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annotate doc/interpreter/io.txi @ 11147:a81994607ca0
document textscan and textread
| author | John W. Eaton <jwe@octave.org> |
|---|---|
| date | Sat, 23 Oct 2010 02:29:35 -0400 |
| parents | 3450551f591e |
| children | c9ec943ebe65 |
| rev | line source |
|---|---|
| 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 |
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222 @DOCSTRING(textread) |
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223 |
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224 @DOCSTRING(textscan) |
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225 |
| 6666 | 226 @menu |
| 227 * Saving Data on Unexpected Exits:: | |
| 228 @end menu | |
| 229 | |
| 230 @node Saving Data on Unexpected Exits | |
| 231 @subsubsection Saving Data on Unexpected Exits | |
| 232 | |
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233 If Octave for some reason exits unexpectedly it will by default save the |
| 6666 | 234 variables available in the workspace to a file in the current directory. |
| 235 By default this file is named @samp{octave-core} and can be loaded | |
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236 into memory with the @code{load} command. While the default behavior |
| 6666 | 237 most often is reasonable it can be changed through the following |
| 238 functions. | |
| 3294 | 239 |
| 3372 | 240 @DOCSTRING(crash_dumps_octave_core) |
| 3294 | 241 |
| 4449 | 242 @DOCSTRING(sighup_dumps_octave_core) |
| 243 | |
| 244 @DOCSTRING(sigterm_dumps_octave_core) | |
| 245 | |
| 5287 | 246 @DOCSTRING(octave_core_file_options) |
| 3294 | 247 |
| 6550 | 248 @DOCSTRING(octave_core_file_limit) |
| 249 | |
| 250 @DOCSTRING(octave_core_file_name) | |
| 251 | |
| 6788 | 252 @node Rational Approximations |
| 253 @subsection Rational Approximations | |
| 254 | |
| 255 @DOCSTRING(rat) | |
| 256 | |
| 257 @DOCSTRING(rats) | |
| 6550 | 258 |
| 4167 | 259 @node C-Style I/O Functions |
| 3294 | 260 @section C-Style I/O Functions |
| 261 | |
| 262 Octave's C-style input and output functions provide most of the | |
| 263 functionality of the C programming language's standard I/O library. The | |
| 264 argument lists for some of the input functions are slightly different, | |
| 265 however, because Octave has no way of passing arguments by reference. | |
| 266 | |
| 267 In the following, @var{file} refers to a file name and @code{fid} refers | |
| 268 to an integer file number, as returned by @code{fopen}. | |
| 269 | |
| 270 There are three files that are always available. Although these files | |
| 271 can be accessed using their corresponding numeric file ids, you should | |
| 272 always use the symbolic names given in the table below, since it will | |
| 273 make your programs easier to understand. | |
| 274 | |
| 3372 | 275 @DOCSTRING(stdin) |
| 3294 | 276 |
| 3372 | 277 @DOCSTRING(stdout) |
| 3294 | 278 |
| 3372 | 279 @DOCSTRING(stderr) |
| 3294 | 280 |
| 281 @menu | |
| 282 * Opening and Closing Files:: | |
| 283 * Simple Output:: | |
| 284 * Line-Oriented Input:: | |
| 285 * Formatted Output:: | |
| 286 * Output Conversion for Matrices:: | |
| 287 * Output Conversion Syntax:: | |
| 288 * Table of Output Conversions:: | |
| 289 * Integer Conversions:: | |
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290 * Floating-Point Conversions:: |
| 3294 | 291 * Other Output Conversions:: |
| 292 * Formatted Input:: | |
| 293 * Input Conversion Syntax:: | |
| 294 * Table of Input Conversions:: | |
| 295 * Numeric Input Conversions:: | |
| 296 * String Input Conversions:: | |
| 297 * Binary I/O:: | |
| 298 * Temporary Files:: | |
| 299 * EOF and Errors:: | |
| 300 * File Positioning:: | |
| 301 @end menu | |
| 302 | |
| 4167 | 303 @node Opening and Closing Files |
| 3294 | 304 @subsection Opening and Closing Files |
| 305 | |
| 6666 | 306 When reading data from a file it must be opened for reading first, and |
| 307 likewise when writing to a file. The @code{fopen} function returns a | |
| 308 pointer to an open file that is ready to be read or written. Once all | |
| 309 data has been read from or written to the opened file it should be closed. | |
| 310 The @code{fclose} function does this. The following code illustrates | |
| 311 the basic pattern for writing to a file, but a very similar pattern is | |
| 312 used when reading a file. | |
| 313 | |
| 314 @example | |
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315 @group |
| 6666 | 316 filename = "myfile.txt"; |
| 317 fid = fopen (filename, "w"); | |
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318 # Do the actual I/O here@dots{} |
| 6666 | 319 fclose (fid); |
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320 @end group |
| 6666 | 321 @end example |
| 322 | |
| 3372 | 323 @DOCSTRING(fopen) |
| 3294 | 324 |
| 3372 | 325 @DOCSTRING(fclose) |
| 3294 | 326 |
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327 @DOCSTRING(is_valid_file_id) |
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328 |
| 4167 | 329 @node Simple Output |
| 3294 | 330 @subsection Simple Output |
| 331 | |
| 6666 | 332 Once a file has been opened for writing a string can be written to the |
| 333 file using the @code{fputs} function. The following example shows | |
| 334 how to write the string @samp{Free Software is needed for Free Science} | |
| 335 to the file @samp{free.txt}. | |
| 336 | |
| 337 @example | |
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338 @group |
| 6666 | 339 filename = "free.txt"; |
| 340 fid = fopen (filename, "w"); | |
| 341 fputs (fid, "Free Software is needed for Free Science"); | |
| 342 fclose (fid); | |
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343 @end group |
| 6666 | 344 @end example |
| 345 | |
| 3372 | 346 @DOCSTRING(fputs) |
| 3294 | 347 |
| 6666 | 348 A function much similar to @code{fputs} is available for writing data |
| 349 to the screen. The @code{puts} function works just like @code{fputs} | |
| 350 except it doesn't take a file pointer as its input. | |
| 351 | |
| 3372 | 352 @DOCSTRING(puts) |
| 3294 | 353 |
| 4167 | 354 @node Line-Oriented Input |
| 3294 | 355 @subsection Line-Oriented Input |
| 356 | |
| 6666 | 357 To read from a file it must be opened for reading using @code{fopen}. |
| 358 Then a line can be read from the file using @code{fgetl} as the following | |
| 359 code illustrates | |
| 360 | |
| 361 @example | |
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362 @group |
| 6666 | 363 fid = fopen ("free.txt"); |
| 364 txt = fgetl (fid) | |
| 365 @print{} Free Software is needed for Free Science | |
| 366 fclose (fid); | |
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367 @end group |
| 6666 | 368 @end example |
| 369 | |
| 370 @noindent | |
| 371 This of course assumes that the file @samp{free.txt} exists and contains | |
| 372 the line @samp{Free Software is needed for Free Science}. | |
| 373 | |
| 3372 | 374 @DOCSTRING(fgetl) |
| 3294 | 375 |
| 3372 | 376 @DOCSTRING(fgets) |
| 3294 | 377 |
| 9701 | 378 @DOCSTRING(fskipl) |
| 379 | |
| 4167 | 380 @node Formatted Output |
| 3294 | 381 @subsection Formatted Output |
| 382 | |
| 383 This section describes how to call @code{printf} and related functions. | |
| 384 | |
| 385 The following functions are available for formatted output. They are | |
| 386 modelled after the C language functions of the same name, but they | |
| 387 interpret the format template differently in order to improve the | |
| 388 performance of printing vector and matrix values. | |
| 389 | |
| 3372 | 390 @DOCSTRING(printf) |
| 3294 | 391 |
| 3372 | 392 @DOCSTRING(fprintf) |
| 3294 | 393 |
| 3372 | 394 @DOCSTRING(sprintf) |
| 3294 | 395 |
| 396 The @code{printf} function can be used to print any number of arguments. | |
| 397 The template string argument you supply in a call provides | |
| 398 information not only about the number of additional arguments, but also | |
| 399 about their types and what style should be used for printing them. | |
| 400 | |
| 401 Ordinary characters in the template string are simply written to the | |
| 402 output stream as-is, while @dfn{conversion specifications} introduced by | |
| 403 a @samp{%} character in the template cause subsequent arguments to be | |
| 404 formatted and written to the output stream. For example, | |
| 405 @cindex conversion specifications (@code{printf}) | |
| 406 | |
| 6670 | 407 @example |
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408 @group |
| 3294 | 409 pct = 37; |
| 410 filename = "foo.txt"; | |
| 7031 | 411 printf ("Processed %d%% of `%s'.\nPlease be patient.\n", |
| 412 pct, filename); | |
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413 @end group |
| 6670 | 414 @end example |
| 3294 | 415 |
| 416 @noindent | |
| 417 produces output like | |
| 418 | |
| 6670 | 419 @example |
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420 @group |
| 7031 | 421 Processed 37% of `foo.txt'. |
| 3294 | 422 Please be patient. |
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423 @end group |
| 6670 | 424 @end example |
| 3294 | 425 |
| 426 This example shows the use of the @samp{%d} conversion to specify that a | |
| 427 scalar argument should be printed in decimal notation, the @samp{%s} | |
| 428 conversion to specify printing of a string argument, and the @samp{%%} | |
| 429 conversion to print a literal @samp{%} character. | |
| 430 | |
| 431 There are also conversions for printing an integer argument as an | |
| 432 unsigned value in octal, decimal, or hexadecimal radix (@samp{%o}, | |
| 433 @samp{%u}, or @samp{%x}, respectively); or as a character value | |
| 434 (@samp{%c}). | |
| 435 | |
| 436 Floating-point numbers can be printed in normal, fixed-point notation | |
| 437 using the @samp{%f} conversion or in exponential notation using the | |
| 438 @samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e} | |
| 439 or @samp{%f} format, depending on what is more appropriate for the | |
| 440 magnitude of the particular number. | |
| 441 | |
| 442 You can control formatting more precisely by writing @dfn{modifiers} | |
| 443 between the @samp{%} and the character that indicates which conversion | |
| 444 to apply. These slightly alter the ordinary behavior of the conversion. | |
| 445 For example, most conversion specifications permit you to specify a | |
| 446 minimum field width and a flag indicating whether you want the result | |
| 447 left- or right-justified within the field. | |
| 448 | |
| 449 The specific flags and modifiers that are permitted and their | |
| 450 interpretation vary depending on the particular conversion. They're all | |
| 451 described in more detail in the following sections. | |
| 452 | |
| 4167 | 453 @node Output Conversion for Matrices |
| 3294 | 454 @subsection Output Conversion for Matrices |
| 455 | |
| 456 When given a matrix value, Octave's formatted output functions cycle | |
| 457 through the format template until all the values in the matrix have been | |
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458 printed. For example: |
| 3294 | 459 |
| 460 @example | |
| 461 @group | |
| 462 printf ("%4.2f %10.2e %8.4g\n", hilb (3)); | |
| 463 | |
| 464 @print{} 1.00 5.00e-01 0.3333 | |
| 465 @print{} 0.50 3.33e-01 0.25 | |
| 466 @print{} 0.33 2.50e-01 0.2 | |
| 467 @end group | |
| 468 @end example | |
| 469 | |
| 470 If more than one value is to be printed in a single call, the output | |
| 471 functions do not return to the beginning of the format template when | |
| 472 moving on from one value to the next. This can lead to confusing output | |
| 473 if the number of elements in the matrices are not exact multiples of the | |
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474 number of conversions in the format template. For example: |
| 3294 | 475 |
| 476 @example | |
| 477 @group | |
| 478 printf ("%4.2f %10.2e %8.4g\n", [1, 2], [3, 4]); | |
| 479 | |
| 480 @print{} 1.00 2.00e+00 3 | |
| 481 @print{} 4.00 | |
| 482 @end group | |
| 483 @end example | |
| 484 | |
| 485 If this is not what you want, use a series of calls instead of just one. | |
| 486 | |
| 4167 | 487 @node Output Conversion Syntax |
| 3294 | 488 @subsection Output Conversion Syntax |
| 489 | |
| 490 This section provides details about the precise syntax of conversion | |
| 491 specifications that can appear in a @code{printf} template | |
| 492 string. | |
| 493 | |
| 494 Characters in the template string that are not part of a | |
| 495 conversion specification are printed as-is to the output stream. | |
| 496 | |
| 497 The conversion specifications in a @code{printf} template string have | |
| 498 the general form: | |
| 499 | |
| 6670 | 500 @example |
| 3294 | 501 % @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion} |
| 6670 | 502 @end example |
| 3294 | 503 |
| 504 For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-} | |
| 505 is a flag, @samp{10} specifies the field width, the precision is | |
| 506 @samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies | |
| 507 the conversion style. (This particular type specifier says to print a | |
| 508 numeric argument in decimal notation, with a minimum of 8 digits | |
| 509 left-justified in a field at least 10 characters wide.) | |
| 510 | |
| 511 In more detail, output conversion specifications consist of an | |
| 512 initial @samp{%} character followed in sequence by: | |
| 513 | |
| 514 @itemize @bullet | |
| 515 @item | |
| 516 Zero or more @dfn{flag characters} that modify the normal behavior of | |
| 517 the conversion specification. | |
| 518 @cindex flag character (@code{printf}) | |
| 519 | |
| 520 @item | |
| 521 An optional decimal integer specifying the @dfn{minimum field width}. | |
| 522 If the normal conversion produces fewer characters than this, the field | |
| 523 is padded with spaces to the specified width. This is a @emph{minimum} | |
| 524 value; if the normal conversion produces more characters than this, the | |
| 525 field is @emph{not} truncated. Normally, the output is right-justified | |
| 526 within the field. | |
| 527 @cindex minimum field width (@code{printf}) | |
| 528 | |
| 529 You can also specify a field width of @samp{*}. This means that the | |
| 530 next argument in the argument list (before the actual value to be | |
| 531 printed) is used as the field width. The value is rounded to the | |
| 532 nearest integer. If the value is negative, this means to set the | |
| 533 @samp{-} flag (see below) and to use the absolute value as the field | |
| 534 width. | |
| 535 | |
| 536 @item | |
| 537 An optional @dfn{precision} to specify the number of digits to be | |
| 538 written for the numeric conversions. If the precision is specified, it | |
| 539 consists of a period (@samp{.}) followed optionally by a decimal integer | |
| 540 (which defaults to zero if omitted). | |
| 541 @cindex precision (@code{printf}) | |
| 542 | |
| 543 You can also specify a precision of @samp{*}. This means that the next | |
| 544 argument in the argument list (before the actual value to be printed) is | |
| 545 used as the precision. The value must be an integer, and is ignored | |
| 546 if it is negative. | |
| 547 | |
| 548 @item | |
| 549 An optional @dfn{type modifier character}. This character is ignored by | |
| 550 Octave's @code{printf} function, but is recognized to provide | |
| 551 compatibility with the C language @code{printf}. | |
| 552 | |
| 553 @item | |
| 554 A character that specifies the conversion to be applied. | |
| 555 @end itemize | |
| 556 | |
| 557 The exact options that are permitted and how they are interpreted vary | |
| 558 between the different conversion specifiers. See the descriptions of the | |
| 559 individual conversions for information about the particular options that | |
| 560 they use. | |
| 561 | |
| 4167 | 562 @node Table of Output Conversions |
| 3294 | 563 @subsection Table of Output Conversions |
| 564 @cindex output conversions, for @code{printf} | |
| 565 | |
| 566 Here is a table summarizing what all the different conversions do: | |
| 567 | |
| 568 @table @asis | |
| 569 @item @samp{%d}, @samp{%i} | |
| 570 Print an integer as a signed decimal number. @xref{Integer | |
| 571 Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for | |
| 572 output, but are different when used with @code{scanf} for input | |
| 573 (@pxref{Table of Input Conversions}). | |
| 574 | |
| 575 @item @samp{%o} | |
| 576 Print an integer as an unsigned octal number. @xref{Integer | |
| 577 Conversions}, for details. | |
| 578 | |
| 579 @item @samp{%u} | |
| 580 Print an integer as an unsigned decimal number. @xref{Integer | |
| 581 Conversions}, for details. | |
| 582 | |
| 583 @item @samp{%x}, @samp{%X} | |
| 584 Print an integer as an unsigned hexadecimal number. @samp{%x} uses | |
| 585 lower-case letters and @samp{%X} uses upper-case. @xref{Integer | |
| 586 Conversions}, for details. | |
| 587 | |
| 588 @item @samp{%f} | |
| 589 Print a floating-point number in normal (fixed-point) notation. | |
| 590 @xref{Floating-Point Conversions}, for details. | |
| 591 | |
| 592 @item @samp{%e}, @samp{%E} | |
| 593 Print a floating-point number in exponential notation. @samp{%e} uses | |
| 594 lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point | |
| 595 Conversions}, for details. | |
| 596 | |
| 597 @item @samp{%g}, @samp{%G} | |
| 598 Print a floating-point number in either normal (fixed-point) or | |
| 599 exponential notation, whichever is more appropriate for its magnitude. | |
| 600 @samp{%g} uses lower-case letters and @samp{%G} uses upper-case. | |
| 601 @xref{Floating-Point Conversions}, for details. | |
| 602 | |
| 603 @item @samp{%c} | |
| 604 Print a single character. @xref{Other Output Conversions}. | |
| 605 | |
| 606 @item @samp{%s} | |
| 607 Print a string. @xref{Other Output Conversions}. | |
| 608 | |
| 609 @item @samp{%%} | |
| 610 Print a literal @samp{%} character. @xref{Other Output Conversions}. | |
| 611 @end table | |
| 612 | |
| 613 If the syntax of a conversion specification is invalid, unpredictable | |
| 614 things will happen, so don't do this. If there aren't enough function | |
| 615 arguments provided to supply values for all the conversion | |
| 616 specifications in the template string, or if the arguments are not of | |
| 617 the correct types, the results are unpredictable. If you supply more | |
| 618 arguments than conversion specifications, the extra argument values are | |
| 619 simply ignored; this is sometimes useful. | |
| 620 | |
| 4167 | 621 @node Integer Conversions |
| 3294 | 622 @subsection Integer Conversions |
| 623 | |
| 624 This section describes the options for the @samp{%d}, @samp{%i}, | |
| 625 @samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion | |
| 626 specifications. These conversions print integers in various formats. | |
| 627 | |
| 628 The @samp{%d} and @samp{%i} conversion specifications both print an | |
| 629 numeric argument as a signed decimal number; while @samp{%o}, | |
| 630 @samp{%u}, and @samp{%x} print the argument as an unsigned octal, | |
| 631 decimal, or hexadecimal number (respectively). The @samp{%X} conversion | |
| 632 specification is just like @samp{%x} except that it uses the characters | |
| 633 @samp{ABCDEF} as digits instead of @samp{abcdef}. | |
| 634 | |
| 635 The following flags are meaningful: | |
| 636 | |
| 637 @table @asis | |
| 638 @item @samp{-} | |
| 639 Left-justify the result in the field (instead of the normal | |
| 640 right-justification). | |
| 641 | |
| 642 @item @samp{+} | |
| 643 For the signed @samp{%d} and @samp{%i} conversions, print a | |
| 644 plus sign if the value is positive. | |
| 645 | |
| 646 @item @samp{ } | |
| 647 For the signed @samp{%d} and @samp{%i} conversions, if the result | |
| 648 doesn't start with a plus or minus sign, prefix it with a space | |
| 649 character instead. Since the @samp{+} flag ensures that the result | |
| 650 includes a sign, this flag is ignored if you supply both of them. | |
| 651 | |
| 652 @item @samp{#} | |
| 653 For the @samp{%o} conversion, this forces the leading digit to be | |
| 654 @samp{0}, as if by increasing the precision. For @samp{%x} or | |
| 655 @samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively) | |
| 656 to the result. This doesn't do anything useful for the @samp{%d}, | |
| 657 @samp{%i}, or @samp{%u} conversions. | |
| 658 | |
| 659 @item @samp{0} | |
| 660 Pad the field with zeros instead of spaces. The zeros are placed after | |
| 661 any indication of sign or base. This flag is ignored if the @samp{-} | |
| 662 flag is also specified, or if a precision is specified. | |
| 663 @end table | |
| 664 | |
| 665 If a precision is supplied, it specifies the minimum number of digits to | |
| 666 appear; leading zeros are produced if necessary. If you don't specify a | |
| 667 precision, the number is printed with as many digits as it needs. If | |
| 668 you convert a value of zero with an explicit precision of zero, then no | |
| 669 characters at all are produced. | |
| 670 | |
| 4167 | 671 @node Floating-Point Conversions |
| 3294 | 672 @subsection Floating-Point Conversions |
| 673 | |
| 674 This section discusses the conversion specifications for floating-point | |
| 675 numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G} | |
| 676 conversions. | |
| 677 | |
| 678 The @samp{%f} conversion prints its argument in fixed-point notation, | |
| 679 producing output of the form | |
| 680 @w{[@code{-}]@var{ddd}@code{.}@var{ddd}}, | |
| 681 where the number of digits following the decimal point is controlled | |
| 682 by the precision you specify. | |
| 683 | |
| 684 The @samp{%e} conversion prints its argument in exponential notation, | |
| 685 producing output of the form | |
| 686 @w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}. | |
| 687 Again, the number of digits following the decimal point is controlled by | |
| 688 the precision. The exponent always contains at least two digits. The | |
| 689 @samp{%E} conversion is similar but the exponent is marked with the letter | |
| 690 @samp{E} instead of @samp{e}. | |
| 691 | |
| 692 The @samp{%g} and @samp{%G} conversions print the argument in the style | |
| 693 of @samp{%e} or @samp{%E} (respectively) if the exponent would be less | |
| 694 than -4 or greater than or equal to the precision; otherwise they use the | |
| 695 @samp{%f} style. Trailing zeros are removed from the fractional portion | |
| 696 of the result and a decimal-point character appears only if it is | |
| 697 followed by a digit. | |
| 698 | |
| 699 The following flags can be used to modify the behavior: | |
| 700 | |
| 701 @c Not @samp so we can have ` ' as an item. | |
| 702 @table @asis | |
| 703 @item @samp{-} | |
| 704 Left-justify the result in the field. Normally the result is | |
| 705 right-justified. | |
| 706 | |
| 707 @item @samp{+} | |
| 708 Always include a plus or minus sign in the result. | |
| 709 | |
| 710 @item @samp{ } | |
| 711 If the result doesn't start with a plus or minus sign, prefix it with a | |
| 712 space instead. Since the @samp{+} flag ensures that the result includes | |
| 713 a sign, this flag is ignored if you supply both of them. | |
| 714 | |
| 715 @item @samp{#} | |
| 716 Specifies that the result should always include a decimal point, even | |
| 717 if no digits follow it. For the @samp{%g} and @samp{%G} conversions, | |
| 718 this also forces trailing zeros after the decimal point to be left | |
| 719 in place where they would otherwise be removed. | |
| 720 | |
| 721 @item @samp{0} | |
| 722 Pad the field with zeros instead of spaces; the zeros are placed | |
| 723 after any sign. This flag is ignored if the @samp{-} flag is also | |
| 724 specified. | |
| 725 @end table | |
| 726 | |
| 727 The precision specifies how many digits follow the decimal-point | |
| 728 character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For | |
| 729 these conversions, the default precision is @code{6}. If the precision | |
| 730 is explicitly @code{0}, this suppresses the decimal point character | |
| 731 entirely. For the @samp{%g} and @samp{%G} conversions, the precision | |
| 732 specifies how many significant digits to print. Significant digits are | |
| 733 the first digit before the decimal point, and all the digits after it. | |
| 734 If the precision is @code{0} or not specified for @samp{%g} or | |
| 735 @samp{%G}, it is treated like a value of @code{1}. If the value being | |
| 736 printed cannot be expressed precisely in the specified number of digits, | |
| 737 the value is rounded to the nearest number that fits. | |
| 738 | |
| 4167 | 739 @node Other Output Conversions |
| 3294 | 740 @subsection Other Output Conversions |
| 741 | |
| 742 This section describes miscellaneous conversions for @code{printf}. | |
| 743 | |
| 744 The @samp{%c} conversion prints a single character. The @samp{-} | |
| 745 flag can be used to specify left-justification in the field, but no | |
| 746 other flags are defined, and no precision or type modifier can be given. | |
| 747 For example: | |
| 748 | |
| 6670 | 749 @example |
| 3294 | 750 printf ("%c%c%c%c%c", "h", "e", "l", "l", "o"); |
| 6670 | 751 @end example |
| 3294 | 752 |
| 753 @noindent | |
| 754 prints @samp{hello}. | |
| 755 | |
| 756 The @samp{%s} conversion prints a string. The corresponding argument | |
| 757 must be a string. A precision can be specified to indicate the maximum | |
| 758 number of characters to write; otherwise characters in the string up to | |
| 759 but not including the terminating null character are written to the | |
| 760 output stream. The @samp{-} flag can be used to specify | |
| 761 left-justification in the field, but no other flags or type modifiers | |
| 762 are defined for this conversion. For example: | |
| 763 | |
| 6670 | 764 @example |
| 3294 | 765 printf ("%3s%-6s", "no", "where"); |
| 6670 | 766 @end example |
| 3294 | 767 |
| 768 @noindent | |
| 769 prints @samp{ nowhere } (note the leading and trailing spaces). | |
| 770 | |
| 4167 | 771 @node Formatted Input |
| 3294 | 772 @subsection Formatted Input |
| 773 | |
| 774 Octave provides the @code{scanf}, @code{fscanf}, and @code{sscanf} | |
| 775 functions to read formatted input. There are two forms of each of these | |
| 776 functions. One can be used to extract vectors of data from a file, and | |
| 777 the other is more `C-like'. | |
| 778 | |
| 3428 | 779 @DOCSTRING(fscanf) |
| 3294 | 780 |
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782 |
| 3372 | 783 @DOCSTRING(sscanf) |
| 3294 | 784 |
| 785 Calls to @code{scanf} are superficially similar to calls to | |
| 786 @code{printf} in that arbitrary arguments are read under the control of | |
| 787 a template string. While the syntax of the conversion specifications in | |
| 788 the template is very similar to that for @code{printf}, the | |
| 789 interpretation of the template is oriented more towards free-format | |
| 790 input and simple pattern matching, rather than fixed-field formatting. | |
| 791 For example, most @code{scanf} conversions skip over any amount of | |
| 792 ``white space'' (including spaces, tabs, and newlines) in the input | |
| 793 file, and there is no concept of precision for the numeric input | |
| 794 conversions as there is for the corresponding output conversions. | |
| 795 Ordinarily, non-whitespace characters in the template are expected to | |
| 796 match characters in the input stream exactly. | |
| 797 @cindex conversion specifications (@code{scanf}) | |
| 798 | |
| 799 When a @dfn{matching failure} occurs, @code{scanf} returns immediately, | |
| 800 leaving the first non-matching character as the next character to be | |
| 801 read from the stream, and @code{scanf} returns all the items that were | |
| 802 successfully converted. | |
| 803 @cindex matching failure, in @code{scanf} | |
| 804 | |
| 805 The formatted input functions are not used as frequently as the | |
| 806 formatted output functions. Partly, this is because it takes some care | |
| 807 to use them properly. Another reason is that it is difficult to recover | |
| 808 from a matching error. | |
| 809 | |
| 4167 | 810 @node Input Conversion Syntax |
| 3294 | 811 @subsection Input Conversion Syntax |
| 812 | |
| 813 A @code{scanf} template string is a string that contains ordinary | |
| 814 multibyte characters interspersed with conversion specifications that | |
| 815 start with @samp{%}. | |
| 816 | |
| 817 Any whitespace character in the template causes any number of whitespace | |
| 818 characters in the input stream to be read and discarded. The whitespace | |
| 819 characters that are matched need not be exactly the same whitespace | |
| 820 characters that appear in the template string. For example, write | |
| 821 @samp{ , } in the template to recognize a comma with optional whitespace | |
| 822 before and after. | |
| 823 | |
| 824 Other characters in the template string that are not part of conversion | |
| 825 specifications must match characters in the input stream exactly; if | |
| 826 this is not the case, a matching failure occurs. | |
| 827 | |
| 828 The conversion specifications in a @code{scanf} template string | |
| 829 have the general form: | |
| 830 | |
| 6670 | 831 @example |
| 3294 | 832 % @var{flags} @var{width} @var{type} @var{conversion} |
| 6670 | 833 @end example |
| 3294 | 834 |
| 835 In more detail, an input conversion specification consists of an initial | |
| 836 @samp{%} character followed in sequence by: | |
| 837 | |
| 838 @itemize @bullet | |
| 839 @item | |
| 840 An optional @dfn{flag character} @samp{*}, which says to ignore the text | |
| 841 read for this specification. When @code{scanf} finds a conversion | |
| 842 specification that uses this flag, it reads input as directed by the | |
| 843 rest of the conversion specification, but it discards this input, does | |
| 844 not return any value, and does not increment the count of | |
| 845 successful assignments. | |
| 846 @cindex flag character (@code{scanf}) | |
| 847 | |
| 848 @item | |
| 849 An optional decimal integer that specifies the @dfn{maximum field | |
| 850 width}. Reading of characters from the input stream stops either when | |
| 851 this maximum is reached or when a non-matching character is found, | |
| 852 whichever happens first. Most conversions discard initial whitespace | |
| 853 characters, and these discarded characters don't count towards the | |
| 854 maximum field width. Conversions that do not discard initial whitespace | |
| 855 are explicitly documented. | |
| 856 @cindex maximum field width (@code{scanf}) | |
| 857 | |
| 858 @item | |
| 859 An optional type modifier character. This character is ignored by | |
| 860 Octave's @code{scanf} function, but is recognized to provide | |
| 861 compatibility with the C language @code{scanf}. | |
| 862 | |
| 863 @item | |
| 864 A character that specifies the conversion to be applied. | |
| 865 @end itemize | |
| 866 | |
| 867 The exact options that are permitted and how they are interpreted vary | |
| 868 between the different conversion specifiers. See the descriptions of the | |
| 869 individual conversions for information about the particular options that | |
| 870 they allow. | |
| 871 | |
| 4167 | 872 @node Table of Input Conversions |
| 3294 | 873 @subsection Table of Input Conversions |
| 874 @cindex input conversions, for @code{scanf} | |
| 875 | |
| 876 Here is a table that summarizes the various conversion specifications: | |
| 877 | |
| 878 @table @asis | |
| 879 @item @samp{%d} | |
| 880 Matches an optionally signed integer written in decimal. @xref{Numeric | |
| 881 Input Conversions}. | |
| 882 | |
| 883 @item @samp{%i} | |
| 884 Matches an optionally signed integer in any of the formats that the C | |
| 885 language defines for specifying an integer constant. @xref{Numeric | |
| 886 Input Conversions}. | |
| 887 | |
| 888 @item @samp{%o} | |
| 889 Matches an unsigned integer written in octal radix. | |
| 890 @xref{Numeric Input Conversions}. | |
| 891 | |
| 892 @item @samp{%u} | |
| 893 Matches an unsigned integer written in decimal radix. | |
| 894 @xref{Numeric Input Conversions}. | |
| 895 | |
| 896 @item @samp{%x}, @samp{%X} | |
| 897 Matches an unsigned integer written in hexadecimal radix. | |
| 898 @xref{Numeric Input Conversions}. | |
| 899 | |
| 900 @item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G} | |
| 901 Matches an optionally signed floating-point number. @xref{Numeric Input | |
| 902 Conversions}. | |
| 903 | |
| 904 @item @samp{%s} | |
| 905 Matches a string containing only non-whitespace characters. | |
| 906 @xref{String Input Conversions}. | |
| 907 | |
| 908 @item @samp{%c} | |
| 909 Matches a string of one or more characters; the number of characters | |
| 910 read is controlled by the maximum field width given for the conversion. | |
| 911 @xref{String Input Conversions}. | |
| 912 | |
| 913 @item @samp{%%} | |
| 914 This matches a literal @samp{%} character in the input stream. No | |
| 915 corresponding argument is used. | |
| 916 @end table | |
| 917 | |
| 918 If the syntax of a conversion specification is invalid, the behavior is | |
| 919 undefined. If there aren't enough function arguments provided to supply | |
| 920 addresses for all the conversion specifications in the template strings | |
| 921 that perform assignments, or if the arguments are not of the correct | |
| 922 types, the behavior is also undefined. On the other hand, extra | |
| 923 arguments are simply ignored. | |
| 924 | |
| 4167 | 925 @node Numeric Input Conversions |
| 3294 | 926 @subsection Numeric Input Conversions |
| 927 | |
| 928 This section describes the @code{scanf} conversions for reading numeric | |
| 929 values. | |
| 930 | |
| 931 The @samp{%d} conversion matches an optionally signed integer in decimal | |
| 932 radix. | |
| 933 | |
| 934 The @samp{%i} conversion matches an optionally signed integer in any of | |
| 935 the formats that the C language defines for specifying an integer | |
| 936 constant. | |
| 937 | |
| 938 For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012} | |
| 939 could be read in as integers under the @samp{%i} conversion. Each of | |
| 940 these specifies a number with decimal value @code{10}. | |
| 941 | |
| 942 The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned | |
| 943 integers in octal, decimal, and hexadecimal radices, respectively. | |
| 944 | |
| 945 The @samp{%X} conversion is identical to the @samp{%x} conversion. They | |
| 946 both permit either uppercase or lowercase letters to be used as digits. | |
| 947 | |
| 948 Unlike the C language @code{scanf}, Octave ignores the @samp{h}, | |
| 949 @samp{l}, and @samp{L} modifiers. | |
| 950 | |
| 4167 | 951 @node String Input Conversions |
| 3294 | 952 @subsection String Input Conversions |
| 953 | |
| 954 This section describes the @code{scanf} input conversions for reading | |
| 955 string and character values: @samp{%s} and @samp{%c}. | |
| 956 | |
| 957 The @samp{%c} conversion is the simplest: it matches a fixed number of | |
| 958 characters, always. The maximum field with says how many characters to | |
| 959 read; if you don't specify the maximum, the default is 1. This | |
| 960 conversion does not skip over initial whitespace characters. It reads | |
| 961 precisely the next @var{n} characters, and fails if it cannot get that | |
| 962 many. | |
| 963 | |
| 964 The @samp{%s} conversion matches a string of non-whitespace characters. | |
| 965 It skips and discards initial whitespace, but stops when it encounters | |
| 966 more whitespace after having read something. | |
| 967 | |
| 968 For example, reading the input: | |
| 969 | |
| 6670 | 970 @example |
| 3294 | 971 hello, world |
| 6670 | 972 @end example |
| 3294 | 973 |
| 974 @noindent | |
| 975 with the conversion @samp{%10c} produces @code{" hello, wo"}, but | |
| 976 reading the same input with the conversion @samp{%10s} produces | |
| 977 @code{"hello,"}. | |
| 978 | |
| 4167 | 979 @node Binary I/O |
| 3294 | 980 @subsection Binary I/O |
| 981 | |
| 982 Octave can read and write binary data using the functions @code{fread} | |
| 983 and @code{fwrite}, which are patterned after the standard C functions | |
| 6939 | 984 with the same names. They are able to automatically swap the byte order |
| 985 of integer data and convert among the supported floating point formats | |
| 3294 | 986 as the data are read. |
| 987 | |
| 3372 | 988 @DOCSTRING(fread) |
| 3294 | 989 |
| 3372 | 990 @DOCSTRING(fwrite) |
| 3294 | 991 |
| 4167 | 992 @node Temporary Files |
| 3294 | 993 @subsection Temporary Files |
| 994 | |
| 6666 | 995 Sometimes one needs to write data to a file that is only temporary. |
| 996 This is most commonly used when an external program launched from | |
| 997 within Octave needs to access data. When Octave exits all temporary | |
| 998 files will be deleted, so this step need not be executed manually. | |
| 999 | |
| 4328 | 1000 @DOCSTRING(mkstemp) |
| 1001 | |
| 1002 @DOCSTRING(tmpfile) | |
| 1003 | |
| 3372 | 1004 @DOCSTRING(tmpnam) |
| 3294 | 1005 |
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1006 @DOCSTRING(octave_tmp_file_name) |
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1007 |
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1008 @node EOF and Errors, File Positioning, Temporary Files, C-Style I/O Functions |
| 3294 | 1009 @subsection End of File and Errors |
| 1010 | |
| 6666 | 1011 Once a file has been opened its status can be acquired. As an example |
| 1012 the @code{feof} functions determines if the end of the file has been | |
| 1013 reached. This can be very useful when reading small parts of a file | |
| 1014 at a time. The following example shows how to read one line at a time | |
| 1015 from a file until the end has been reached. | |
| 1016 | |
| 1017 @example | |
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1018 @group |
| 6666 | 1019 filename = "myfile.txt"; |
| 1020 fid = fopen (filename, "r"); | |
| 1021 while (! feof (fid) ) | |
| 1022 text_line = fgetl (fid); | |
| 1023 endwhile | |
| 1024 fclose (fid); | |
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1025 @end group |
| 6666 | 1026 @end example |
| 1027 | |
| 1028 @noindent | |
| 1029 Note that in some situations it is more efficient to read the entire | |
| 1030 contents of a file and then process it, than it is to read it line by | |
| 1031 line. This has the potential advantage of removing the loop in the | |
| 1032 above code. | |
| 1033 | |
| 3372 | 1034 @DOCSTRING(feof) |
| 3294 | 1035 |
| 3372 | 1036 @DOCSTRING(ferror) |
| 3294 | 1037 |
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1038 @DOCSTRING(fclear) |
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1039 |
| 3372 | 1040 @DOCSTRING(freport) |
| 3294 | 1041 |
| 4167 | 1042 @node File Positioning |
| 3294 | 1043 @subsection File Positioning |
| 1044 | |
| 1045 Three functions are available for setting and determining the position of | |
| 1046 the file pointer for a given file. | |
| 1047 | |
| 3372 | 1048 @DOCSTRING(ftell) |
| 1049 | |
| 1050 @DOCSTRING(fseek) | |
| 3294 | 1051 |
| 3372 | 1052 @DOCSTRING(SEEK_SET) |
| 3294 | 1053 |
| 3372 | 1054 @DOCSTRING(frewind) |
| 3294 | 1055 |
| 1056 The following example stores the current file position in the variable | |
| 1057 @code{marker}, moves the pointer to the beginning of the file, reads | |
| 1058 four characters, and then returns to the original position. | |
| 1059 | |
| 1060 @example | |
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1061 @group |
| 3294 | 1062 marker = ftell (myfile); |
| 1063 frewind (myfile); | |
| 1064 fourch = fgets (myfile, 4); | |
| 1065 fseek (myfile, marker, SEEK_SET); | |
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1066 @end group |
| 3294 | 1067 @end example |
| 1068 |