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<p>
<a name="Evaluation-in-a-Different-Context"></a>
Previous: <a rel="previous" accesskey="p" href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name">Calling a Function by its Name</a>,
Up: <a rel="up" accesskey="u" href="Evaluation.html#Evaluation">Evaluation</a>
<hr>
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<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>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>Consider how you might write <code>save</code> and <code>load</code> as
m-files. For example,
<pre class="example"> function create_data
x = linspace (0, 10, 10);
y = sin (x);
save mydata x y
endfunction
</pre>
<p>With <code>evalin</code>, you could write <code>save</code> as follows:
<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>
<p class="noindent">Here, ‘<samp><span class="samp">caller</span></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>You later want to load the values back from <code>mydata</code>
in a different context:
<pre class="example"> function process_data
load mydata
... do work ...
endfunction
</pre>
<p class="noindent">With <code>assignin</code>, you could write <code>load</code> as follows:
<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>
<p class="noindent">Here, ‘<samp><span class="samp">caller</span></samp>’ is the <code>process_data</code> function.
<p>You can set and use variables at the command prompt
using the context ‘<samp><span class="samp">base</span></samp>’ rather than ‘<samp><span class="samp">caller</span></samp>’.
<p>These functions are rarely used in practice. One
example is the <code>fail (‘</code><samp><span class="samp">code</span></samp><code>’, ‘</code><samp><span class="samp">pattern</span></samp><code>’)</code> function
which evaluates ‘<samp><span class="samp">code</span></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><span class="samp">caller</span></samp>’ context and <code>evalin</code> is not needed.
<!-- parse.cc -->
<p><a name="doc_002devalin"></a>
<div class="defun">
— Built-in Function: <b>evalin</b> (<var>context, try, catch</var>)<var><a name="index-evalin-552"></a></var><br>
<blockquote><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></blockquote></div>
<!-- parse.cc -->
<p><a name="doc_002dassignin"></a>
<div class="defun">
— Built-in Function: <b>assignin</b> (<var>context, varname, value</var>)<var><a name="index-assignin-553"></a></var><br>
<blockquote><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></blockquote></div>
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