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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><em></em> <strong>eval</strong> <em>(<var>try</var>)</em></dt>
<dt><a name="index-eval-1"></a><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.
</p>
<p>If execution fails, evaluate the optional string <var>catch</var>.
</p>
<p>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>, <a href="#XREFevalc">evalc</a>, <a href="Evaluation-in-a-Different-Context.html#XREFassignin">assignin</a>, <a href="Calling-a-Function-by-its-Name.html#XREFfeval">feval</a>.
</p></dd></dl>
<p>The <code>evalc</code> function additionally captures any console output
produced by the evaluated expression.
</p>
<a name="XREFevalc"></a><dl>
<dt><a name="index-evalc"></a><em><var>s</var> =</em> <strong>evalc</strong> <em>(<var>try</var>)</em></dt>
<dt><a name="index-evalc-1"></a><em><var>s</var> =</em> <strong>evalc</strong> <em>(<var>try</var>, <var>catch</var>)</em></dt>
<dd><p>Parse and evaluate the string <var>try</var> as if it were an Octave program,
while capturing the output into the return variable <var>s</var>.
</p>
<p>If execution fails, evaluate the optional string <var>catch</var>.
</p>
<p>This function behaves like <code>eval</code>, but any output or warning messages
which would normally be written to the console are captured and returned in
the string <var>s</var>.
</p>
<p>The <code>diary</code> is disabled during the execution of this function. When
<code>system</code> is used, any output produced by external programs is
<em>not</em> captured, unless their output is captured by the <code>system</code>
function itself.
</p>
<div class="example">
<pre class="example">s = evalc ("t = 42"), t
⇒ s = t = 42
⇒ t = 42
</pre></div>
<p><strong>See also:</strong> <a href="#XREFeval">eval</a>, <a href="Diary-and-Echo-Commands.html#XREFdiary">diary</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">
</td></tr>
<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">
</td></tr>
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