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<a name="Evaluation-in-a-Different-Context"></a>
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<p>
Previous: <a href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" accesskey="p" rel="prev">Calling a Function by its Name</a>, Up: <a href="Evaluation.html#Evaluation" 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#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Evaluation-in-a-Different-Context-1"></a>
<h3 class="section">9.2 Evaluation in a Different Context</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>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>save</code> and <code>load</code> as
m-files. For example:
</p>
<div class="example">
<pre class="example">function create_data
x = linspace (0, 10, 10);
y = sin (x);
save mydata x y
endfunction
</pre></div>
<p>With <code>evalin</code>, you could write <code>save</code> as follows:
</p>
<div class="example">
<pre class="example">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>
<p>Here, ‘<samp>caller</samp>’ is the <code>create_data</code> function and <code>name1</code>
is the string <code>"x"</code>, which evaluates simply as the value of <code>x</code>.
</p>
<p>You later want to load the values back from <code>mydata</code>
in a different context:
</p>
<div class="example">
<pre class="example">function process_data
load mydata
… do work …
endfunction
</pre></div>
<p>With <code>assignin</code>, you could write <code>load</code> as follows:
</p>
<div class="example">
<pre class="example">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>
<p>Here, ‘<samp>caller</samp>’ is the <code>process_data</code> function.
</p>
<p>You can set and use variables at the command prompt
using the context ‘<samp>base</samp>’ rather than ‘<samp>caller</samp>’.
</p>
<p>These functions are rarely used in practice. One
example is the <code>fail (‘<samp>code</samp>’, ‘<samp>pattern</samp>’)</code> function
which evaluates ‘<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>save</code> and <code>load</code>
are written in C++ where all Octave variables
are in the ‘<samp>caller</samp>’ context and <code>evalin</code> is not needed.
</p>
<a name="XREFevalin"></a><dl>
<dt><a name="index-evalin"></a><em></em> <strong>evalin</strong> <em>(<var>context</var>, <var>try</var>)</em></dt>
<dt><a name="index-evalin-1"></a><em></em> <strong>evalin</strong> <em>(<var>context</var>, <var>try</var>, <var>catch</var>)</em></dt>
<dd><p>Like <code>eval</code>, except that the expressions are evaluated in the context
<var>context</var>, which may be either <code>"caller"</code> or <code>"base"</code>.
</p>
<p><strong>See also:</strong> <a href="Evaluation.html#XREFeval">eval</a>, <a href="#XREFassignin">assignin</a>.
</p></dd></dl>
<a name="XREFassignin"></a><dl>
<dt><a name="index-assignin"></a><em></em> <strong>assignin</strong> <em>(<var>context</var>, <var>varname</var>, <var>value</var>)</em></dt>
<dd><p>Assign <var>value</var> to <var>varname</var> in context <var>context</var>, which
may be either <code>"base"</code> or <code>"caller"</code>.
</p>
<p><strong>See also:</strong> <a href="#XREFevalin">evalin</a>.
</p></dd></dl>
<hr>
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