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<a name="Evaluation"></a>
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<p>
Next: <a href="Statements.html#Statements" accesskey="n" rel="next">Statements</a>, Previous: <a href="Expressions.html#Expressions" accesskey="p" rel="prev">Expressions</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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<a name="Evaluation-1"></a>
<h2 class="chapter">9 Evaluation</h2>
<p>Normally, you evaluate expressions simply by typing them at the Octave
prompt, or by asking Octave to interpret commands that you have saved in
a file.
</p>
<p>Sometimes, you may find it necessary to evaluate an expression that has
been computed and stored in a string, which is exactly what the
<code>eval</code> function lets you do.
</p>
<a name="XREFeval"></a><dl>
<dt><a name="index-eval"></a>Built-in Function: <em></em> <strong>eval</strong> <em>(<var>try</var>)</em></dt>
<dt><a name="index-eval-1"></a>Built-in Function: <em></em> <strong>eval</strong> <em>(<var>try</var>, <var>catch</var>)</em></dt>
<dd><p>Parse the string <var>try</var> and evaluate it as if it were an Octave
program. If that fails, evaluate the optional string <var>catch</var>.
The string <var>try</var> is evaluated in the current context,
so any results remain available after <code>eval</code> returns.
</p>
<p>The following example creates the variable <var>A</var> with the approximate
value of 3.1416 in the current workspace.
</p>
<div class="example">
<pre class="example">eval ("A = acos(-1);");
</pre></div>
<p>If an error occurs during the evaluation of <var>try</var> then the <var>catch</var>
string is evaluated, as the following example shows:
</p>
<div class="example">
<pre class="example">eval ('error ("This is a bad example");',
'printf ("This error occurred:\n%s\n", lasterr ());');
-| This error occurred:
This is a bad example
</pre></div>
<p>Programming Note: if you are only using <code>eval</code> as an error-capturing
mechanism, rather than for the execution of arbitrary code strings,
Consider using try/catch blocks or unwind_protect/unwind_protect_cleanup
blocks instead. These techniques have higher performance and don’t introduce
the security considerations that the evaluation of arbitrary code does.
</p>
<p><strong>See also:</strong> <a href="Evaluation-in-a-Different-Context.html#XREFevalin">evalin</a>.
</p></dd></dl>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">• <a href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" accesskey="1">Calling a Function by its Name</a>:</td><td> </td><td align="left" valign="top">
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<tr><td align="left" valign="top">• <a href="Evaluation-in-a-Different-Context.html#Evaluation-in-a-Different-Context" accesskey="2">Evaluation in a Different Context</a>:</td><td> </td><td align="left" valign="top">
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