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<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>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.
<!-- eval src/oct-parse.cc -->
<p><a name="doc_002deval"></a>
<div class="defun">
— Built-in Function: <b>eval</b> (<var>try</var>)<var><a name="index-eval-680"></a></var><br>
— Built-in Function: <b>eval</b> (<var>try, catch</var>)<var><a name="index-eval-681"></a></var><br>
<blockquote><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>The following example makes the variable <var>a</var> with the approximate
value 3.1416 available.
<pre class="example"> eval("a = acos(-1);");
</pre>
<p>If an error occurs during the evaluation of <var>try</var> the <var>catch</var>
string is evaluated, as the following example shows:
<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>
<p>Consider using try/catch blocks instead if you are only using <code>eval</code>
as an error-capturing mechanism rather than for the execution of arbitrary
code strings.
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<p class="noindent"><strong>See also:</strong> <a href="doc_002devalin.html#doc_002devalin">evalin</a>.
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