1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
|
<!DOCTYPE html>
<html>
<!-- Created by GNU Texinfo 7.1.1, https://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Evaluation in a Different Context (GNU Octave (version 10.3.0))</title>
<meta name="description" content="Evaluation in a Different Context (GNU Octave (version 10.3.0))">
<meta name="keywords" content="Evaluation in a Different Context (GNU Octave (version 10.3.0))">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta name="viewport" content="width=device-width,initial-scale=1">
<link href="index.html" rel="start" title="Top">
<link href="Concept-Index.html" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Evaluation.html" rel="up" title="Evaluation">
<link href="Calling-a-Function-by-its-Name.html" rel="prev" title="Calling a Function by its Name">
<style type="text/css">
<!--
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
div.example {margin-left: 3.2em}
span:hover a.copiable-link {visibility: visible}
strong.def-name {font-family: monospace; font-weight: bold; font-size: larger}
-->
</style>
<link rel="stylesheet" type="text/css" href="octave.css">
</head>
<body lang="en">
<div class="section-level-extent" id="Evaluation-in-a-Different-Context">
<div class="nav-panel">
<p>
Previous: <a href="Calling-a-Function-by-its-Name.html" accesskey="p" rel="prev">Calling a Function by its Name</a>, Up: <a href="Evaluation.html" accesskey="u" rel="up">Evaluation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<h3 class="section" id="Evaluation-in-a-Different-Context-1"><span>9.2 Evaluation in a Different Context<a class="copiable-link" href="#Evaluation-in-a-Different-Context-1"> ¶</a></span></h3>
<p>Before you evaluate an expression you need to substitute
the values of the variables used in the expression. These
are stored in the symbol table. Whenever the interpreter
starts a new function it saves the current symbol table
and creates a new one, initializing it with the list of
function parameters and a couple of predefined variables
such as <code class="code">nargin</code>. Expressions inside the function use the
new symbol table.
</p>
<p>Sometimes you want to write a function so that when you
call it, it modifies variables in your own context. This
allows you to use a pass-by-name style of function,
which is similar to using a pointer in programming languages such
as C.
</p>
<p>Consider how you might write <code class="code">save</code> and <code class="code">load</code> as
m-files. For example:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function create_data
x = linspace (0, 10, 10);
y = sin (x);
save mydata x y
endfunction
</pre></div></div>
<p>With <code class="code">evalin</code>, you could write <code class="code">save</code> as follows:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function save (file, name1, name2)
f = open_save_file (file);
save_var (f, name1, evalin ("caller", name1));
save_var (f, name2, evalin ("caller", name2));
endfunction
</pre></div></div>
<p>Here, ‘<samp class="samp">caller</samp>’ is the <code class="code">create_data</code> function and <code class="code">name1</code>
is the string <code class="code">"x"</code>, which evaluates simply as the value of <code class="code">x</code>.
</p>
<p>You later want to load the values back from <code class="code">mydata</code>
in a different context:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function process_data
load mydata
... do work ...
endfunction
</pre></div></div>
<p>With <code class="code">assignin</code>, you could write <code class="code">load</code> as follows:
</p>
<div class="example">
<div class="group"><pre class="example-preformatted">function load (file)
f = open_load_file (file);
[name, val] = load_var (f);
assignin ("caller", name, val);
[name, val] = load_var (f);
assignin ("caller", name, val);
endfunction
</pre></div></div>
<p>Here, ‘<samp class="samp">caller</samp>’ is the <code class="code">process_data</code> function.
</p>
<p>You can set and use variables at the command prompt
using the context ‘<samp class="samp">base</samp>’ rather than ‘<samp class="samp">caller</samp>’.
</p>
<p>These functions are rarely used in practice. One
example is the <code class="code">fail (‘<samp class="samp">code</samp>’, ‘<samp class="samp">pattern</samp>’)</code> function
which evaluates ‘<samp class="samp">code</samp>’ in the caller’s context and
checks that the error message it produces matches
the given pattern. Other examples such as <code class="code">save</code> and <code class="code">load</code>
are written in C++ where all Octave variables
are in the ‘<samp class="samp">caller</samp>’ context and <code class="code">evalin</code> is not needed.
</p>
<a class="anchor" id="XREFevalin"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-evalin"><span><strong class="def-name">evalin</strong> <code class="def-code-arguments">(<var class="var">context</var>, <var class="var">try</var>)</code><a class="copiable-link" href="#index-evalin"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-evalin-1"><span><strong class="def-name">evalin</strong> <code class="def-code-arguments">(<var class="var">context</var>, <var class="var">try</var>, <var class="var">catch</var>)</code><a class="copiable-link" href="#index-evalin-1"> ¶</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-evalin-2"><span><code class="def-type">[<var class="var">var1</var>, …] =</code> <strong class="def-name">evalin</strong> <code class="def-code-arguments">(…)</code><a class="copiable-link" href="#index-evalin-2"> ¶</a></span></dt>
<dd><p>Like <code class="code">eval</code>, except that the expressions are evaluated in the context
<var class="var">context</var>, which may be either <code class="code">"caller"</code> or <code class="code">"base"</code>.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="Evaluation.html#XREFeval">eval</a>, <a class="ref" href="#XREFassignin">assignin</a>.
</p></dd></dl>
<a class="anchor" id="XREFassignin"></a><span style="display:block; margin-top:-4.5ex;"> </span>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-assignin"><span><strong class="def-name">assignin</strong> <code class="def-code-arguments">(<var class="var">context</var>, <var class="var">varname</var>, <var class="var">value</var>)</code><a class="copiable-link" href="#index-assignin"> ¶</a></span></dt>
<dd><p>Assign <var class="var">value</var> to <var class="var">varname</var> in context <var class="var">context</var>, which
may be either <code class="code">"base"</code> or <code class="code">"caller"</code>.
</p>
<p><strong class="strong">See also:</strong> <a class="ref" href="#XREFevalin">evalin</a>.
</p></dd></dl>
</div>
<hr>
<div class="nav-panel">
<p>
Previous: <a href="Calling-a-Function-by-its-Name.html">Calling a Function by its Name</a>, Up: <a href="Evaluation.html">Evaluation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
</div>
</body>
</html>
|
