<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Evaluation (GNU Octave (version 5.1.0))</title> <meta name="description" content="Evaluation (GNU Octave (version 5.1.0))"> <meta name="keywords" content="Evaluation (GNU Octave (version 5.1.0))"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <link href="index.html#Top" rel="start" title="Top"> <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="index.html#Top" rel="up" title="Top"> <link href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" rel="next" title="Calling a Function by its Name"> <link href="Operator-Precedence.html#Operator-Precedence" rel="prev" title="Operator Precedence"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> <link rel="stylesheet" type="text/css" href="octave.css"> </head> <body lang="en"> <a name="Evaluation"></a> <div class="header"> <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> </div> <hr> <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> </table> <hr> <div class="header"> <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> </div> </body> </html>