} or | |
| 667 @code{while} loop that encloses it. The @code{break} statement may only | |
| 668 be used within the body of a loop. The following example finds the | |
| 669 smallest divisor of a given integer, and also identifies prime numbers: | |
| 670 | |
| 671 @example | |
| 672 @group | |
| 673 num = 103; | |
| 674 div = 2; | |
| 675 while (div*div <= num) | |
| 676 if (rem (num, div) == 0) | |
| 677 break; | |
| 678 endif | |
| 679 div++; | |
| 680 endwhile | |
| 681 if (rem (num, div) == 0) | |
| 682 printf ("Smallest divisor of %d is %d\n", num, div) | |
| 683 else | |
| 684 printf ("%d is prime\n", num); | |
| 685 endif | |
| 686 @end group | |
| 687 @end example | |
| 688 | |
| 689 When the remainder is zero in the first @code{while} statement, Octave | |
| 690 immediately @dfn{breaks out} of the loop. This means that Octave | |
| 691 proceeds immediately to the statement following the loop and continues | |
| 692 processing. (This is very different from the @code{exit} statement | |
| 693 which stops the entire Octave program.) | |
| 694 | |
| 695 Here is another program equivalent to the previous one. It illustrates | |
| 696 how the @var{condition} of a @code{while} statement could just as well | |
| 697 be replaced with a @code{break} inside an @code{if}: | |
| 698 | |
| 699 @example | |
| 700 @group | |
| 701 num = 103; | |
| 702 div = 2; | |
| 703 while (1) | |
| 704 if (rem (num, div) == 0) | |
| 705 printf ("Smallest divisor of %d is %d\n", num, div); | |
| 706 break; | |
| 707 endif | |
| 708 div++; | |
| 709 if (div*div > num) | |
| 710 printf ("%d is prime\n", num); | |
| 711 break; | |
| 712 endif | |
| 713 endwhile | |
| 714 @end group | |
| 715 @end example | |
| 716 | |
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717 @node The @code{continue} Statement |
| 3294 | 718 @section The @code{continue} Statement |
| 719 @cindex @code{continue} statement | |
| 720 | |
| 721 The @code{continue} statement, like @code{break}, is used only inside | |
| 722 @code{for} or @code{while} loops. It skips over the rest of the loop | |
| 723 body, causing the next cycle around the loop to begin immediately. | |
| 724 Contrast this with @code{break}, which jumps out of the loop altogether. | |
| 725 Here is an example: | |
| 726 | |
| 727 @example | |
| 728 @group | |
| 729 # print elements of a vector of random | |
| 730 # integers that are even. | |
| 731 | |
| 732 # first, create a row vector of 10 random | |
| 733 # integers with values between 0 and 100: | |
| 734 | |
| 735 vec = round (rand (1, 10) * 100); | |
| 736 | |
| 737 # print what we're interested in: | |
| 738 | |
| 739 for x = vec | |
| 740 if (rem (x, 2) != 0) | |
| 741 continue; | |
| 742 endif | |
| 743 printf ("%d\n", x); | |
| 744 endfor | |
| 745 @end group | |
| 746 @end example | |
| 747 | |
| 748 If one of the elements of @var{vec} is an odd number, this example skips | |
| 749 the print statement for that element, and continues back to the first | |
| 750 statement in the loop. | |
| 751 | |
| 752 This is not a practical example of the @code{continue} statement, but it | |
| 753 should give you a clear understanding of how it works. Normally, one | |
| 754 would probably write the loop like this: | |
| 755 | |
| 756 @example | |
| 757 @group | |
| 758 for x = vec | |
| 759 if (rem (x, 2) == 0) | |
| 760 printf ("%d\n", x); | |
| 761 endif | |
| 762 endfor | |
| 763 @end group | |
| 764 @end example | |
| 765 | |
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766 @node The @code{unwind_protect} Statement |
| 3294 | 767 @section The @code{unwind_protect} Statement |
| 768 @cindex @code{unwind_protect} statement | |
| 769 @cindex @code{unwind_protect_cleanup} | |
| 770 @cindex @code{end_unwind_protect} | |
| 771 | |
| 772 Octave supports a limited form of exception handling modelled after the | |
| 773 unwind-protect form of Lisp. | |
| 774 | |
| 775 The general form of an @code{unwind_protect} block looks like this: | |
| 776 | |
| 777 @example | |
| 778 @group | |
| 779 unwind_protect | |
| 780 @var{body} | |
| 781 unwind_protect_cleanup | |
| 782 @var{cleanup} | |
| 783 end_unwind_protect | |
| 784 @end group | |
| 785 @end example | |
| 786 | |
| 787 @noindent | |
| 6637 | 788 where @var{body} and @var{cleanup} are both optional and may contain any |
| 3294 | 789 Octave expressions or commands. The statements in @var{cleanup} are |
| 790 guaranteed to be executed regardless of how control exits @var{body}. | |
| 791 | |
| 792 This is useful to protect temporary changes to global variables from | |
| 793 possible errors. For example, the following code will always restore | |
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794 the original value of the global variable @code{frobnosticate} |
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795 even if an error occurs in the first part of the @code{unwind_protect} |
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796 block. |
| 3294 | 797 |
| 798 @example | |
| 799 @group | |
| 6501 | 800 save_frobnosticate = frobnosticate; |
| 3294 | 801 unwind_protect |
| 6501 | 802 frobnosticate = true; |
| 803 @dots{} | |
| 3294 | 804 unwind_protect_cleanup |
| 6501 | 805 frobnosticate = save_frobnosticate; |
| 3294 | 806 end_unwind_protect |
| 807 @end group | |
| 808 @end example | |
| 809 | |
| 6637 | 810 @noindent |
| 6501 | 811 Without @code{unwind_protect}, the value of @var{frobnosticate} |
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813 of the @code{unwind_protect} block because evaluation would stop at the |
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814 point of the error and the statement to restore the value would not be |
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815 executed. |
| 3294 | 816 |
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817 @node The @code{try} Statement |
| 3294 | 818 @section The @code{try} Statement |
| 819 @cindex @code{try} statement | |
| 820 @cindex @code{catch} | |
| 821 @cindex @code{end_try_catch} | |
| 822 | |
| 823 In addition to unwind_protect, Octave supports another limited form of | |
| 824 exception handling. | |
| 825 | |
| 826 The general form of a @code{try} block looks like this: | |
| 827 | |
| 828 @example | |
| 829 @group | |
| 830 try | |
| 831 @var{body} | |
| 832 catch | |
| 833 @var{cleanup} | |
| 834 end_try_catch | |
| 835 @end group | |
| 836 @end example | |
| 837 | |
| 6637 | 838 @noindent |
| 839 where @var{body} and @var{cleanup} are both optional and may contain any | |
| 3294 | 840 Octave expressions or commands. The statements in @var{cleanup} are |
| 841 only executed if an error occurs in @var{body}. | |
| 842 | |
| 843 No warnings or error messages are printed while @var{body} is | |
| 844 executing. If an error does occur during the execution of @var{body}, | |
| 4699 | 845 @var{cleanup} can use the function @code{lasterr} to access the text |
| 846 of the message that would have been printed. This is the same | |
| 847 as @code{eval (@var{try}, @var{catch})} but it is more efficient since | |
| 848 the commands do not need to be parsed each time the @var{try} and | |
| 6667 | 849 @var{catch} statements are evaluated. @xref{Errors and Warnings}, for more |
| 4699 | 850 information about the @code{lasterr} function. |
| 3294 | 851 |
| 852 @cindex continuation lines | |
| 853 @cindex @code{...} continuation marker | |
| 854 @cindex @code{\} continuation marker | |
| 855 | |
| 4167 | 856 @node Continuation Lines |
| 3294 | 857 @section Continuation Lines |
| 858 | |
| 859 In the Octave language, most statements end with a newline character and | |
| 860 you must tell Octave to ignore the newline character in order to | |
| 861 continue a statement from one line to the next. Lines that end with the | |
| 862 characters @code{...} or @code{\} are joined with the following line | |
| 863 before they are divided into tokens by Octave's parser. For example, | |
| 864 the lines | |
| 865 | |
| 866 @example | |
| 867 @group | |
| 868 x = long_variable_name ... | |
| 869 + longer_variable_name \ | |
| 870 - 42 | |
| 871 @end group | |
| 872 @end example | |
| 873 | |
| 874 @noindent | |
| 875 form a single statement. The backslash character on the second line | |
| 6637 | 876 above is interpreted as a continuation character, @emph{not} as a division |
| 3294 | 877 operator. |
| 878 | |
| 879 For continuation lines that do not occur inside string constants, | |
| 880 whitespace and comments may appear between the continuation marker and | |
| 881 the newline character. For example, the statement | |
| 882 | |
| 883 @example | |
| 884 @group | |
| 885 x = long_variable_name ... # comment one | |
| 886 + longer_variable_name \ # comment two | |
| 887 - 42 # last comment | |
| 888 @end group | |
| 889 @end example | |
| 890 | |
| 891 @noindent | |
| 892 is equivalent to the one shown above. Inside string constants, the | |
| 893 continuation marker must appear at the end of the line just before the | |
| 894 newline character. | |
| 895 | |
| 896 Input that occurs inside parentheses can be continued to the next line | |
| 897 without having to use a continuation marker. For example, it is | |
| 898 possible to write statements like | |
| 899 | |
| 900 @example | |
| 901 @group | |
| 902 if (fine_dining_destination == on_a_boat | |
| 903 || fine_dining_destination == on_a_train) | |
| 4541 | 904 seuss (i, will, not, eat, them, sam, i, am, i, |
| 3294 | 905 will, not, eat, green, eggs, and, ham); |
| 906 endif | |
| 907 @end group | |
| 908 @end example | |
| 909 | |
| 910 @noindent | |
| 911 without having to add to the clutter with continuation markers. |