Mercurial > octave
annotate doc/interpreter/eval.txi @ 12578:f5a780d675a1
Clean up operator and function indices in documentation.
author | Rik <octave@nomad.inbox5.com> |
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date | Wed, 06 Apr 2011 11:44:21 -0700 |
parents | fd0a3ac60b0e |
children | 72c96de7a403 |
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11523 | 1 @c Copyright (C) 1996-2011 John W. Eaton |
7018 | 2 @c |
3 @c This file is part of Octave. | |
4 @c | |
5 @c Octave is free software; you can redistribute it and/or modify it | |
6 @c under the terms of the GNU General Public License as published by the | |
7 @c Free Software Foundation; either version 3 of the License, or (at | |
8 @c your option) any later version. | |
9 @c | |
10 @c Octave is distributed in the hope that it will be useful, but WITHOUT | |
11 @c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
12 @c FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
13 @c for more details. | |
14 @c | |
15 @c You should have received a copy of the GNU General Public License | |
16 @c along with Octave; see the file COPYING. If not, see | |
17 @c <http://www.gnu.org/licenses/>. | |
3294 | 18 |
4167 | 19 @node Evaluation |
3294 | 20 @chapter Evaluation |
21 | |
22 Normally, you evaluate expressions simply by typing them at the Octave | |
23 prompt, or by asking Octave to interpret commands that you have saved in | |
24 a file. | |
25 | |
26 Sometimes, you may find it necessary to evaluate an expression that has | |
6641 | 27 been computed and stored in a string, which is exactly what the |
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28 @code{eval} function lets you do. |
3294 | 29 |
3371 | 30 @DOCSTRING(eval) |
3294 | 31 |
6641 | 32 @menu |
33 * Calling a Function by its Name:: | |
34 * Evaluation in a Different Context:: | |
35 @end menu | |
36 | |
37 @node Calling a Function by its Name | |
38 @section Calling a Function by its Name | |
3294 | 39 |
7001 | 40 The @code{feval} function allows you to call a function from a string |
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41 containing its name. This is useful when writing a function that needs to |
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42 call user-supplied functions. The @code{feval} function takes the name |
6641 | 43 of the function to call as its first argument, and the remaining |
44 arguments are given to the function. | |
45 | |
46 The following example is a simple-minded function using @code{feval} | |
47 that finds the root of a user-supplied function of one variable using | |
48 Newton's method. | |
3294 | 49 |
50 @example | |
51 function result = newtroot (fname, x) | |
52 | |
53 # usage: newtroot (fname, x) | |
54 # | |
55 # fname : a string naming a function f(x). | |
56 # x : initial guess | |
57 | |
58 delta = tol = sqrt (eps); | |
59 maxit = 200; | |
60 fx = feval (fname, x); | |
61 for i = 1:maxit | |
62 if (abs (fx) < tol) | |
63 result = x; | |
64 return; | |
65 else | |
66 fx_new = feval (fname, x + delta); | |
67 deriv = (fx_new - fx) / delta; | |
68 x = x - fx / deriv; | |
69 fx = fx_new; | |
70 endif | |
71 endfor | |
72 | |
73 result = x; | |
74 | |
75 endfunction | |
76 @end example | |
77 | |
78 Note that this is only meant to be an example of calling user-supplied | |
79 functions and should not be taken too seriously. In addition to using a | |
80 more robust algorithm, any serious code would check the number and type | |
81 of all the arguments, ensure that the supplied function really was a | |
6641 | 82 function, etc. @xref{Predicates for Numeric Objects}, for example, |
83 for a list of predicates for numeric objects, and see @ref{Status of | |
3294 | 84 Variables}, for a description of the @code{exist} function. |
6549 | 85 |
6641 | 86 @DOCSTRING(feval) |
87 | |
6863 | 88 A similar function @code{run} exists for calling user script files, that |
89 are not necessarily on the user path | |
90 | |
91 @DOCSTRING(run) | |
92 | |
6641 | 93 @node Evaluation in a Different Context |
94 @section Evaluation in a Different Context | |
95 | |
96 Before you evaluate an expression you need to substitute | |
97 the values of the variables used in the expression. These | |
98 are stored in the symbol table. Whenever the interpreter | |
99 starts a new function it saves the current symbol table | |
100 and creates a new one, initializing it with the list of | |
101 function parameters and a couple of predefined variables | |
102 such as @code{nargin}. Expressions inside the function use the | |
103 new symbol table. | |
104 | |
105 Sometimes you want to write a function so that when you | |
106 call it, it modifies variables in your own context. This | |
107 allows you to use a pass-by-name style of function, | |
108 which is similar to using a pointer in programming languages such | |
109 as C. | |
110 | |
111 Consider how you might write @code{save} and @code{load} as | |
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112 m-files. For example: |
6641 | 113 |
114 @example | |
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115 @group |
6641 | 116 function create_data |
117 x = linspace (0, 10, 10); | |
118 y = sin (x); | |
119 save mydata x y | |
120 endfunction | |
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121 @end group |
6641 | 122 @end example |
123 | |
8828 | 124 With @code{evalin}, you could write @code{save} as follows: |
6641 | 125 |
126 @example | |
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127 @group |
6641 | 128 function save (file, name1, name2) |
129 f = open_save_file (file); | |
130 save_var(f, name1, evalin ("caller", name1)); | |
131 save_var(f, name2, evalin ("caller", name2)); | |
132 endfunction | |
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133 @end group |
6641 | 134 @end example |
135 | |
136 @noindent | |
137 Here, @samp{caller} is the @code{create_data} function and @code{name1} | |
138 is the string @code{"x"}, which evaluates simply as the value of @code{x}. | |
139 | |
140 You later want to load the values back from @code{mydata} | |
141 in a different context: | |
142 | |
143 @example | |
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144 @group |
6641 | 145 function process_data |
146 load mydata | |
147 @dots{} do work @dots{} | |
148 endfunction | |
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149 @end group |
6641 | 150 @end example |
151 | |
152 @noindent | |
153 With @code{assignin}, you could write @code{load} as follows: | |
154 | |
155 @example | |
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156 @group |
6641 | 157 function load (file) |
158 f = open_load_file (file); | |
159 [name, val] = load_var (f); | |
160 assignin ("caller", name, val); | |
161 [name, val] = load_var (f); | |
162 assignin ("caller", name, val); | |
163 endfunction | |
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164 @end group |
6641 | 165 @end example |
166 | |
167 @noindent | |
168 Here, @samp{caller} is the @code{process_data} function. | |
169 | |
170 You can set and use variables at the command prompt | |
171 using the context @samp{base} rather than @samp{caller}. | |
172 | |
173 These functions are rarely used in practice. One | |
174 example is the @code{fail (@samp{code}, @samp{pattern})} function | |
175 which evaluates @samp{code} in the caller's context and | |
176 checks that the error message it produces matches | |
177 the given pattern. Other examples such as @code{save} and @code{load} | |
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178 are written in C++ where all Octave variables |
6641 | 179 are in the @samp{caller} context and @code{evalin} is not needed. |
180 | |
6549 | 181 @DOCSTRING(evalin) |
182 | |
183 @DOCSTRING(assignin) |