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1 @c Copyright (C) 1996, 1997, 2007 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 Variables |
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6 @chapter Variables |
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7 @cindex variables, user-defined |
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8 @cindex user-defined variables |
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9 |
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10 Variables let you give names to values and refer to them later. You have |
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11 already seen variables in many of the examples. The name of a variable |
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12 must be a sequence of letters, digits and underscores, but it may not begin |
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13 with a digit. Octave does not enforce a limit on the length of variable |
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14 names, but it is seldom useful to have variables with names longer than |
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15 about 30 characters. The following are all valid variable names |
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16 |
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17 @cindex job hunting |
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18 @cindex getting a good job |
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19 @cindex flying high and fast |
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20 @example |
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21 @group |
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22 x |
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23 x15 |
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24 __foo_bar_baz__ |
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25 fucnrdthsucngtagdjb |
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26 @end group |
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27 @end example |
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28 |
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29 @noindent |
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30 However, names like @code{__foo_bar_baz__} that begin and end with two |
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31 underscores are understood to be reserved for internal use by Octave. |
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32 You should not use them in code you write, except to access Octave's |
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33 documented internal variables and built-in symbolic constants. |
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34 |
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35 Case is significant in variable names. The symbols @code{a} and |
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36 @code{A} are distinct variables. |
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37 |
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38 A variable name is a valid expression by itself. It represents the |
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39 variable's current value. Variables are given new values with |
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40 @dfn{assignment operators} and @dfn{increment operators}. |
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41 @xref{Assignment Ops, ,Assignment Expressions}. |
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42 |
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43 A number of variables have special built-in meanings. For example, |
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44 @code{ans} holds the current working directory, and @code{pi} names the |
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45 ratio of the circumference of a circle to its diameter. @xref{Summary of |
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46 Built-in Variables}, for a list of all the predefined variables. Some |
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47 of these built-in symbols are constants and may not be changed. Others |
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48 can be used and assigned just like all other variables, but their values |
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49 are also used or changed automatically by Octave. |
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50 |
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51 Variables in Octave do not have fixed types, so it is possible to first |
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52 store a numeric value in a variable and then to later use the same name |
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53 to hold a string value in the same program. Variables may not be used |
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54 before they have been given a value. Doing so results in an error. |
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55 |
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56 @DOCSTRING(isvarname) |
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57 |
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58 @menu |
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59 * Global Variables:: |
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60 * Persistent Variables:: |
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61 * Status of Variables:: |
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62 * Summary of Built-in Variables:: |
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63 * Defaults from the Environment:: |
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64 @end menu |
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65 |
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66 @node Global Variables |
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67 @section Global Variables |
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68 @cindex global variables |
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69 @cindex @code{global} statement |
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70 @cindex variables, global |
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71 |
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72 A variable that has been declared @dfn{global} may be accessed from |
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73 within a function body without having to pass it as a formal parameter. |
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74 |
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75 A variable may be declared global using a @code{global} declaration |
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76 statement. The following statements are all global declarations. |
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77 |
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78 @example |
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79 @group |
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80 global a |
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81 global a b |
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82 global c = 2 |
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83 global d = 3 e f = 5 |
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84 @end group |
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85 @end example |
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86 |
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87 A global variable may only be initialized once in a @code{global} |
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88 statement. For example, after executing the following code |
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89 |
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90 @example |
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91 @group |
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92 global gvar = 1 |
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93 global gvar = 2 |
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94 @end group |
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95 @end example |
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96 |
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97 @noindent |
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98 the value of the global variable @code{gvar} is 1, not 2. Issuing a |
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99 @samp{clear gvar} command does not change the above behavior, but |
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100 @samp{clear all} does. |
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101 |
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102 It is necessary declare a variable as global within a function body in |
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103 order to access it. For example, |
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104 |
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105 @example |
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106 @group |
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107 global x |
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108 function f () |
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109 x = 1; |
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110 endfunction |
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111 f () |
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112 @end group |
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113 @end example |
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114 |
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115 @noindent |
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116 does @emph{not} set the value of the global variable @code{x} to 1. In |
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117 order to change the value of the global variable @code{x}, you must also |
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118 declare it to be global within the function body, like this |
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119 |
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120 @example |
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121 @group |
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122 function f () |
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123 global x; |
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124 x = 1; |
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125 endfunction |
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126 @end group |
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127 @end example |
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128 |
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129 Passing a global variable in a function parameter list will |
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130 make a local copy and not modify the global value. For example, given |
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131 the function |
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132 |
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133 @example |
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134 @group |
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135 function f (x) |
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136 x = 0 |
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137 endfunction |
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138 @end group |
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139 @end example |
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140 |
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141 @noindent |
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142 and the definition of @code{x} as a global variable at the top level, |
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143 |
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144 @example |
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145 global x = 13 |
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146 @end example |
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147 |
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148 @noindent |
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149 the expression |
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150 |
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151 @example |
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152 f (x) |
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153 @end example |
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154 |
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155 @noindent |
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156 will display the value of @code{x} from inside the function as 0, |
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157 but the value of @code{x} at the top level remains unchanged, because |
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158 the function works with a @emph{copy} of its argument. |
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159 |
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160 @DOCSTRING(isglobal) |
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161 |
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162 @node Persistent Variables |
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163 @section Persistent Variables |
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164 @cindex persistent variables |
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165 @cindex @code{persistent} statement |
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166 @cindex variables, persistent |
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167 |
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168 A variable that has been declared @dfn{persistent} within a function |
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169 will retain its contents in memory between subsequent calls to the |
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170 same function. The difference between persistent variables and global |
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171 variables is that persistent variables are local in scope to a |
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172 particular function and are not visible elsewhere. |
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173 |
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174 The following example uses a persistent variable to create a function |
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175 that prints the number of times it has been called. |
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176 |
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177 @example |
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178 @group |
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179 function count_calls () |
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180 persistent calls = 0; |
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181 printf ("'count_calls' has been called %d times\n", ++calls); |
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182 endfunction |
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183 |
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184 for i = 1:3 |
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185 count_calls (); |
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186 endfor |
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187 |
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188 @print{} 'count_calls' has been called 1 times |
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189 @print{} 'count_calls' has been called 2 times |
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190 @print{} 'count_calls' has been called 3 times |
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191 @end group |
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192 @end example |
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193 |
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194 As the example shows, a variable may be declared persistent using a |
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195 @code{persistent} declaration statement. The following statements are |
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196 all persistent declarations. |
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197 |
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198 @example |
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199 @group |
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200 persistent a |
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201 persistent a b |
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202 persistent c = 2 |
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203 persistent d = 3 e f = 5 |
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204 @end group |
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205 @end example |
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206 |
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207 The behavior of persistent variables is equivalent to the behavior of |
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208 static variables in C. The command @code{static} in octave is also |
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209 recognized and is equivalent to @code{persistent}. |
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210 |
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211 Like global variables, a persistent variable may only be initialized once. |
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212 For example, after executing the following code |
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213 |
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214 @example |
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215 @group |
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216 persistent pvar = 1 |
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217 persistent pvar = 2 |
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218 @end group |
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219 @end example |
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220 |
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221 @noindent |
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222 the value of the persistent variable @code{pvar} is 1, not 2. |
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223 |
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224 If a persistent variable is declared but not initialized to a specific |
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225 value, it will contain an empty matrix. So, it is also possible to |
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226 initialize a persistent variable by checking whether it is empty, as the |
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227 following example illustrates. |
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228 |
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229 @example |
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230 @group |
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231 function count_calls () |
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232 persistent calls; |
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233 if (isempty (calls)) |
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234 calls = 0; |
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235 endif |
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236 printf ("'count_calls' has been called %d times\n", ++calls); |
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237 endfunction |
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238 @end group |
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239 @end example |
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240 |
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241 @noindent |
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242 This implementation behaves in exactly the same way as the previous |
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243 implementation of @code{count_calls}. |
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244 |
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245 The value of a persistent variable is kept in memory until it is |
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246 explicitly cleared. Assuming that the implementation of @code{count_calls} |
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247 is saved on disc, we get the following behaviour. |
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248 |
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249 @example |
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250 @group |
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251 for i = 1:2 |
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252 count_calls (); |
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253 endfor |
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254 @print{} 'count_calls' has been called 1 times |
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255 @print{} 'count_calls' has been called 2 times |
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256 |
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257 clear |
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258 for i = 1:2 |
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259 count_calls(); |
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260 endfor |
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261 @print{} 'count_calls' has been called 3 times |
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262 @print{} 'count_calls' has been called 4 times |
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263 |
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264 clear all |
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265 for i = 1:2 |
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266 count_calls(); |
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267 endfor |
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268 @print{} 'count_calls' has been called 1 times |
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269 @print{} 'count_calls' has been called 2 times |
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270 |
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271 clear count_calls |
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272 for i = 1:2 |
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273 count_calls(); |
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274 endfor |
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275 @print{} 'count_calls' has been called 1 times |
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276 @print{} 'count_calls' has been called 2 times |
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277 @end group |
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278 @end example |
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279 |
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280 @noindent |
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281 That is, the persistent variable is only removed from memory when the |
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282 function containing the variable is removed. Note that if the function |
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283 definition is typed directly into the Octave prompt, the persistent |
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284 variable will be cleared by a simple @code{clear} command as the entire |
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285 function definition will be removed from memory. If you do not want |
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286 a persistent variable to be removed from memory even if the function is |
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287 cleared, you should use the @code{mlock} function as described in |
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288 @xref{Function Locking}. |
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289 |
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290 @node Status of Variables |
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291 @section Status of Variables |
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292 |
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293 When creating simple one-shot programs it can be very convenient to |
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294 see which variables are available at the prompt. The function @code{who} |
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295 and its siblings @code{whos} and @code{whos_line_format} will show |
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296 different information about what is in memory, as the following shows. |
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297 |
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298 @example |
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299 str = "A random string"; |
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300 who -variables |
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301 @print{} *** local user variables: |
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302 @print{} |
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303 @print{} __nargin__ str |
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304 @end example |
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305 |
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306 @DOCSTRING(who) |
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307 |
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308 @DOCSTRING(whos) |
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309 |
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310 @DOCSTRING(whos_line_format) |
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311 |
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312 Instead of displaying which variables are in memory, it is possible |
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313 to determine if a given variable is available. That way it is possible |
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314 to alter the behaviour of a program depending on the existence of a |
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315 variable. The following example illustrates this. |
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316 |
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317 @example |
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318 if (! exist ("meaning", "var")) |
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319 disp ("The program has no 'meaning'"); |
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320 endif |
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321 @end example |
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322 |
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323 @DOCSTRING(exist) |
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324 |
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325 Usually Octave will manage the memory, but sometimes it can be practical |
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326 to remove variables from memory manually. This is usually needed when |
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327 working with large variables that fill a substantial part of the memory. |
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328 On a computer that uses the IEEE floating point format, the following |
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329 program allocates a matrix that requires around 128 MB memory. |
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330 |
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331 @example |
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332 large_matrix = zeros (4000, 4000); |
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333 @end example |
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334 |
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335 @noindent |
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336 Since having this variable in memory might slow down other computations, |
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337 it can be necessary to remove it manually from memory. The @code{clear} |
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338 function allows this. |
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339 |
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340 @DOCSTRING(clear) |
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341 |
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342 Information about a function or variable such as it's location in the |
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343 file system can also be acquired from within Octave. This is usually |
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344 only useful during development of programs, and not within a program. |
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345 |
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346 @DOCSTRING(document) |
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347 |
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348 @DOCSTRING(type) |
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349 |
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350 @DOCSTRING(which) |
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351 |
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352 @node Summary of Built-in Variables |
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353 @section Summary of Built-in Variables |
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354 |
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355 Here is a summary of all of Octave's built-in variables along with |
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356 cross references to additional information and their default values. In |
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357 the following table @var{octave-home} stands for the root directory |
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358 where all of Octave is installed (the default is @file{@value{OCTAVEHOME}}, |
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359 @var{version} stands for the Octave version number (for example, |
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360 @value{VERSION}) and @var{arch} stands for the type of system for which |
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361 Octave was compiled (for example, @code{x86_64-unknown-linux-gnu}). |
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362 |
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363 @vtable @code |
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364 @item EDITOR |
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365 @xref{Commands For History}. |
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366 |
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367 Default value: @code{"emacs"}. |
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368 |
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369 @item EXEC_PATH |
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370 @xref{Controlling Subprocesses}. |
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371 |
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372 Default value: @code{":$PATH"}. |
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373 |
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374 @item OCTAVE_HOME |
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375 |
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376 Default value: @code{"@value{OCTAVEHOME}"}. |
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377 |
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378 @item PAGER |
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379 @xref{Input and Output}. |
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380 |
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381 Default value: @code{"less", or "more"}. |
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382 |
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383 @item PS1 |
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384 @xref{Customizing the Prompt}. |
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385 |
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386 Default value: @code{"\s:\#> "}. |
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387 |
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388 @item PS2 |
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389 @xref{Customizing the Prompt}. |
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390 |
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391 Default value: @code{"> "}. |
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392 |
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393 @item PS4 |
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394 @xref{Customizing the Prompt}. |
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395 |
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396 Default value: @code{"+ "}. |
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397 |
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398 @item beep_on_error |
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399 @xref{Errors and Warnings}. |
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400 |
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401 Default value: 0. |
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402 |
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403 @item completion_append_char |
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404 @xref{Commands For Completion}. |
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405 |
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406 Default value: @code{" "}. |
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407 |
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408 @item default_save_options |
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409 @xref{Simple File I/O}. |
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410 |
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411 Default value: @code{"ascii"}. |
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412 |
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413 @item crash_dumps_octave_core |
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414 @xref{Simple File I/O}. |
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415 |
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416 Default value: 1. |
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417 |
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418 @item fixed_point_format |
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419 @xref{Matrices}. |
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420 |
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421 Default value: 0. |
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422 |
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423 @item gnuplot_binary |
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424 @xref{Three-Dimensional Plotting}. |
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425 |
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426 Default value: @code{"gnuplot"}. |
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427 |
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428 @item history_file |
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429 @xref{Commands For History}. |
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430 |
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431 Default value: @code{"~/.octave_hist"}. |
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432 |
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433 @item history_size |
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434 @xref{Commands For History}. |
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435 |
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436 Default value: 1024. |
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437 |
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438 @item ignore_function_time_stamp |
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439 @xref{Function Files}. |
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440 |
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441 Default value: @code{"system"}. |
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442 |
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443 @item max_recursion_depth |
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444 @xref{Recursion}. |
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445 |
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446 Default value: 256. |
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447 |
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448 @item output_max_field_width |
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449 @xref{Matrices}. |
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450 |
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451 Default value: 10. |
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452 |
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453 @item output_precision |
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454 @xref{Matrices}. |
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455 |
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456 Default value: 5. |
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457 |
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458 @item page_screen_output |
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459 @xref{Input and Output}. |
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460 |
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461 Default value: 1. |
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462 |
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463 @item print_answer_id_name |
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464 @xref{Terminal Output}. |
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465 |
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466 Default value: 1. |
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467 |
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468 @item print_empty_dimensions |
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469 @xref{Empty Matrices}. |
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470 |
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471 Default value: 1. |
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472 |
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473 @item return_last_computed_value |
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474 @xref{Returning From a Function}. |
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475 |
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476 Default value: 0. |
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477 |
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478 @item save_precision |
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479 @xref{Simple File I/O}. |
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480 |
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481 Default value: 17. |
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482 |
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483 @item saving_history |
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484 @xref{Commands For History}. |
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485 |
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486 Default value: 1. |
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487 |
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488 @item sighup_dumps_octave_core |
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489 @xref{Simple File I/O}. |
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490 |
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491 Default value: 1. |
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492 |
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493 @item sigterm_dumps_octave_core |
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494 @xref{Simple File I/O}. |
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495 |
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496 Default value: 1. |
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497 |
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498 @item silent_functions |
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499 @xref{Defining Functions}. |
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500 |
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501 Default value: 0. |
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502 |
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503 @item split_long_rows |
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504 @xref{Matrices}. |
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505 |
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506 Default value: 1. |
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507 |
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508 @item struct_levels_to_print |
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509 @xref{Data Structures}. |
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510 |
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511 Default value: 2. |
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512 |
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513 @item suppress_verbose_help_message |
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514 @xref{Getting Help}. |
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515 |
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516 Default value: 1. |
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517 @end vtable |
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518 |
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519 |
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520 @node Defaults from the Environment |
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521 @section Defaults from the Environment |
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522 |
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523 Octave uses the values of the following environment variables to set the |
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524 default values for the corresponding built-in or internal variables. |
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525 In addition, the values from the environment may be overridden by |
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526 command-line arguments. @xref{Command Line Options}. |
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527 |
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528 @vtable @code |
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529 @item EDITOR |
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530 @xref{Commands For History}. |
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531 |
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532 Built-in variable: @code{EDITOR}. |
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533 |
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534 @item OCTAVE_EXEC_PATH |
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535 @xref{Controlling Subprocesses}. |
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536 |
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537 Built-in variable: @code{EXEC_PATH}. |
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538 Command-line argument: @code{--exec-path}. |
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539 |
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540 @item OCTAVE_PATH |
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541 @xref{Function Files}. |
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542 |
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543 Internal variable changed by function @code{path}. |
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544 Command-line argument: @code{--path}. |
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545 |
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546 @item OCTAVE_INFO_FILE |
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547 @xref{Getting Help}. |
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548 |
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549 Internal variable changed by function @code{info_file}. |
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550 Command-line argument: @code{--info-file}. |
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551 |
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552 @item OCTAVE_INFO_PROGRAM |
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553 @xref{Getting Help}. |
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554 |
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555 Internal variable changed by function @code{info_program}. |
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556 Command-line argument: @code{--info-program}. |
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557 |
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558 @item OCTAVE_HISTSIZE |
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559 @xref{Commands For History}. |
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560 |
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561 Built-in variable: @code{history_size}. |
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562 |
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563 @item OCTAVE_HISTFILE |
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|
564 @xref{Commands For History}. |
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565 |
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566 Built-in variable: @code{history_file}. |
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567 @end vtable |
