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1 @c Copyright (C) 1996, 1997 John W. Eaton |
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2 @c This is part of the Octave manual. |
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3 @c For copying conditions, see the file gpl.texi. |
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4 |
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5 @node Input and Output |
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6 @chapter Input and Output |
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7 |
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8 Octave supports several ways of reading and writing data to or from the |
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9 prompt or a file. The most simple functions for data Input and Output |
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10 (I/O) are easy to use, but only provides a limited control of how |
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11 data is processed. For more control, a set of functions modelled |
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12 after the C standard library are also provided by Octave. |
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13 |
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14 @menu |
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15 * Basic Input and Output:: |
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16 * C-Style I/O Functions:: |
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17 @end menu |
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18 |
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19 @node Basic Input and Output |
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20 @section Basic Input and Output |
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21 |
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22 @c We could use a two-line introduction here... |
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23 |
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24 @menu |
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25 * Terminal Output:: |
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26 * Terminal Input:: |
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27 * Simple File I/O:: |
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28 @end menu |
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29 |
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30 @node Terminal Output |
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31 @subsection Terminal Output |
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32 |
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33 Since Octave normally prints the value of an expression as soon as it |
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34 has been evaluated, the simplest of all I/O functions is a simple |
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35 expression. For example, the following expression will display the |
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36 value of @samp{pi} |
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37 |
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38 @example |
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39 pi |
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40 @print{} pi = 3.1416 |
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41 @end example |
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42 |
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43 This works well as long as it is acceptable to have the name of the |
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44 variable (or @samp{ans}) printed along with the value. To print the |
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45 value of a variable without printing its name, use the function |
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46 @code{disp}. |
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47 |
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48 The @code{format} command offers some control over the way Octave prints |
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49 values with @code{disp} and through the normal echoing mechanism. |
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50 |
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51 @DOCSTRING(ans) |
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52 |
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53 @DOCSTRING(disp) |
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54 |
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55 @DOCSTRING(format) |
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56 |
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57 @DOCSTRING(print_answer_id_name) |
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58 |
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59 @menu |
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60 * Paging Screen Output:: |
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61 @end menu |
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62 |
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63 @node Paging Screen Output |
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64 @subsubsection Paging Screen Output |
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65 |
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66 When running interactively, Octave normally sends any output intended |
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67 for your terminal that is more than one screen long to a paging program, |
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68 such as @code{less} or @code{more}. This avoids the problem of having a |
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69 large volume of output stream by before you can read it. With |
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70 @code{less} (and some versions of @code{more}) you can also scan forward |
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71 and backward, and search for specific items. |
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72 |
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73 Normally, no output is displayed by the pager until just before Octave |
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74 is ready to print the top level prompt, or read from the standard input |
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75 (for example, by using the @code{fscanf} or @code{scanf} functions). |
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76 This means that there may be some delay before any output appears on |
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77 your screen if you have asked Octave to perform a significant amount of |
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78 work with a single command statement. The function @code{fflush} may be |
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79 used to force output to be sent to the pager (or any other stream) |
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80 immediately. |
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81 |
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82 You can select the program to run as the pager using the @code{PAGER} |
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83 function, and you can turn paging off by using the function |
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84 @code{more}. |
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85 |
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86 @DOCSTRING(more) |
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87 |
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88 @DOCSTRING(PAGER) |
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89 |
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90 @DOCSTRING(PAGER_FLAGS) |
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91 |
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92 @DOCSTRING(page_screen_output) |
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93 |
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94 @DOCSTRING(page_output_immediately) |
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95 |
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96 @DOCSTRING(fflush) |
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97 |
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98 @c FIXME -- maybe this would be a good place to describe the |
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99 @c following message: |
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100 @c |
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101 @c warning: connection to external pager (pid = 9334) lost -- |
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102 @c warning: pending computations and output may be lost |
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103 @c warning: broken pipe |
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104 |
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105 @node Terminal Input |
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106 @subsection Terminal Input |
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107 |
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108 Octave has three functions that make it easy to prompt users for |
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109 input. The @code{input} and @code{menu} functions are normally |
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110 used for managing an interactive dialog with a user, and the |
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111 @code{keyboard} function is normally used for doing simple debugging. |
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112 |
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113 @DOCSTRING(input) |
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114 |
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115 @DOCSTRING(menu) |
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116 |
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117 For @code{input}, the normal command line history and editing functions |
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118 are available at the prompt. |
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119 |
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120 Octave also has a function that makes it possible to get a single |
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121 character from the keyboard without requiring the user to type a |
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122 carriage return. |
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123 |
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124 @DOCSTRING(kbhit) |
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125 |
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126 @node Simple File I/O |
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127 @subsection Simple File I/O |
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128 |
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129 @cindex saving data |
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130 @cindex loading data |
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131 The @code{save} and @code{load} commands allow data to be written to and |
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132 read from disk files in various formats. The default format of files |
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133 written by the @code{save} command can be controlled using the functions |
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134 @code{default_save_options} and @code{save_precision}. |
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135 |
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136 As an example the following code creates a 3-by-3 matrix and saves it |
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137 to the file @samp{myfile.mat}. |
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138 |
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139 @example |
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140 A = [ 1:3; 4:6; 7:9 ]; |
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141 save myfile.mat A |
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142 @end example |
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143 |
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144 Once one or more variables have been saved to a file, they can be |
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145 read into memory using the @code{load} command. |
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146 |
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147 @example |
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148 load myfile.mat |
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149 A |
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150 @print{} A = |
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151 @print{} |
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152 @print{} 1 2 3 |
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153 @print{} 4 5 6 |
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154 @print{} 7 8 9 |
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155 @end example |
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156 |
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157 @DOCSTRING(save) |
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158 |
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159 @DOCSTRING(load) |
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160 |
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161 There are three functions that modify the behavior of @code{save}. |
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162 |
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163 @DOCSTRING(default_save_options) |
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164 |
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165 @DOCSTRING(save_precision) |
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166 |
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167 @DOCSTRING(save_header_format_string) |
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168 |
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169 @DOCSTRING(native_float_format) |
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170 |
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171 It is possible to write data to a file in a way much similar to the |
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172 @code{disp} function for writing data to the screen. The @code{fdisp} |
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173 works just like @code{disp} except its first argument is a file pointer |
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174 as created by @code{fopen}. As an example, the following code writes |
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175 to data @samp{myfile.txt}. |
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176 |
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177 @example |
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178 fid = fopen ("myfile.txt", "w"); |
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179 fdisp (fid, "3/8 is "); |
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180 fdisp (fid, 3/8); |
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181 fclose (fid); |
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182 @end example |
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183 |
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184 @noindent |
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185 @xref{Opening and Closing Files}, for details on how to use @code{fopen} |
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186 and @code{fclose}. |
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187 |
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188 @DOCSTRING(fdisp) |
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189 |
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190 @menu |
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191 * Saving Data on Unexpected Exits:: |
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192 @end menu |
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193 |
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194 @node Saving Data on Unexpected Exits |
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195 @subsubsection Saving Data on Unexpected Exits |
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196 |
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197 If Octave for some reason exits unexpected it will by default save the |
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198 variables available in the workspace to a file in the current directory. |
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199 By default this file is named @samp{octave-core} and can be loaded |
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200 into memory with the @code{load} command. While the default behaviour |
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201 most often is reasonable it can be changed through the following |
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202 functions. |
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203 |
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204 @DOCSTRING(crash_dumps_octave_core) |
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205 |
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206 @DOCSTRING(sighup_dumps_octave_core) |
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207 |
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208 @DOCSTRING(sigterm_dumps_octave_core) |
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209 |
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210 @DOCSTRING(octave_core_file_options) |
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211 |
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212 @DOCSTRING(octave_core_file_limit) |
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213 |
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214 @DOCSTRING(octave_core_file_name) |
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215 |
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216 |
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217 @node C-Style I/O Functions |
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218 @section C-Style I/O Functions |
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219 |
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220 Octave's C-style input and output functions provide most of the |
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221 functionality of the C programming language's standard I/O library. The |
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222 argument lists for some of the input functions are slightly different, |
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223 however, because Octave has no way of passing arguments by reference. |
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224 |
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225 In the following, @var{file} refers to a file name and @code{fid} refers |
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226 to an integer file number, as returned by @code{fopen}. |
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227 |
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228 There are three files that are always available. Although these files |
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229 can be accessed using their corresponding numeric file ids, you should |
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230 always use the symbolic names given in the table below, since it will |
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231 make your programs easier to understand. |
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232 |
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233 @DOCSTRING(stdin) |
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234 |
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235 @DOCSTRING(stdout) |
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236 |
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237 @DOCSTRING(stderr) |
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238 |
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239 @menu |
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240 * Opening and Closing Files:: |
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241 * Simple Output:: |
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242 * Line-Oriented Input:: |
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243 * Formatted Output:: |
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244 * Output Conversion for Matrices:: |
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245 * Output Conversion Syntax:: |
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246 * Table of Output Conversions:: |
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247 * Integer Conversions:: |
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248 * Floating-Point Conversions:: Other Output Conversions:: |
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249 * Other Output Conversions:: |
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250 * Formatted Input:: |
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251 * Input Conversion Syntax:: |
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252 * Table of Input Conversions:: |
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253 * Numeric Input Conversions:: |
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254 * String Input Conversions:: |
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255 * Binary I/O:: |
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256 * Temporary Files:: |
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257 * EOF and Errors:: |
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258 * File Positioning:: |
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259 @end menu |
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260 |
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261 @node Opening and Closing Files |
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262 @subsection Opening and Closing Files |
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263 |
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264 When reading data from a file it must be opened for reading first, and |
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265 likewise when writing to a file. The @code{fopen} function returns a |
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266 pointer to an open file that is ready to be read or written. Once all |
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267 data has been read from or written to the opened file it should be closed. |
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268 The @code{fclose} function does this. The following code illustrates |
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269 the basic pattern for writing to a file, but a very similar pattern is |
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270 used when reading a file. |
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271 |
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272 @example |
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273 filename = "myfile.txt"; |
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274 fid = fopen (filename, "w"); |
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275 # Do the actual I/O here... |
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276 fclose (fid); |
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277 @end example |
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278 |
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279 @DOCSTRING(fopen) |
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280 |
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281 @DOCSTRING(fclose) |
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282 |
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283 @node Simple Output |
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284 @subsection Simple Output |
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285 |
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286 Once a file has been opened for writing a string can be written to the |
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287 file using the @code{fputs} function. The following example shows |
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288 how to write the string @samp{Free Software is needed for Free Science} |
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289 to the file @samp{free.txt}. |
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290 |
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291 @example |
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292 filename = "free.txt"; |
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293 fid = fopen (filename, "w"); |
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294 fputs (fid, "Free Software is needed for Free Science"); |
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295 fclose (fid); |
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296 @end example |
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297 |
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298 @DOCSTRING(fputs) |
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299 |
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300 A function much similar to @code{fputs} is available for writing data |
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301 to the screen. The @code{puts} function works just like @code{fputs} |
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302 except it doesn't take a file pointer as its input. |
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303 |
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304 @DOCSTRING(puts) |
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305 |
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306 @node Line-Oriented Input |
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307 @subsection Line-Oriented Input |
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308 |
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309 To read from a file it must be opened for reading using @code{fopen}. |
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310 Then a line can be read from the file using @code{fgetl} as the following |
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311 code illustrates |
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312 |
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313 @example |
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314 fid = fopen ("free.txt"); |
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315 txt = fgetl (fid) |
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316 @print{} Free Software is needed for Free Science |
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317 fclose (fid); |
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318 @end example |
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319 |
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320 @noindent |
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321 This of course assumes that the file @samp{free.txt} exists and contains |
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322 the line @samp{Free Software is needed for Free Science}. |
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323 |
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324 @DOCSTRING(fgetl) |
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325 |
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326 @DOCSTRING(fgets) |
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327 |
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328 @node Formatted Output |
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329 @subsection Formatted Output |
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330 |
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331 This section describes how to call @code{printf} and related functions. |
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332 |
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333 The following functions are available for formatted output. They are |
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334 modelled after the C language functions of the same name, but they |
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335 interpret the format template differently in order to improve the |
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336 performance of printing vector and matrix values. |
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337 |
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338 @DOCSTRING(printf) |
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339 |
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340 @DOCSTRING(fprintf) |
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341 |
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342 @DOCSTRING(sprintf) |
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343 |
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344 The @code{printf} function can be used to print any number of arguments. |
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345 The template string argument you supply in a call provides |
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346 information not only about the number of additional arguments, but also |
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347 about their types and what style should be used for printing them. |
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348 |
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349 Ordinary characters in the template string are simply written to the |
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350 output stream as-is, while @dfn{conversion specifications} introduced by |
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351 a @samp{%} character in the template cause subsequent arguments to be |
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352 formatted and written to the output stream. For example, |
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353 @cindex conversion specifications (@code{printf}) |
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354 |
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355 @example |
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356 pct = 37; |
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357 filename = "foo.txt"; |
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358 printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n", |
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359 filename, pct); |
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360 @end example |
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361 |
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362 @noindent |
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363 produces output like |
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364 |
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365 @example |
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366 Processing of `foo.txt' is 37% finished. |
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367 Please be patient. |
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368 @end example |
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369 |
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370 This example shows the use of the @samp{%d} conversion to specify that a |
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371 scalar argument should be printed in decimal notation, the @samp{%s} |
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372 conversion to specify printing of a string argument, and the @samp{%%} |
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373 conversion to print a literal @samp{%} character. |
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374 |
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375 There are also conversions for printing an integer argument as an |
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376 unsigned value in octal, decimal, or hexadecimal radix (@samp{%o}, |
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377 @samp{%u}, or @samp{%x}, respectively); or as a character value |
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378 (@samp{%c}). |
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379 |
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380 Floating-point numbers can be printed in normal, fixed-point notation |
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381 using the @samp{%f} conversion or in exponential notation using the |
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382 @samp{%e} conversion. The @samp{%g} conversion uses either @samp{%e} |
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383 or @samp{%f} format, depending on what is more appropriate for the |
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384 magnitude of the particular number. |
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385 |
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386 You can control formatting more precisely by writing @dfn{modifiers} |
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387 between the @samp{%} and the character that indicates which conversion |
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388 to apply. These slightly alter the ordinary behavior of the conversion. |
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389 For example, most conversion specifications permit you to specify a |
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390 minimum field width and a flag indicating whether you want the result |
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391 left- or right-justified within the field. |
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392 |
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393 The specific flags and modifiers that are permitted and their |
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394 interpretation vary depending on the particular conversion. They're all |
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395 described in more detail in the following sections. |
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396 |
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397 @node Output Conversion for Matrices |
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398 @subsection Output Conversion for Matrices |
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399 |
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400 When given a matrix value, Octave's formatted output functions cycle |
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401 through the format template until all the values in the matrix have been |
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402 printed. For example, |
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403 |
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404 @example |
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405 @group |
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406 printf ("%4.2f %10.2e %8.4g\n", hilb (3)); |
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407 |
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408 @print{} 1.00 5.00e-01 0.3333 |
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409 @print{} 0.50 3.33e-01 0.25 |
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410 @print{} 0.33 2.50e-01 0.2 |
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411 @end group |
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412 @end example |
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413 |
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414 If more than one value is to be printed in a single call, the output |
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415 functions do not return to the beginning of the format template when |
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416 moving on from one value to the next. This can lead to confusing output |
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417 if the number of elements in the matrices are not exact multiples of the |
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418 number of conversions in the format template. For example, |
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419 |
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420 @example |
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421 @group |
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422 printf ("%4.2f %10.2e %8.4g\n", [1, 2], [3, 4]); |
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423 |
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424 @print{} 1.00 2.00e+00 3 |
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425 @print{} 4.00 |
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426 @end group |
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427 @end example |
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428 |
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429 If this is not what you want, use a series of calls instead of just one. |
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430 |
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431 @node Output Conversion Syntax |
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432 @subsection Output Conversion Syntax |
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433 |
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434 This section provides details about the precise syntax of conversion |
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435 specifications that can appear in a @code{printf} template |
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436 string. |
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437 |
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438 Characters in the template string that are not part of a |
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439 conversion specification are printed as-is to the output stream. |
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440 |
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441 The conversion specifications in a @code{printf} template string have |
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442 the general form: |
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443 |
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444 @example |
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445 % @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion} |
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446 @end example |
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447 |
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448 For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-} |
|
|
449 is a flag, @samp{10} specifies the field width, the precision is |
|
|
450 @samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies |
|
|
451 the conversion style. (This particular type specifier says to print a |
|
|
452 numeric argument in decimal notation, with a minimum of 8 digits |
|
|
453 left-justified in a field at least 10 characters wide.) |
|
|
454 |
|
|
455 In more detail, output conversion specifications consist of an |
|
|
456 initial @samp{%} character followed in sequence by: |
|
|
457 |
|
|
458 @itemize @bullet |
|
|
459 @item |
|
|
460 Zero or more @dfn{flag characters} that modify the normal behavior of |
|
|
461 the conversion specification. |
|
|
462 @cindex flag character (@code{printf}) |
|
|
463 |
|
|
464 @item |
|
|
465 An optional decimal integer specifying the @dfn{minimum field width}. |
|
|
466 If the normal conversion produces fewer characters than this, the field |
|
|
467 is padded with spaces to the specified width. This is a @emph{minimum} |
|
|
468 value; if the normal conversion produces more characters than this, the |
|
|
469 field is @emph{not} truncated. Normally, the output is right-justified |
|
|
470 within the field. |
|
|
471 @cindex minimum field width (@code{printf}) |
|
|
472 |
|
|
473 You can also specify a field width of @samp{*}. This means that the |
|
|
474 next argument in the argument list (before the actual value to be |
|
|
475 printed) is used as the field width. The value is rounded to the |
|
|
476 nearest integer. If the value is negative, this means to set the |
|
|
477 @samp{-} flag (see below) and to use the absolute value as the field |
|
|
478 width. |
|
|
479 |
|
|
480 @item |
|
|
481 An optional @dfn{precision} to specify the number of digits to be |
|
|
482 written for the numeric conversions. If the precision is specified, it |
|
|
483 consists of a period (@samp{.}) followed optionally by a decimal integer |
|
|
484 (which defaults to zero if omitted). |
|
|
485 @cindex precision (@code{printf}) |
|
|
486 |
|
|
487 You can also specify a precision of @samp{*}. This means that the next |
|
|
488 argument in the argument list (before the actual value to be printed) is |
|
|
489 used as the precision. The value must be an integer, and is ignored |
|
|
490 if it is negative. |
|
|
491 |
|
|
492 @item |
|
|
493 An optional @dfn{type modifier character}. This character is ignored by |
|
|
494 Octave's @code{printf} function, but is recognized to provide |
|
|
495 compatibility with the C language @code{printf}. |
|
|
496 |
|
|
497 @item |
|
|
498 A character that specifies the conversion to be applied. |
|
|
499 @end itemize |
|
|
500 |
|
|
501 The exact options that are permitted and how they are interpreted vary |
|
|
502 between the different conversion specifiers. See the descriptions of the |
|
|
503 individual conversions for information about the particular options that |
|
|
504 they use. |
|
|
505 |
|
4167
|
506 @node Table of Output Conversions |
|
3294
|
507 @subsection Table of Output Conversions |
|
|
508 @cindex output conversions, for @code{printf} |
|
|
509 |
|
|
510 Here is a table summarizing what all the different conversions do: |
|
|
511 |
|
|
512 @table @asis |
|
|
513 @item @samp{%d}, @samp{%i} |
|
|
514 Print an integer as a signed decimal number. @xref{Integer |
|
|
515 Conversions}, for details. @samp{%d} and @samp{%i} are synonymous for |
|
|
516 output, but are different when used with @code{scanf} for input |
|
|
517 (@pxref{Table of Input Conversions}). |
|
|
518 |
|
|
519 @item @samp{%o} |
|
|
520 Print an integer as an unsigned octal number. @xref{Integer |
|
|
521 Conversions}, for details. |
|
|
522 |
|
|
523 @item @samp{%u} |
|
|
524 Print an integer as an unsigned decimal number. @xref{Integer |
|
|
525 Conversions}, for details. |
|
|
526 |
|
|
527 @item @samp{%x}, @samp{%X} |
|
|
528 Print an integer as an unsigned hexadecimal number. @samp{%x} uses |
|
|
529 lower-case letters and @samp{%X} uses upper-case. @xref{Integer |
|
|
530 Conversions}, for details. |
|
|
531 |
|
|
532 @item @samp{%f} |
|
|
533 Print a floating-point number in normal (fixed-point) notation. |
|
|
534 @xref{Floating-Point Conversions}, for details. |
|
|
535 |
|
|
536 @item @samp{%e}, @samp{%E} |
|
|
537 Print a floating-point number in exponential notation. @samp{%e} uses |
|
|
538 lower-case letters and @samp{%E} uses upper-case. @xref{Floating-Point |
|
|
539 Conversions}, for details. |
|
|
540 |
|
|
541 @item @samp{%g}, @samp{%G} |
|
|
542 Print a floating-point number in either normal (fixed-point) or |
|
|
543 exponential notation, whichever is more appropriate for its magnitude. |
|
|
544 @samp{%g} uses lower-case letters and @samp{%G} uses upper-case. |
|
|
545 @xref{Floating-Point Conversions}, for details. |
|
|
546 |
|
|
547 @item @samp{%c} |
|
|
548 Print a single character. @xref{Other Output Conversions}. |
|
|
549 |
|
|
550 @item @samp{%s} |
|
|
551 Print a string. @xref{Other Output Conversions}. |
|
|
552 |
|
|
553 @item @samp{%%} |
|
|
554 Print a literal @samp{%} character. @xref{Other Output Conversions}. |
|
|
555 @end table |
|
|
556 |
|
|
557 If the syntax of a conversion specification is invalid, unpredictable |
|
|
558 things will happen, so don't do this. If there aren't enough function |
|
|
559 arguments provided to supply values for all the conversion |
|
|
560 specifications in the template string, or if the arguments are not of |
|
|
561 the correct types, the results are unpredictable. If you supply more |
|
|
562 arguments than conversion specifications, the extra argument values are |
|
|
563 simply ignored; this is sometimes useful. |
|
|
564 |
|
4167
|
565 @node Integer Conversions |
|
3294
|
566 @subsection Integer Conversions |
|
|
567 |
|
|
568 This section describes the options for the @samp{%d}, @samp{%i}, |
|
|
569 @samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion |
|
|
570 specifications. These conversions print integers in various formats. |
|
|
571 |
|
|
572 The @samp{%d} and @samp{%i} conversion specifications both print an |
|
|
573 numeric argument as a signed decimal number; while @samp{%o}, |
|
|
574 @samp{%u}, and @samp{%x} print the argument as an unsigned octal, |
|
|
575 decimal, or hexadecimal number (respectively). The @samp{%X} conversion |
|
|
576 specification is just like @samp{%x} except that it uses the characters |
|
|
577 @samp{ABCDEF} as digits instead of @samp{abcdef}. |
|
|
578 |
|
|
579 The following flags are meaningful: |
|
|
580 |
|
|
581 @table @asis |
|
|
582 @item @samp{-} |
|
|
583 Left-justify the result in the field (instead of the normal |
|
|
584 right-justification). |
|
|
585 |
|
|
586 @item @samp{+} |
|
|
587 For the signed @samp{%d} and @samp{%i} conversions, print a |
|
|
588 plus sign if the value is positive. |
|
|
589 |
|
|
590 @item @samp{ } |
|
|
591 For the signed @samp{%d} and @samp{%i} conversions, if the result |
|
|
592 doesn't start with a plus or minus sign, prefix it with a space |
|
|
593 character instead. Since the @samp{+} flag ensures that the result |
|
|
594 includes a sign, this flag is ignored if you supply both of them. |
|
|
595 |
|
|
596 @item @samp{#} |
|
|
597 For the @samp{%o} conversion, this forces the leading digit to be |
|
|
598 @samp{0}, as if by increasing the precision. For @samp{%x} or |
|
|
599 @samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively) |
|
|
600 to the result. This doesn't do anything useful for the @samp{%d}, |
|
|
601 @samp{%i}, or @samp{%u} conversions. |
|
|
602 |
|
|
603 @item @samp{0} |
|
|
604 Pad the field with zeros instead of spaces. The zeros are placed after |
|
|
605 any indication of sign or base. This flag is ignored if the @samp{-} |
|
|
606 flag is also specified, or if a precision is specified. |
|
|
607 @end table |
|
|
608 |
|
|
609 If a precision is supplied, it specifies the minimum number of digits to |
|
|
610 appear; leading zeros are produced if necessary. If you don't specify a |
|
|
611 precision, the number is printed with as many digits as it needs. If |
|
|
612 you convert a value of zero with an explicit precision of zero, then no |
|
|
613 characters at all are produced. |
|
|
614 |
|
4167
|
615 @node Floating-Point Conversions |
|
3294
|
616 @subsection Floating-Point Conversions |
|
|
617 |
|
|
618 This section discusses the conversion specifications for floating-point |
|
|
619 numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G} |
|
|
620 conversions. |
|
|
621 |
|
|
622 The @samp{%f} conversion prints its argument in fixed-point notation, |
|
|
623 producing output of the form |
|
|
624 @w{[@code{-}]@var{ddd}@code{.}@var{ddd}}, |
|
|
625 where the number of digits following the decimal point is controlled |
|
|
626 by the precision you specify. |
|
|
627 |
|
|
628 The @samp{%e} conversion prints its argument in exponential notation, |
|
|
629 producing output of the form |
|
|
630 @w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}. |
|
|
631 Again, the number of digits following the decimal point is controlled by |
|
|
632 the precision. The exponent always contains at least two digits. The |
|
|
633 @samp{%E} conversion is similar but the exponent is marked with the letter |
|
|
634 @samp{E} instead of @samp{e}. |
|
|
635 |
|
|
636 The @samp{%g} and @samp{%G} conversions print the argument in the style |
|
|
637 of @samp{%e} or @samp{%E} (respectively) if the exponent would be less |
|
|
638 than -4 or greater than or equal to the precision; otherwise they use the |
|
|
639 @samp{%f} style. Trailing zeros are removed from the fractional portion |
|
|
640 of the result and a decimal-point character appears only if it is |
|
|
641 followed by a digit. |
|
|
642 |
|
|
643 The following flags can be used to modify the behavior: |
|
|
644 |
|
|
645 @c Not @samp so we can have ` ' as an item. |
|
|
646 @table @asis |
|
|
647 @item @samp{-} |
|
|
648 Left-justify the result in the field. Normally the result is |
|
|
649 right-justified. |
|
|
650 |
|
|
651 @item @samp{+} |
|
|
652 Always include a plus or minus sign in the result. |
|
|
653 |
|
|
654 @item @samp{ } |
|
|
655 If the result doesn't start with a plus or minus sign, prefix it with a |
|
|
656 space instead. Since the @samp{+} flag ensures that the result includes |
|
|
657 a sign, this flag is ignored if you supply both of them. |
|
|
658 |
|
|
659 @item @samp{#} |
|
|
660 Specifies that the result should always include a decimal point, even |
|
|
661 if no digits follow it. For the @samp{%g} and @samp{%G} conversions, |
|
|
662 this also forces trailing zeros after the decimal point to be left |
|
|
663 in place where they would otherwise be removed. |
|
|
664 |
|
|
665 @item @samp{0} |
|
|
666 Pad the field with zeros instead of spaces; the zeros are placed |
|
|
667 after any sign. This flag is ignored if the @samp{-} flag is also |
|
|
668 specified. |
|
|
669 @end table |
|
|
670 |
|
|
671 The precision specifies how many digits follow the decimal-point |
|
|
672 character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions. For |
|
|
673 these conversions, the default precision is @code{6}. If the precision |
|
|
674 is explicitly @code{0}, this suppresses the decimal point character |
|
|
675 entirely. For the @samp{%g} and @samp{%G} conversions, the precision |
|
|
676 specifies how many significant digits to print. Significant digits are |
|
|
677 the first digit before the decimal point, and all the digits after it. |
|
|
678 If the precision is @code{0} or not specified for @samp{%g} or |
|
|
679 @samp{%G}, it is treated like a value of @code{1}. If the value being |
|
|
680 printed cannot be expressed precisely in the specified number of digits, |
|
|
681 the value is rounded to the nearest number that fits. |
|
|
682 |
|
4167
|
683 @node Other Output Conversions |
|
3294
|
684 @subsection Other Output Conversions |
|
|
685 |
|
|
686 This section describes miscellaneous conversions for @code{printf}. |
|
|
687 |
|
|
688 The @samp{%c} conversion prints a single character. The @samp{-} |
|
|
689 flag can be used to specify left-justification in the field, but no |
|
|
690 other flags are defined, and no precision or type modifier can be given. |
|
|
691 For example: |
|
|
692 |
|
6670
|
693 @example |
|
3294
|
694 printf ("%c%c%c%c%c", "h", "e", "l", "l", "o"); |
|
6670
|
695 @end example |
|
3294
|
696 |
|
|
697 @noindent |
|
|
698 prints @samp{hello}. |
|
|
699 |
|
|
700 The @samp{%s} conversion prints a string. The corresponding argument |
|
|
701 must be a string. A precision can be specified to indicate the maximum |
|
|
702 number of characters to write; otherwise characters in the string up to |
|
|
703 but not including the terminating null character are written to the |
|
|
704 output stream. The @samp{-} flag can be used to specify |
|
|
705 left-justification in the field, but no other flags or type modifiers |
|
|
706 are defined for this conversion. For example: |
|
|
707 |
|
6670
|
708 @example |
|
3294
|
709 printf ("%3s%-6s", "no", "where"); |
|
6670
|
710 @end example |
|
3294
|
711 |
|
|
712 @noindent |
|
|
713 prints @samp{ nowhere } (note the leading and trailing spaces). |
|
|
714 |
|
4167
|
715 @node Formatted Input |
|
3294
|
716 @subsection Formatted Input |
|
|
717 |
|
|
718 Octave provides the @code{scanf}, @code{fscanf}, and @code{sscanf} |
|
|
719 functions to read formatted input. There are two forms of each of these |
|
|
720 functions. One can be used to extract vectors of data from a file, and |
|
|
721 the other is more `C-like'. |
|
|
722 |
|
3428
|
723 @DOCSTRING(fscanf) |
|
3294
|
724 |
|
3372
|
725 @DOCSTRING(sscanf) |
|
3294
|
726 |
|
|
727 Calls to @code{scanf} are superficially similar to calls to |
|
|
728 @code{printf} in that arbitrary arguments are read under the control of |
|
|
729 a template string. While the syntax of the conversion specifications in |
|
|
730 the template is very similar to that for @code{printf}, the |
|
|
731 interpretation of the template is oriented more towards free-format |
|
|
732 input and simple pattern matching, rather than fixed-field formatting. |
|
|
733 For example, most @code{scanf} conversions skip over any amount of |
|
|
734 ``white space'' (including spaces, tabs, and newlines) in the input |
|
|
735 file, and there is no concept of precision for the numeric input |
|
|
736 conversions as there is for the corresponding output conversions. |
|
|
737 Ordinarily, non-whitespace characters in the template are expected to |
|
|
738 match characters in the input stream exactly. |
|
|
739 @cindex conversion specifications (@code{scanf}) |
|
|
740 |
|
|
741 When a @dfn{matching failure} occurs, @code{scanf} returns immediately, |
|
|
742 leaving the first non-matching character as the next character to be |
|
|
743 read from the stream, and @code{scanf} returns all the items that were |
|
|
744 successfully converted. |
|
|
745 @cindex matching failure, in @code{scanf} |
|
|
746 |
|
|
747 The formatted input functions are not used as frequently as the |
|
|
748 formatted output functions. Partly, this is because it takes some care |
|
|
749 to use them properly. Another reason is that it is difficult to recover |
|
|
750 from a matching error. |
|
|
751 |
|
4167
|
752 @node Input Conversion Syntax |
|
3294
|
753 @subsection Input Conversion Syntax |
|
|
754 |
|
|
755 A @code{scanf} template string is a string that contains ordinary |
|
|
756 multibyte characters interspersed with conversion specifications that |
|
|
757 start with @samp{%}. |
|
|
758 |
|
|
759 Any whitespace character in the template causes any number of whitespace |
|
|
760 characters in the input stream to be read and discarded. The whitespace |
|
|
761 characters that are matched need not be exactly the same whitespace |
|
|
762 characters that appear in the template string. For example, write |
|
|
763 @samp{ , } in the template to recognize a comma with optional whitespace |
|
|
764 before and after. |
|
|
765 |
|
|
766 Other characters in the template string that are not part of conversion |
|
|
767 specifications must match characters in the input stream exactly; if |
|
|
768 this is not the case, a matching failure occurs. |
|
|
769 |
|
|
770 The conversion specifications in a @code{scanf} template string |
|
|
771 have the general form: |
|
|
772 |
|
6670
|
773 @example |
|
3294
|
774 % @var{flags} @var{width} @var{type} @var{conversion} |
|
6670
|
775 @end example |
|
3294
|
776 |
|
|
777 In more detail, an input conversion specification consists of an initial |
|
|
778 @samp{%} character followed in sequence by: |
|
|
779 |
|
|
780 @itemize @bullet |
|
|
781 @item |
|
|
782 An optional @dfn{flag character} @samp{*}, which says to ignore the text |
|
|
783 read for this specification. When @code{scanf} finds a conversion |
|
|
784 specification that uses this flag, it reads input as directed by the |
|
|
785 rest of the conversion specification, but it discards this input, does |
|
|
786 not return any value, and does not increment the count of |
|
|
787 successful assignments. |
|
|
788 @cindex flag character (@code{scanf}) |
|
|
789 |
|
|
790 @item |
|
|
791 An optional decimal integer that specifies the @dfn{maximum field |
|
|
792 width}. Reading of characters from the input stream stops either when |
|
|
793 this maximum is reached or when a non-matching character is found, |
|
|
794 whichever happens first. Most conversions discard initial whitespace |
|
|
795 characters, and these discarded characters don't count towards the |
|
|
796 maximum field width. Conversions that do not discard initial whitespace |
|
|
797 are explicitly documented. |
|
|
798 @cindex maximum field width (@code{scanf}) |
|
|
799 |
|
|
800 @item |
|
|
801 An optional type modifier character. This character is ignored by |
|
|
802 Octave's @code{scanf} function, but is recognized to provide |
|
|
803 compatibility with the C language @code{scanf}. |
|
|
804 |
|
|
805 @item |
|
|
806 A character that specifies the conversion to be applied. |
|
|
807 @end itemize |
|
|
808 |
|
|
809 The exact options that are permitted and how they are interpreted vary |
|
|
810 between the different conversion specifiers. See the descriptions of the |
|
|
811 individual conversions for information about the particular options that |
|
|
812 they allow. |
|
|
813 |
|
4167
|
814 @node Table of Input Conversions |
|
3294
|
815 @subsection Table of Input Conversions |
|
|
816 @cindex input conversions, for @code{scanf} |
|
|
817 |
|
|
818 Here is a table that summarizes the various conversion specifications: |
|
|
819 |
|
|
820 @table @asis |
|
|
821 @item @samp{%d} |
|
|
822 Matches an optionally signed integer written in decimal. @xref{Numeric |
|
|
823 Input Conversions}. |
|
|
824 |
|
|
825 @item @samp{%i} |
|
|
826 Matches an optionally signed integer in any of the formats that the C |
|
|
827 language defines for specifying an integer constant. @xref{Numeric |
|
|
828 Input Conversions}. |
|
|
829 |
|
|
830 @item @samp{%o} |
|
|
831 Matches an unsigned integer written in octal radix. |
|
|
832 @xref{Numeric Input Conversions}. |
|
|
833 |
|
|
834 @item @samp{%u} |
|
|
835 Matches an unsigned integer written in decimal radix. |
|
|
836 @xref{Numeric Input Conversions}. |
|
|
837 |
|
|
838 @item @samp{%x}, @samp{%X} |
|
|
839 Matches an unsigned integer written in hexadecimal radix. |
|
|
840 @xref{Numeric Input Conversions}. |
|
|
841 |
|
|
842 @item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G} |
|
|
843 Matches an optionally signed floating-point number. @xref{Numeric Input |
|
|
844 Conversions}. |
|
|
845 |
|
|
846 @item @samp{%s} |
|
|
847 Matches a string containing only non-whitespace characters. |
|
|
848 @xref{String Input Conversions}. |
|
|
849 |
|
|
850 @item @samp{%c} |
|
|
851 Matches a string of one or more characters; the number of characters |
|
|
852 read is controlled by the maximum field width given for the conversion. |
|
|
853 @xref{String Input Conversions}. |
|
|
854 |
|
|
855 @item @samp{%%} |
|
|
856 This matches a literal @samp{%} character in the input stream. No |
|
|
857 corresponding argument is used. |
|
|
858 @end table |
|
|
859 |
|
|
860 If the syntax of a conversion specification is invalid, the behavior is |
|
|
861 undefined. If there aren't enough function arguments provided to supply |
|
|
862 addresses for all the conversion specifications in the template strings |
|
|
863 that perform assignments, or if the arguments are not of the correct |
|
|
864 types, the behavior is also undefined. On the other hand, extra |
|
|
865 arguments are simply ignored. |
|
|
866 |
|
4167
|
867 @node Numeric Input Conversions |
|
3294
|
868 @subsection Numeric Input Conversions |
|
|
869 |
|
|
870 This section describes the @code{scanf} conversions for reading numeric |
|
|
871 values. |
|
|
872 |
|
|
873 The @samp{%d} conversion matches an optionally signed integer in decimal |
|
|
874 radix. |
|
|
875 |
|
|
876 The @samp{%i} conversion matches an optionally signed integer in any of |
|
|
877 the formats that the C language defines for specifying an integer |
|
|
878 constant. |
|
|
879 |
|
|
880 For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012} |
|
|
881 could be read in as integers under the @samp{%i} conversion. Each of |
|
|
882 these specifies a number with decimal value @code{10}. |
|
|
883 |
|
|
884 The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned |
|
|
885 integers in octal, decimal, and hexadecimal radices, respectively. |
|
|
886 |
|
|
887 The @samp{%X} conversion is identical to the @samp{%x} conversion. They |
|
|
888 both permit either uppercase or lowercase letters to be used as digits. |
|
|
889 |
|
|
890 Unlike the C language @code{scanf}, Octave ignores the @samp{h}, |
|
|
891 @samp{l}, and @samp{L} modifiers. |
|
|
892 |
|
4167
|
893 @node String Input Conversions |
|
3294
|
894 @subsection String Input Conversions |
|
|
895 |
|
|
896 This section describes the @code{scanf} input conversions for reading |
|
|
897 string and character values: @samp{%s} and @samp{%c}. |
|
|
898 |
|
|
899 The @samp{%c} conversion is the simplest: it matches a fixed number of |
|
|
900 characters, always. The maximum field with says how many characters to |
|
|
901 read; if you don't specify the maximum, the default is 1. This |
|
|
902 conversion does not skip over initial whitespace characters. It reads |
|
|
903 precisely the next @var{n} characters, and fails if it cannot get that |
|
|
904 many. |
|
|
905 |
|
|
906 The @samp{%s} conversion matches a string of non-whitespace characters. |
|
|
907 It skips and discards initial whitespace, but stops when it encounters |
|
|
908 more whitespace after having read something. |
|
|
909 |
|
|
910 For example, reading the input: |
|
|
911 |
|
6670
|
912 @example |
|
3294
|
913 hello, world |
|
6670
|
914 @end example |
|
3294
|
915 |
|
|
916 @noindent |
|
|
917 with the conversion @samp{%10c} produces @code{" hello, wo"}, but |
|
|
918 reading the same input with the conversion @samp{%10s} produces |
|
|
919 @code{"hello,"}. |
|
|
920 |
|
4167
|
921 @node Binary I/O |
|
3294
|
922 @subsection Binary I/O |
|
|
923 |
|
|
924 Octave can read and write binary data using the functions @code{fread} |
|
|
925 and @code{fwrite}, which are patterned after the standard C functions |
|
|
926 with the same names. The are able to automatically swap the byte order |
|
|
927 of integer data and convert among ths supported floating point formats |
|
|
928 as the data are read. |
|
|
929 |
|
3372
|
930 @DOCSTRING(fread) |
|
3294
|
931 |
|
3372
|
932 @DOCSTRING(fwrite) |
|
3294
|
933 |
|
4167
|
934 @node Temporary Files |
|
3294
|
935 @subsection Temporary Files |
|
|
936 |
|
6666
|
937 Sometimes one needs to write data to a file that is only temporary. |
|
|
938 This is most commonly used when an external program launched from |
|
|
939 within Octave needs to access data. When Octave exits all temporary |
|
|
940 files will be deleted, so this step need not be executed manually. |
|
|
941 |
|
4328
|
942 @DOCSTRING(mkstemp) |
|
|
943 |
|
|
944 @DOCSTRING(tmpfile) |
|
|
945 |
|
3372
|
946 @DOCSTRING(tmpnam) |
|
3294
|
947 |
|
4167
|
948 @node EOF and Errors |
|
3294
|
949 @subsection End of File and Errors |
|
|
950 |
|
6666
|
951 Once a file has been opened its status can be acquired. As an example |
|
|
952 the @code{feof} functions determines if the end of the file has been |
|
|
953 reached. This can be very useful when reading small parts of a file |
|
|
954 at a time. The following example shows how to read one line at a time |
|
|
955 from a file until the end has been reached. |
|
|
956 |
|
|
957 @example |
|
|
958 filename = "myfile.txt"; |
|
|
959 fid = fopen (filename, "r"); |
|
|
960 while (! feof (fid) ) |
|
|
961 text_line = fgetl (fid); |
|
|
962 endwhile |
|
|
963 fclose (fid); |
|
|
964 @end example |
|
|
965 |
|
|
966 @noindent |
|
|
967 Note that in some situations it is more efficient to read the entire |
|
|
968 contents of a file and then process it, than it is to read it line by |
|
|
969 line. This has the potential advantage of removing the loop in the |
|
|
970 above code. |
|
|
971 |
|
3372
|
972 @DOCSTRING(feof) |
|
3294
|
973 |
|
3372
|
974 @DOCSTRING(ferror) |
|
3294
|
975 |
|
3372
|
976 @DOCSTRING(freport) |
|
3294
|
977 |
|
4167
|
978 @node File Positioning |
|
3294
|
979 @subsection File Positioning |
|
|
980 |
|
|
981 Three functions are available for setting and determining the position of |
|
|
982 the file pointer for a given file. |
|
|
983 |
|
3372
|
984 @DOCSTRING(ftell) |
|
|
985 |
|
|
986 @DOCSTRING(fseek) |
|
3294
|
987 |
|
3372
|
988 @DOCSTRING(SEEK_SET) |
|
3294
|
989 |
|
3372
|
990 @DOCSTRING(frewind) |
|
3294
|
991 |
|
|
992 The following example stores the current file position in the variable |
|
|
993 @code{marker}, moves the pointer to the beginning of the file, reads |
|
|
994 four characters, and then returns to the original position. |
|
|
995 |
|
|
996 @example |
|
|
997 marker = ftell (myfile); |
|
|
998 frewind (myfile); |
|
|
999 fourch = fgets (myfile, 4); |
|
|
1000 fseek (myfile, marker, SEEK_SET); |
|
|
1001 @end example |
|
|
1002 |
