Mercurial > hg > octave-lyh
annotate doc/interpreter/io.txi @ 11148:c9ec943ebe65
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| author | Ben Abbott <bpabbott@mac.com> |
|---|---|
| date | Sat, 23 Oct 2010 02:31:08 -0400 |
| parents | a81994607ca0 |
| children | fd0a3ac60b0e |
| rev | line source |
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| 8920 | 1 @c Copyright (C) 1996, 1997, 2007, 2008, 2009 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. | |
| 9 @c | |
| 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. | |
| 14 @c | |
| 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 |
| 4169 | 19 @node Input and Output |
| 3294 | 20 @chapter Input and Output |
| 21 | |
| 6666 | 22 Octave supports several ways of reading and writing data to or from the |
| 8828 | 23 prompt or a file. The simplest functions for data Input and Output |
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24 (I/O) are easy to use, but only provides limited control of how |
| 6666 | 25 data is processed. For more control, a set of functions modelled |
| 26 after the C standard library are also provided by Octave. | |
| 27 | |
| 28 @menu | |
| 29 * Basic Input and Output:: | |
| 30 * C-Style I/O Functions:: | |
| 31 @end menu | |
| 32 | |
| 33 @node Basic Input and Output | |
| 34 @section Basic Input and Output | |
| 35 | |
| 36 @c We could use a two-line introduction here... | |
| 37 | |
| 38 @menu | |
| 39 * Terminal Output:: | |
| 40 * Terminal Input:: | |
| 41 * Simple File I/O:: | |
| 6788 | 42 * Rational Approximations:: |
| 6666 | 43 @end menu |
| 44 | |
| 45 @node Terminal Output | |
| 46 @subsection Terminal Output | |
| 47 | |
| 48 Since Octave normally prints the value of an expression as soon as it | |
| 49 has been evaluated, the simplest of all I/O functions is a simple | |
| 50 expression. For example, the following expression will display the | |
| 51 value of @samp{pi} | |
| 52 | |
| 53 @example | |
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54 @group |
| 6666 | 55 pi |
| 56 @print{} pi = 3.1416 | |
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57 @end group |
| 6666 | 58 @end example |
| 59 | |
| 60 This works well as long as it is acceptable to have the name of the | |
| 61 variable (or @samp{ans}) printed along with the value. To print the | |
| 62 value of a variable without printing its name, use the function | |
| 63 @code{disp}. | |
| 64 | |
| 65 The @code{format} command offers some control over the way Octave prints | |
| 66 values with @code{disp} and through the normal echoing mechanism. | |
| 67 | |
| 68 @DOCSTRING(disp) | |
| 69 | |
| 70 @DOCSTRING(format) | |
| 71 | |
| 72 @menu | |
| 73 * Paging Screen Output:: | |
| 74 @end menu | |
| 75 | |
| 76 @node Paging Screen Output | |
| 77 @subsubsection Paging Screen Output | |
| 78 | |
| 3294 | 79 When running interactively, Octave normally sends any output intended |
| 80 for your terminal that is more than one screen long to a paging program, | |
| 81 such as @code{less} or @code{more}. This avoids the problem of having a | |
| 82 large volume of output stream by before you can read it. With | |
| 83 @code{less} (and some versions of @code{more}) you can also scan forward | |
| 84 and backward, and search for specific items. | |
| 85 | |
| 86 Normally, no output is displayed by the pager until just before Octave | |
| 87 is ready to print the top level prompt, or read from the standard input | |
| 88 (for example, by using the @code{fscanf} or @code{scanf} functions). | |
| 89 This means that there may be some delay before any output appears on | |
| 90 your screen if you have asked Octave to perform a significant amount of | |
| 91 work with a single command statement. The function @code{fflush} may be | |
| 92 used to force output to be sent to the pager (or any other stream) | |
| 93 immediately. | |
| 94 | |
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95 You can select the program to run as the pager using the @env{PAGER} |
| 6666 | 96 function, and you can turn paging off by using the function |
| 97 @code{more}. | |
| 3294 | 98 |
| 3372 | 99 @DOCSTRING(more) |
| 3294 | 100 |
| 3372 | 101 @DOCSTRING(PAGER) |
| 3294 | 102 |
| 6549 | 103 @DOCSTRING(PAGER_FLAGS) |
| 104 | |
| 3372 | 105 @DOCSTRING(page_screen_output) |
| 3294 | 106 |
| 3372 | 107 @DOCSTRING(page_output_immediately) |
| 3294 | 108 |
| 3372 | 109 @DOCSTRING(fflush) |
| 3294 | 110 |
| 5775 | 111 @c FIXME -- maybe this would be a good place to describe the |
| 3294 | 112 @c following message: |
| 113 @c | |
| 114 @c warning: connection to external pager (pid = 9334) lost -- | |
| 115 @c warning: pending computations and output may be lost | |
| 116 @c warning: broken pipe | |
| 117 | |
| 4167 | 118 @node Terminal Input |
| 3294 | 119 @subsection Terminal Input |
| 120 | |
| 121 Octave has three functions that make it easy to prompt users for | |
| 122 input. The @code{input} and @code{menu} functions are normally | |
| 123 used for managing an interactive dialog with a user, and the | |
| 124 @code{keyboard} function is normally used for doing simple debugging. | |
| 125 | |
| 3372 | 126 @DOCSTRING(input) |
| 3294 | 127 |
| 3372 | 128 @DOCSTRING(menu) |
| 3294 | 129 |
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130 @DOCSTRING(yes_or_no) |
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131 |
| 6646 | 132 For @code{input}, the normal command line history and editing functions |
| 133 are available at the prompt. | |
| 3294 | 134 |
| 135 Octave also has a function that makes it possible to get a single | |
| 136 character from the keyboard without requiring the user to type a | |
| 137 carriage return. | |
| 138 | |
| 3372 | 139 @DOCSTRING(kbhit) |
| 3294 | 140 |
| 4167 | 141 @node Simple File I/O |
| 3294 | 142 @subsection Simple File I/O |
| 143 | |
| 5225 | 144 @cindex saving data |
| 145 @cindex loading data | |
| 3294 | 146 The @code{save} and @code{load} commands allow data to be written to and |
| 147 read from disk files in various formats. The default format of files | |
| 6666 | 148 written by the @code{save} command can be controlled using the functions |
| 149 @code{default_save_options} and @code{save_precision}. | |
| 150 | |
| 151 As an example the following code creates a 3-by-3 matrix and saves it | |
| 152 to the file @samp{myfile.mat}. | |
| 153 | |
| 154 @example | |
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155 @group |
| 6666 | 156 A = [ 1:3; 4:6; 7:9 ]; |
| 157 save myfile.mat A | |
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158 @end group |
| 6666 | 159 @end example |
| 3294 | 160 |
| 6666 | 161 Once one or more variables have been saved to a file, they can be |
| 162 read into memory using the @code{load} command. | |
| 163 | |
| 164 @example | |
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165 @group |
| 6666 | 166 load myfile.mat |
| 167 A | |
| 168 @print{} A = | |
| 169 @print{} | |
| 170 @print{} 1 2 3 | |
| 171 @print{} 4 5 6 | |
| 172 @print{} 7 8 9 | |
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173 @end group |
| 6666 | 174 @end example |
| 3294 | 175 |
| 3372 | 176 @DOCSTRING(save) |
| 3294 | 177 |
| 6666 | 178 @DOCSTRING(load) |
| 179 | |
| 180 There are three functions that modify the behavior of @code{save}. | |
| 181 | |
| 182 @DOCSTRING(default_save_options) | |
| 183 | |
| 184 @DOCSTRING(save_precision) | |
| 185 | |
| 186 @DOCSTRING(save_header_format_string) | |
| 187 | |
| 188 @DOCSTRING(native_float_format) | |
| 189 | |
| 8828 | 190 It is possible to write data to a file in a similar way to the |
| 6666 | 191 @code{disp} function for writing data to the screen. The @code{fdisp} |
| 192 works just like @code{disp} except its first argument is a file pointer | |
| 193 as created by @code{fopen}. As an example, the following code writes | |
| 194 to data @samp{myfile.txt}. | |
| 195 | |
| 196 @example | |
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197 @group |
| 6666 | 198 fid = fopen ("myfile.txt", "w"); |
| 199 fdisp (fid, "3/8 is "); | |
| 200 fdisp (fid, 3/8); | |
| 201 fclose (fid); | |
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202 @end group |
| 6666 | 203 @end example |
| 204 | |
| 205 @noindent | |
| 206 @xref{Opening and Closing Files}, for details on how to use @code{fopen} | |
| 207 and @code{fclose}. | |
| 208 | |
| 209 @DOCSTRING(fdisp) | |
| 210 | |
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211 Octave can also read and write matrices text files such as comma |
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212 separated lists. |
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213 |
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214 @DOCSTRING(dlmwrite) |
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215 |
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216 @DOCSTRING(dlmread) |
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217 |
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218 @DOCSTRING(csvwrite) |
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219 |
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220 @DOCSTRING(csvread) |
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221 |
| 11148 | 222 Formatted data from can be read from, or written to, text files as well. |
| 223 | |
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224 @DOCSTRING(textread) |
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225 |
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226 @DOCSTRING(textscan) |
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227 |
| 6666 | 228 @menu |
| 229 * Saving Data on Unexpected Exits:: | |
| 230 @end menu | |
| 231 | |
| 232 @node Saving Data on Unexpected Exits | |
| 233 @subsubsection Saving Data on Unexpected Exits | |
| 234 | |
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235 If Octave for some reason exits unexpectedly it will by default save the |
| 6666 | 236 variables available in the workspace to a file in the current directory. |
| 237 By default this file is named @samp{octave-core} and can be loaded | |
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238 into memory with the @code{load} command. While the default behavior |
| 6666 | 239 most often is reasonable it can be changed through the following |
| 240 functions. | |
| 3294 | 241 |
| 3372 | 242 @DOCSTRING(crash_dumps_octave_core) |
| 3294 | 243 |
| 4449 | 244 @DOCSTRING(sighup_dumps_octave_core) |
| 245 | |
| 246 @DOCSTRING(sigterm_dumps_octave_core) | |
| 247 | |
| 5287 | 248 @DOCSTRING(octave_core_file_options) |
| 3294 | 249 |
| 6550 | 250 @DOCSTRING(octave_core_file_limit) |
| 251 | |
| 252 @DOCSTRING(octave_core_file_name) | |
| 253 | |
| 6788 | 254 @node Rational Approximations |
| 255 @subsection Rational Approximations | |
| 256 | |
| 257 @DOCSTRING(rat) | |
| 258 | |
| 259 @DOCSTRING(rats) | |
| 6550 | 260 |
| 4167 | 261 @node C-Style I/O Functions |
| 3294 | 262 @section C-Style I/O Functions |
| 263 | |
| 264 Octave's C-style input and output functions provide most of the | |
| 265 functionality of the C programming language's standard I/O library. The | |
| 266 argument lists for some of the input functions are slightly different, | |
| 267 however, because Octave has no way of passing arguments by reference. | |
| 268 | |
| 269 In the following, @var{file} refers to a file name and @code{fid} refers | |
| 270 to an integer file number, as returned by @code{fopen}. | |
| 271 | |
| 272 There are three files that are always available. Although these files | |
| 273 can be accessed using their corresponding numeric file ids, you should | |
| 274 always use the symbolic names given in the table below, since it will | |
| 275 make your programs easier to understand. | |
| 276 | |
| 3372 | 277 @DOCSTRING(stdin) |
| 3294 | 278 |
| 3372 | 279 @DOCSTRING(stdout) |
| 3294 | 280 |
| 3372 | 281 @DOCSTRING(stderr) |
| 3294 | 282 |
| 283 @menu | |
| 284 * Opening and Closing Files:: | |
| 285 * Simple Output:: | |
| 286 * Line-Oriented Input:: | |
| 287 * Formatted Output:: | |
| 288 * Output Conversion for Matrices:: | |
| 289 * Output Conversion Syntax:: | |
| 290 * Table of Output Conversions:: | |
| 291 * Integer Conversions:: | |
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292 * Floating-Point Conversions:: |
| 3294 | 293 * Other Output Conversions:: |
| 294 * Formatted Input:: | |
| 295 * Input Conversion Syntax:: | |
| 296 * Table of Input Conversions:: | |
| 297 * Numeric Input Conversions:: | |
| 298 * String Input Conversions:: | |
| 299 * Binary I/O:: | |
| 300 * Temporary Files:: | |
| 301 * EOF and Errors:: | |
| 302 * File Positioning:: | |
| 303 @end menu | |
| 304 | |
| 4167 | 305 @node Opening and Closing Files |
| 3294 | 306 @subsection Opening and Closing Files |
| 307 | |
| 6666 | 308 When reading data from a file it must be opened for reading first, and |
| 309 likewise when writing to a file. The @code{fopen} function returns a | |
| 310 pointer to an open file that is ready to be read or written. Once all | |
| 311 data has been read from or written to the opened file it should be closed. | |
| 312 The @code{fclose} function does this. The following code illustrates | |
| 313 the basic pattern for writing to a file, but a very similar pattern is | |
| 314 used when reading a file. | |
| 315 | |
| 316 @example | |
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317 @group |
| 6666 | 318 filename = "myfile.txt"; |
| 319 fid = fopen (filename, "w"); | |
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320 # Do the actual I/O here@dots{} |
| 6666 | 321 fclose (fid); |
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322 @end group |
| 6666 | 323 @end example |
| 324 | |
| 3372 | 325 @DOCSTRING(fopen) |
| 3294 | 326 |
| 3372 | 327 @DOCSTRING(fclose) |
| 3294 | 328 |
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329 @DOCSTRING(is_valid_file_id) |
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330 |
| 4167 | 331 @node Simple Output |
| 3294 | 332 @subsection Simple Output |
| 333 | |
| 6666 | 334 Once a file has been opened for writing a string can be written to the |
| 335 file using the @code{fputs} function. The following example shows | |
| 336 how to write the string @samp{Free Software is needed for Free Science} | |
| 337 to the file @samp{free.txt}. | |
| 338 | |
| 339 @example | |
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340 @group |
| 6666 | 341 filename = "free.txt"; |
| 342 fid = fopen (filename, "w"); | |
| 343 fputs (fid, "Free Software is needed for Free Science"); | |
| 344 fclose (fid); | |
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345 @end group |
| 6666 | 346 @end example |
| 347 | |
| 3372 | 348 @DOCSTRING(fputs) |
| 3294 | 349 |
| 6666 | 350 A function much similar to @code{fputs} is available for writing data |
| 351 to the screen. The @code{puts} function works just like @code{fputs} | |
| 352 except it doesn't take a file pointer as its input. | |
| 353 | |
| 3372 | 354 @DOCSTRING(puts) |
| 3294 | 355 |
| 4167 | 356 @node Line-Oriented Input |
| 3294 | 357 @subsection Line-Oriented Input |
| 358 | |
| 6666 | 359 To read from a file it must be opened for reading using @code{fopen}. |
| 360 Then a line can be read from the file using @code{fgetl} as the following | |
| 361 code illustrates | |
| 362 | |
| 363 @example | |
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364 @group |
| 6666 | 365 fid = fopen ("free.txt"); |
| 366 txt = fgetl (fid) | |
| 367 @print{} Free Software is needed for Free Science | |
| 368 fclose (fid); | |
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369 @end group |
| 6666 | 370 @end example |
| 371 | |
| 372 @noindent | |
| 373 This of course assumes that the file @samp{free.txt} exists and contains | |
| 374 the line @samp{Free Software is needed for Free Science}. | |
| 375 | |
| 3372 | 376 @DOCSTRING(fgetl) |
| 3294 | 377 |
| 3372 | 378 @DOCSTRING(fgets) |
| 3294 | 379 |
| 9701 | 380 @DOCSTRING(fskipl) |
| 381 | |
| 4167 | 382 @node Formatted Output |
| 3294 | 383 @subsection Formatted Output |
| 384 | |
| 385 This section describes how to call @code{printf} and related functions. | |
| 386 | |
| 387 The following functions are available for formatted output. They are | |
| 388 modelled after the C language functions of the same name, but they | |
| 389 interpret the format template differently in order to improve the | |
| 390 performance of printing vector and matrix values. | |
| 391 | |
| 3372 | 392 @DOCSTRING(printf) |
| 3294 | 393 |
| 3372 | 394 @DOCSTRING(fprintf) |
| 3294 | 395 |
| 3372 | 396 @DOCSTRING(sprintf) |
| 3294 | 397 |
| 398 The @code{printf} function can be used to print any number of arguments. | |
| 399 The template string argument you supply in a call provides | |
| 400 information not only about the number of additional arguments, but also | |
| 401 about their types and what style should be used for printing them. | |
| 402 | |
| 403 Ordinary characters in the template string are simply written to the | |
| 404 output stream as-is, while @dfn{conversion specifications} introduced by | |
| 405 a @samp{%} character in the template cause subsequent arguments to be | |
| 406 formatted and written to the output stream. For example, | |
| 407 @cindex conversion specifications (@code{printf}) | |
| 408 | |
| 6670 | 409 @example |
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410 @group |
| 3294 | 411 pct = 37; |
| 412 filename = "foo.txt"; | |
| 7031 | 413 printf ("Processed %d%% of `%s'.\nPlease be patient.\n", |
| 414 pct, filename); | |
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415 @end group |
| 6670 | 416 @end example |
| 3294 | 417 |
| 418 @noindent | |
| 419 produces output like | |
| 420 | |
| 6670 | 421 @example |
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422 @group |
| 7031 | 423 Processed 37% of `foo.txt'. |
| 3294 | 424 Please be patient. |
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425 @end group |
| 6670 | 426 @end example |
| 3294 | 427 |
| 428 This example shows the use of the @samp{%d} conversion to specify that a | |
| 429 scalar argument should be printed in decimal notation, the @samp{%s} | |
| 430 conversion to specify printing of a string argument, and the @samp{%%} | |
| 431 conversion to print a literal @samp{%} character. | |
| 432 | |
| 433 There are also conversions for printing an integer argument as an | |
| 434 unsigned value in octal, decimal, or hexadecimal radix (@samp{%o}, | |
| 435 @samp{%u}, or @samp{%x}, respectively); or as a character value | |
| 436 (@samp{%c}). | |
| 437 | |
| 438 Floating-point numbers can be printed in normal, fixed-point notation | |
| 439 using the @samp{%f} conversion or in exponential notation using the | |
| 440 @samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e} | |
| 441 or @samp{%f} format, depending on what is more appropriate for the | |
| 442 magnitude of the particular number. | |
| 443 | |
| 444 You can control formatting more precisely by writing @dfn{modifiers} | |
| 445 between the @samp{%} and the character that indicates which conversion | |
| 446 to apply. These slightly alter the ordinary behavior of the conversion. | |
| 447 For example, most conversion specifications permit you to specify a | |
| 448 minimum field width and a flag indicating whether you want the result | |
| 449 left- or right-justified within the field. | |
| 450 | |
| 451 The specific flags and modifiers that are permitted and their | |
| 452 interpretation vary depending on the particular conversion. They're all | |
| 453 described in more detail in the following sections. | |
| 454 | |
| 4167 | 455 @node Output Conversion for Matrices |
| 3294 | 456 @subsection Output Conversion for Matrices |
| 457 | |
| 458 When given a matrix value, Octave's formatted output functions cycle | |
| 459 through the format template until all the values in the matrix have been | |
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460 printed. For example: |
| 3294 | 461 |
| 462 @example | |
| 463 @group | |
| 464 printf ("%4.2f %10.2e %8.4g\n", hilb (3)); | |
| 465 | |
| 466 @print{} 1.00 5.00e-01 0.3333 | |
| 467 @print{} 0.50 3.33e-01 0.25 | |
| 468 @print{} 0.33 2.50e-01 0.2 | |
| 469 @end group | |
| 470 @end example | |
| 471 | |
| 472 If more than one value is to be printed in a single call, the output | |
| 473 functions do not return to the beginning of the format template when | |
| 474 moving on from one value to the next. This can lead to confusing output | |
| 475 if the number of elements in the matrices are not exact multiples of the | |
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476 number of conversions in the format template. For example: |
| 3294 | 477 |
| 478 @example | |
| 479 @group | |
| 480 printf ("%4.2f %10.2e %8.4g\n", [1, 2], [3, 4]); | |
| 481 | |
| 482 @print{} 1.00 2.00e+00 3 | |
| 483 @print{} 4.00 | |
| 484 @end group | |
| 485 @end example | |
| 486 | |
| 487 If this is not what you want, use a series of calls instead of just one. | |
| 488 | |
| 4167 | 489 @node Output Conversion Syntax |
| 3294 | 490 @subsection Output Conversion Syntax |
| 491 | |
| 492 This section provides details about the precise syntax of conversion | |
| 493 specifications that can appear in a @code{printf} template | |
| 494 string. | |
| 495 | |
| 496 Characters in the template string that are not part of a | |
| 497 conversion specification are printed as-is to the output stream. | |
| 498 | |
| 499 The conversion specifications in a @code{printf} template string have | |
| 500 the general form: | |
| 501 | |
| 6670 | 502 @example |
| 3294 | 503 % @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion} |
| 6670 | 504 @end example |
| 3294 | 505 |
| 506 For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-} | |
| 507 is a flag, @samp{10} specifies the field width, the precision is | |
| 508 @samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies | |
| 509 the conversion style. (This particular type specifier says to print a | |
| 510 numeric argument in decimal notation, with a minimum of 8 digits | |
| 511 left-justified in a field at least 10 characters wide.) | |
| 512 | |
| 513 In more detail, output conversion specifications consist of an | |
| 514 initial @samp{%} character followed in sequence by: | |
| 515 | |
| 516 @itemize @bullet | |
| 517 @item | |
| 518 Zero or more @dfn{flag characters} that modify the normal behavior of | |
| 519 the conversion specification. | |
| 520 @cindex flag character (@code{printf}) | |
| 521 | |
| 522 @item | |
| 523 An optional decimal integer specifying the @dfn{minimum field width}. | |
| 524 If the normal conversion produces fewer characters than this, the field | |
| 525 is padded with spaces to the specified width. This is a @emph{minimum} | |
| 526 value; if the normal conversion produces more characters than this, the | |
| 527 field is @emph{not} truncated. Normally, the output is right-justified | |
| 528 within the field. | |
| 529 @cindex minimum field width (@code{printf}) | |
| 530 | |
| 531 You can also specify a field width of @samp{*}. This means that the | |
| 532 next argument in the argument list (before the actual value to be | |
| 533 printed) is used as the field width. The value is rounded to the | |
| 534 nearest integer. If the value is negative, this means to set the | |
| 535 @samp{-} flag (see below) and to use the absolute value as the field | |
| 536 width. | |
| 537 | |
| 538 @item | |
| 539 An optional @dfn{precision} to specify the number of digits to be | |
| 540 written for the numeric conversions. If the precision is specified, it | |
| 541 consists of a period (@samp{.}) followed optionally by a decimal integer | |
| 542 (which defaults to zero if omitted). | |
| 543 @cindex precision (@code{printf}) | |
| 544 | |
| 545 You can also specify a precision of @samp{*}. This means that the next | |
| 546 argument in the argument list (before the actual value to be printed) is | |
| 547 used as the precision. The value must be an integer, and is ignored | |
| 548 if it is negative. | |
| 549 | |
| 550 @item | |
| 551 An optional @dfn{type modifier character}. This character is ignored by | |
| 552 Octave's @code{printf} function, but is recognized to provide | |
| 553 compatibility with the C language @code{printf}. | |
| 554 | |
| 555 @item | |
| 556 A character that specifies the conversion to be applied. | |
| 557 @end itemize | |
| 558 | |
| 559 The exact options that are permitted and how they are interpreted vary | |
| 560 between the different conversion specifiers. See the descriptions of the | |
| 561 individual conversions for information about the particular options that | |
| 562 they use. | |
| 563 | |
| 4167 | 564 @node Table of Output Conversions |
| 3294 | 565 @subsection Table of Output Conversions |
| 566 @cindex output conversions, for @code{printf} | |
| 567 | |
| 568 Here is a table summarizing what all the different conversions do: | |
| 569 | |
| 570 @table @asis | |
| 571 @item @samp{%d}, @samp{%i} | |
| 572 Print an integer as a signed decimal number. @xref{Integer | |
| 573 Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for | |
| 574 output, but are different when used with @code{scanf} for input | |
| 575 (@pxref{Table of Input Conversions}). | |
| 576 | |
| 577 @item @samp{%o} | |
| 578 Print an integer as an unsigned octal number. @xref{Integer | |
| 579 Conversions}, for details. | |
| 580 | |
| 581 @item @samp{%u} | |
| 582 Print an integer as an unsigned decimal number. @xref{Integer | |
| 583 Conversions}, for details. | |
| 584 | |
| 585 @item @samp{%x}, @samp{%X} | |
| 586 Print an integer as an unsigned hexadecimal number. @samp{%x} uses | |
| 587 lower-case letters and @samp{%X} uses upper-case. @xref{Integer | |
| 588 Conversions}, for details. | |
| 589 | |
| 590 @item @samp{%f} | |
| 591 Print a floating-point number in normal (fixed-point) notation. | |
| 592 @xref{Floating-Point Conversions}, for details. | |
| 593 | |
| 594 @item @samp{%e}, @samp{%E} | |
| 595 Print a floating-point number in exponential notation. @samp{%e} uses | |
| 596 lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point | |
| 597 Conversions}, for details. | |
| 598 | |
| 599 @item @samp{%g}, @samp{%G} | |
| 600 Print a floating-point number in either normal (fixed-point) or | |
| 601 exponential notation, whichever is more appropriate for its magnitude. | |
| 602 @samp{%g} uses lower-case letters and @samp{%G} uses upper-case. | |
| 603 @xref{Floating-Point Conversions}, for details. | |
| 604 | |
| 605 @item @samp{%c} | |
| 606 Print a single character. @xref{Other Output Conversions}. | |
| 607 | |
| 608 @item @samp{%s} | |
| 609 Print a string. @xref{Other Output Conversions}. | |
| 610 | |
| 611 @item @samp{%%} | |
| 612 Print a literal @samp{%} character. @xref{Other Output Conversions}. | |
| 613 @end table | |
| 614 | |
| 615 If the syntax of a conversion specification is invalid, unpredictable | |
| 616 things will happen, so don't do this. If there aren't enough function | |
| 617 arguments provided to supply values for all the conversion | |
| 618 specifications in the template string, or if the arguments are not of | |
| 619 the correct types, the results are unpredictable. If you supply more | |
| 620 arguments than conversion specifications, the extra argument values are | |
| 621 simply ignored; this is sometimes useful. | |
| 622 | |
| 4167 | 623 @node Integer Conversions |
| 3294 | 624 @subsection Integer Conversions |
| 625 | |
| 626 This section describes the options for the @samp{%d}, @samp{%i}, | |
| 627 @samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion | |
| 628 specifications. These conversions print integers in various formats. | |
| 629 | |
| 630 The @samp{%d} and @samp{%i} conversion specifications both print an | |
| 631 numeric argument as a signed decimal number; while @samp{%o}, | |
| 632 @samp{%u}, and @samp{%x} print the argument as an unsigned octal, | |
| 633 decimal, or hexadecimal number (respectively). The @samp{%X} conversion | |
| 634 specification is just like @samp{%x} except that it uses the characters | |
| 635 @samp{ABCDEF} as digits instead of @samp{abcdef}. | |
| 636 | |
| 637 The following flags are meaningful: | |
| 638 | |
| 639 @table @asis | |
| 640 @item @samp{-} | |
| 641 Left-justify the result in the field (instead of the normal | |
| 642 right-justification). | |
| 643 | |
| 644 @item @samp{+} | |
| 645 For the signed @samp{%d} and @samp{%i} conversions, print a | |
| 646 plus sign if the value is positive. | |
| 647 | |
| 648 @item @samp{ } | |
| 649 For the signed @samp{%d} and @samp{%i} conversions, if the result | |
| 650 doesn't start with a plus or minus sign, prefix it with a space | |
| 651 character instead. Since the @samp{+} flag ensures that the result | |
| 652 includes a sign, this flag is ignored if you supply both of them. | |
| 653 | |
| 654 @item @samp{#} | |
| 655 For the @samp{%o} conversion, this forces the leading digit to be | |
| 656 @samp{0}, as if by increasing the precision. For @samp{%x} or | |
| 657 @samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively) | |
| 658 to the result. This doesn't do anything useful for the @samp{%d}, | |
| 659 @samp{%i}, or @samp{%u} conversions. | |
| 660 | |
| 661 @item @samp{0} | |
| 662 Pad the field with zeros instead of spaces. The zeros are placed after | |
| 663 any indication of sign or base. This flag is ignored if the @samp{-} | |
| 664 flag is also specified, or if a precision is specified. | |
| 665 @end table | |
| 666 | |
| 667 If a precision is supplied, it specifies the minimum number of digits to | |
| 668 appear; leading zeros are produced if necessary. If you don't specify a | |
| 669 precision, the number is printed with as many digits as it needs. If | |
| 670 you convert a value of zero with an explicit precision of zero, then no | |
| 671 characters at all are produced. | |
| 672 | |
| 4167 | 673 @node Floating-Point Conversions |
| 3294 | 674 @subsection Floating-Point Conversions |
| 675 | |
| 676 This section discusses the conversion specifications for floating-point | |
| 677 numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G} | |
| 678 conversions. | |
| 679 | |
| 680 The @samp{%f} conversion prints its argument in fixed-point notation, | |
| 681 producing output of the form | |
| 682 @w{[@code{-}]@var{ddd}@code{.}@var{ddd}}, | |
| 683 where the number of digits following the decimal point is controlled | |
| 684 by the precision you specify. | |
| 685 | |
| 686 The @samp{%e} conversion prints its argument in exponential notation, | |
| 687 producing output of the form | |
| 688 @w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}. | |
| 689 Again, the number of digits following the decimal point is controlled by | |
| 690 the precision. The exponent always contains at least two digits. The | |
| 691 @samp{%E} conversion is similar but the exponent is marked with the letter | |
| 692 @samp{E} instead of @samp{e}. | |
| 693 | |
| 694 The @samp{%g} and @samp{%G} conversions print the argument in the style | |
| 695 of @samp{%e} or @samp{%E} (respectively) if the exponent would be less | |
| 696 than -4 or greater than or equal to the precision; otherwise they use the | |
| 697 @samp{%f} style. Trailing zeros are removed from the fractional portion | |
| 698 of the result and a decimal-point character appears only if it is | |
| 699 followed by a digit. | |
| 700 | |
| 701 The following flags can be used to modify the behavior: | |
| 702 | |
| 703 @c Not @samp so we can have ` ' as an item. | |
| 704 @table @asis | |
| 705 @item @samp{-} | |
| 706 Left-justify the result in the field. Normally the result is | |
| 707 right-justified. | |
| 708 | |
| 709 @item @samp{+} | |
| 710 Always include a plus or minus sign in the result. | |
| 711 | |
| 712 @item @samp{ } | |
| 713 If the result doesn't start with a plus or minus sign, prefix it with a | |
| 714 space instead. Since the @samp{+} flag ensures that the result includes | |
| 715 a sign, this flag is ignored if you supply both of them. | |
| 716 | |
| 717 @item @samp{#} | |
| 718 Specifies that the result should always include a decimal point, even | |
| 719 if no digits follow it. For the @samp{%g} and @samp{%G} conversions, | |
| 720 this also forces trailing zeros after the decimal point to be left | |
| 721 in place where they would otherwise be removed. | |
| 722 | |
| 723 @item @samp{0} | |
| 724 Pad the field with zeros instead of spaces; the zeros are placed | |
| 725 after any sign. This flag is ignored if the @samp{-} flag is also | |
| 726 specified. | |
| 727 @end table | |
| 728 | |
| 729 The precision specifies how many digits follow the decimal-point | |
| 730 character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For | |
| 731 these conversions, the default precision is @code{6}. If the precision | |
| 732 is explicitly @code{0}, this suppresses the decimal point character | |
| 733 entirely. For the @samp{%g} and @samp{%G} conversions, the precision | |
| 734 specifies how many significant digits to print. Significant digits are | |
| 735 the first digit before the decimal point, and all the digits after it. | |
| 736 If the precision is @code{0} or not specified for @samp{%g} or | |
| 737 @samp{%G}, it is treated like a value of @code{1}. If the value being | |
| 738 printed cannot be expressed precisely in the specified number of digits, | |
| 739 the value is rounded to the nearest number that fits. | |
| 740 | |
| 4167 | 741 @node Other Output Conversions |
| 3294 | 742 @subsection Other Output Conversions |
| 743 | |
| 744 This section describes miscellaneous conversions for @code{printf}. | |
| 745 | |
| 746 The @samp{%c} conversion prints a single character. The @samp{-} | |
| 747 flag can be used to specify left-justification in the field, but no | |
| 748 other flags are defined, and no precision or type modifier can be given. | |
| 749 For example: | |
| 750 | |
| 6670 | 751 @example |
| 3294 | 752 printf ("%c%c%c%c%c", "h", "e", "l", "l", "o"); |
| 6670 | 753 @end example |
| 3294 | 754 |
| 755 @noindent | |
| 756 prints @samp{hello}. | |
| 757 | |
| 758 The @samp{%s} conversion prints a string. The corresponding argument | |
| 759 must be a string. A precision can be specified to indicate the maximum | |
| 760 number of characters to write; otherwise characters in the string up to | |
| 761 but not including the terminating null character are written to the | |
| 762 output stream. The @samp{-} flag can be used to specify | |
| 763 left-justification in the field, but no other flags or type modifiers | |
| 764 are defined for this conversion. For example: | |
| 765 | |
| 6670 | 766 @example |
| 3294 | 767 printf ("%3s%-6s", "no", "where"); |
| 6670 | 768 @end example |
| 3294 | 769 |
| 770 @noindent | |
| 771 prints @samp{ nowhere } (note the leading and trailing spaces). | |
| 772 | |
| 4167 | 773 @node Formatted Input |
| 3294 | 774 @subsection Formatted Input |
| 775 | |
| 776 Octave provides the @code{scanf}, @code{fscanf}, and @code{sscanf} | |
| 777 functions to read formatted input. There are two forms of each of these | |
| 778 functions. One can be used to extract vectors of data from a file, and | |
| 779 the other is more `C-like'. | |
| 780 | |
| 3428 | 781 @DOCSTRING(fscanf) |
| 3294 | 782 |
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784 |
| 3372 | 785 @DOCSTRING(sscanf) |
| 3294 | 786 |
| 787 Calls to @code{scanf} are superficially similar to calls to | |
| 788 @code{printf} in that arbitrary arguments are read under the control of | |
| 789 a template string. While the syntax of the conversion specifications in | |
| 790 the template is very similar to that for @code{printf}, the | |
| 791 interpretation of the template is oriented more towards free-format | |
| 792 input and simple pattern matching, rather than fixed-field formatting. | |
| 793 For example, most @code{scanf} conversions skip over any amount of | |
| 794 ``white space'' (including spaces, tabs, and newlines) in the input | |
| 795 file, and there is no concept of precision for the numeric input | |
| 796 conversions as there is for the corresponding output conversions. | |
| 797 Ordinarily, non-whitespace characters in the template are expected to | |
| 798 match characters in the input stream exactly. | |
| 799 @cindex conversion specifications (@code{scanf}) | |
| 800 | |
| 801 When a @dfn{matching failure} occurs, @code{scanf} returns immediately, | |
| 802 leaving the first non-matching character as the next character to be | |
| 803 read from the stream, and @code{scanf} returns all the items that were | |
| 804 successfully converted. | |
| 805 @cindex matching failure, in @code{scanf} | |
| 806 | |
| 807 The formatted input functions are not used as frequently as the | |
| 808 formatted output functions. Partly, this is because it takes some care | |
| 809 to use them properly. Another reason is that it is difficult to recover | |
| 810 from a matching error. | |
| 811 | |
| 4167 | 812 @node Input Conversion Syntax |
| 3294 | 813 @subsection Input Conversion Syntax |
| 814 | |
| 815 A @code{scanf} template string is a string that contains ordinary | |
| 816 multibyte characters interspersed with conversion specifications that | |
| 817 start with @samp{%}. | |
| 818 | |
| 819 Any whitespace character in the template causes any number of whitespace | |
| 820 characters in the input stream to be read and discarded. The whitespace | |
| 821 characters that are matched need not be exactly the same whitespace | |
| 822 characters that appear in the template string. For example, write | |
| 823 @samp{ , } in the template to recognize a comma with optional whitespace | |
| 824 before and after. | |
| 825 | |
| 826 Other characters in the template string that are not part of conversion | |
| 827 specifications must match characters in the input stream exactly; if | |
| 828 this is not the case, a matching failure occurs. | |
| 829 | |
| 830 The conversion specifications in a @code{scanf} template string | |
| 831 have the general form: | |
| 832 | |
| 6670 | 833 @example |
| 3294 | 834 % @var{flags} @var{width} @var{type} @var{conversion} |
| 6670 | 835 @end example |
| 3294 | 836 |
| 837 In more detail, an input conversion specification consists of an initial | |
| 838 @samp{%} character followed in sequence by: | |
| 839 | |
| 840 @itemize @bullet | |
| 841 @item | |
| 842 An optional @dfn{flag character} @samp{*}, which says to ignore the text | |
| 843 read for this specification. When @code{scanf} finds a conversion | |
| 844 specification that uses this flag, it reads input as directed by the | |
| 845 rest of the conversion specification, but it discards this input, does | |
| 846 not return any value, and does not increment the count of | |
| 847 successful assignments. | |
| 848 @cindex flag character (@code{scanf}) | |
| 849 | |
| 850 @item | |
| 851 An optional decimal integer that specifies the @dfn{maximum field | |
| 852 width}. Reading of characters from the input stream stops either when | |
| 853 this maximum is reached or when a non-matching character is found, | |
| 854 whichever happens first. Most conversions discard initial whitespace | |
| 855 characters, and these discarded characters don't count towards the | |
| 856 maximum field width. Conversions that do not discard initial whitespace | |
| 857 are explicitly documented. | |
| 858 @cindex maximum field width (@code{scanf}) | |
| 859 | |
| 860 @item | |
| 861 An optional type modifier character. This character is ignored by | |
| 862 Octave's @code{scanf} function, but is recognized to provide | |
| 863 compatibility with the C language @code{scanf}. | |
| 864 | |
| 865 @item | |
| 866 A character that specifies the conversion to be applied. | |
| 867 @end itemize | |
| 868 | |
| 869 The exact options that are permitted and how they are interpreted vary | |
| 870 between the different conversion specifiers. See the descriptions of the | |
| 871 individual conversions for information about the particular options that | |
| 872 they allow. | |
| 873 | |
| 4167 | 874 @node Table of Input Conversions |
| 3294 | 875 @subsection Table of Input Conversions |
| 876 @cindex input conversions, for @code{scanf} | |
| 877 | |
| 878 Here is a table that summarizes the various conversion specifications: | |
| 879 | |
| 880 @table @asis | |
| 881 @item @samp{%d} | |
| 882 Matches an optionally signed integer written in decimal. @xref{Numeric | |
| 883 Input Conversions}. | |
| 884 | |
| 885 @item @samp{%i} | |
| 886 Matches an optionally signed integer in any of the formats that the C | |
| 887 language defines for specifying an integer constant. @xref{Numeric | |
| 888 Input Conversions}. | |
| 889 | |
| 890 @item @samp{%o} | |
| 891 Matches an unsigned integer written in octal radix. | |
| 892 @xref{Numeric Input Conversions}. | |
| 893 | |
| 894 @item @samp{%u} | |
| 895 Matches an unsigned integer written in decimal radix. | |
| 896 @xref{Numeric Input Conversions}. | |
| 897 | |
| 898 @item @samp{%x}, @samp{%X} | |
| 899 Matches an unsigned integer written in hexadecimal radix. | |
| 900 @xref{Numeric Input Conversions}. | |
| 901 | |
| 902 @item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G} | |
| 903 Matches an optionally signed floating-point number. @xref{Numeric Input | |
| 904 Conversions}. | |
| 905 | |
| 906 @item @samp{%s} | |
| 907 Matches a string containing only non-whitespace characters. | |
| 908 @xref{String Input Conversions}. | |
| 909 | |
| 910 @item @samp{%c} | |
| 911 Matches a string of one or more characters; the number of characters | |
| 912 read is controlled by the maximum field width given for the conversion. | |
| 913 @xref{String Input Conversions}. | |
| 914 | |
| 915 @item @samp{%%} | |
| 916 This matches a literal @samp{%} character in the input stream. No | |
| 917 corresponding argument is used. | |
| 918 @end table | |
| 919 | |
| 920 If the syntax of a conversion specification is invalid, the behavior is | |
| 921 undefined. If there aren't enough function arguments provided to supply | |
| 922 addresses for all the conversion specifications in the template strings | |
| 923 that perform assignments, or if the arguments are not of the correct | |
| 924 types, the behavior is also undefined. On the other hand, extra | |
| 925 arguments are simply ignored. | |
| 926 | |
| 4167 | 927 @node Numeric Input Conversions |
| 3294 | 928 @subsection Numeric Input Conversions |
| 929 | |
| 930 This section describes the @code{scanf} conversions for reading numeric | |
| 931 values. | |
| 932 | |
| 933 The @samp{%d} conversion matches an optionally signed integer in decimal | |
| 934 radix. | |
| 935 | |
| 936 The @samp{%i} conversion matches an optionally signed integer in any of | |
| 937 the formats that the C language defines for specifying an integer | |
| 938 constant. | |
| 939 | |
| 940 For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012} | |
| 941 could be read in as integers under the @samp{%i} conversion. Each of | |
| 942 these specifies a number with decimal value @code{10}. | |
| 943 | |
| 944 The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned | |
| 945 integers in octal, decimal, and hexadecimal radices, respectively. | |
| 946 | |
| 947 The @samp{%X} conversion is identical to the @samp{%x} conversion. They | |
| 948 both permit either uppercase or lowercase letters to be used as digits. | |
| 949 | |
| 950 Unlike the C language @code{scanf}, Octave ignores the @samp{h}, | |
| 951 @samp{l}, and @samp{L} modifiers. | |
| 952 | |
| 4167 | 953 @node String Input Conversions |
| 3294 | 954 @subsection String Input Conversions |
| 955 | |
| 956 This section describes the @code{scanf} input conversions for reading | |
| 957 string and character values: @samp{%s} and @samp{%c}. | |
| 958 | |
| 959 The @samp{%c} conversion is the simplest: it matches a fixed number of | |
| 960 characters, always. The maximum field with says how many characters to | |
| 961 read; if you don't specify the maximum, the default is 1. This | |
| 962 conversion does not skip over initial whitespace characters. It reads | |
| 963 precisely the next @var{n} characters, and fails if it cannot get that | |
| 964 many. | |
| 965 | |
| 966 The @samp{%s} conversion matches a string of non-whitespace characters. | |
| 967 It skips and discards initial whitespace, but stops when it encounters | |
| 968 more whitespace after having read something. | |
| 969 | |
| 970 For example, reading the input: | |
| 971 | |
| 6670 | 972 @example |
| 3294 | 973 hello, world |
| 6670 | 974 @end example |
| 3294 | 975 |
| 976 @noindent | |
| 977 with the conversion @samp{%10c} produces @code{" hello, wo"}, but | |
| 978 reading the same input with the conversion @samp{%10s} produces | |
| 979 @code{"hello,"}. | |
| 980 | |
| 4167 | 981 @node Binary I/O |
| 3294 | 982 @subsection Binary I/O |
| 983 | |
| 984 Octave can read and write binary data using the functions @code{fread} | |
| 985 and @code{fwrite}, which are patterned after the standard C functions | |
| 6939 | 986 with the same names. They are able to automatically swap the byte order |
| 987 of integer data and convert among the supported floating point formats | |
| 3294 | 988 as the data are read. |
| 989 | |
| 3372 | 990 @DOCSTRING(fread) |
| 3294 | 991 |
| 3372 | 992 @DOCSTRING(fwrite) |
| 3294 | 993 |
| 4167 | 994 @node Temporary Files |
| 3294 | 995 @subsection Temporary Files |
| 996 | |
| 6666 | 997 Sometimes one needs to write data to a file that is only temporary. |
| 998 This is most commonly used when an external program launched from | |
| 999 within Octave needs to access data. When Octave exits all temporary | |
| 1000 files will be deleted, so this step need not be executed manually. | |
| 1001 | |
| 4328 | 1002 @DOCSTRING(mkstemp) |
| 1003 | |
| 1004 @DOCSTRING(tmpfile) | |
| 1005 | |
| 3372 | 1006 @DOCSTRING(tmpnam) |
| 3294 | 1007 |
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1008 @DOCSTRING(octave_tmp_file_name) |
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1009 |
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1010 @node EOF and Errors, File Positioning, Temporary Files, C-Style I/O Functions |
| 3294 | 1011 @subsection End of File and Errors |
| 1012 | |
| 6666 | 1013 Once a file has been opened its status can be acquired. As an example |
| 1014 the @code{feof} functions determines if the end of the file has been | |
| 1015 reached. This can be very useful when reading small parts of a file | |
| 1016 at a time. The following example shows how to read one line at a time | |
| 1017 from a file until the end has been reached. | |
| 1018 | |
| 1019 @example | |
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1020 @group |
| 6666 | 1021 filename = "myfile.txt"; |
| 1022 fid = fopen (filename, "r"); | |
| 1023 while (! feof (fid) ) | |
| 1024 text_line = fgetl (fid); | |
| 1025 endwhile | |
| 1026 fclose (fid); | |
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1027 @end group |
| 6666 | 1028 @end example |
| 1029 | |
| 1030 @noindent | |
| 1031 Note that in some situations it is more efficient to read the entire | |
| 1032 contents of a file and then process it, than it is to read it line by | |
| 1033 line. This has the potential advantage of removing the loop in the | |
| 1034 above code. | |
| 1035 | |
| 3372 | 1036 @DOCSTRING(feof) |
| 3294 | 1037 |
| 3372 | 1038 @DOCSTRING(ferror) |
| 3294 | 1039 |
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1040 @DOCSTRING(fclear) |
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1041 |
| 3372 | 1042 @DOCSTRING(freport) |
| 3294 | 1043 |
| 4167 | 1044 @node File Positioning |
| 3294 | 1045 @subsection File Positioning |
| 1046 | |
| 1047 Three functions are available for setting and determining the position of | |
| 1048 the file pointer for a given file. | |
| 1049 | |
| 3372 | 1050 @DOCSTRING(ftell) |
| 1051 | |
| 1052 @DOCSTRING(fseek) | |
| 3294 | 1053 |
| 3372 | 1054 @DOCSTRING(SEEK_SET) |
| 3294 | 1055 |
| 3372 | 1056 @DOCSTRING(frewind) |
| 3294 | 1057 |
| 1058 The following example stores the current file position in the variable | |
| 1059 @code{marker}, moves the pointer to the beginning of the file, reads | |
| 1060 four characters, and then returns to the original position. | |
| 1061 | |
| 1062 @example | |
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1063 @group |
| 3294 | 1064 marker = ftell (myfile); |
| 1065 frewind (myfile); | |
| 1066 fourch = fgets (myfile, 4); | |
| 1067 fseek (myfile, marker, SEEK_SET); | |
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1068 @end group |
| 3294 | 1069 @end example |
| 1070 |