<!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 in a Different Context (GNU Octave (version 5.1.0))</title> <meta name="description" content="Evaluation in a Different Context (GNU Octave (version 5.1.0))"> <meta name="keywords" content="Evaluation in a Different Context (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="Evaluation.html#Evaluation" rel="up" title="Evaluation"> <link href="Statements.html#Statements" rel="next" title="Statements"> <link href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" rel="prev" title="Calling a Function by its Name"> <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-in-a-Different-Context"></a> <div class="header"> <p> Previous: <a href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" accesskey="p" rel="prev">Calling a Function by its Name</a>, Up: <a href="Evaluation.html#Evaluation" accesskey="u" rel="up">Evaluation</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-in-a-Different-Context-1"></a> <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> <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> <p>Consider how you might write <code>save</code> and <code>load</code> as m-files. For example: </p> <div class="example"> <pre class="example">function create_data x = linspace (0, 10, 10); y = sin (x); save mydata x y endfunction </pre></div> <p>With <code>evalin</code>, you could write <code>save</code> as follows: </p> <div class="example"> <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></div> <p>Here, ‘<samp>caller</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> <p>You later want to load the values back from <code>mydata</code> in a different context: </p> <div class="example"> <pre class="example">function process_data load mydata … do work … endfunction </pre></div> <p>With <code>assignin</code>, you could write <code>load</code> as follows: </p> <div class="example"> <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></div> <p>Here, ‘<samp>caller</samp>’ is the <code>process_data</code> function. </p> <p>You can set and use variables at the command prompt using the context ‘<samp>base</samp>’ rather than ‘<samp>caller</samp>’. </p> <p>These functions are rarely used in practice. One example is the <code>fail (‘<samp>code</samp>’, ‘<samp>pattern</samp>’)</code> function which evaluates ‘<samp>code</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>caller</samp>’ context and <code>evalin</code> is not needed. </p> <a name="XREFevalin"></a><dl> <dt><a name="index-evalin"></a><em></em> <strong>evalin</strong> <em>(<var>context</var>, <var>try</var>)</em></dt> <dt><a name="index-evalin-1"></a><em></em> <strong>evalin</strong> <em>(<var>context</var>, <var>try</var>, <var>catch</var>)</em></dt> <dd><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> <p><strong>See also:</strong> <a href="Evaluation.html#XREFeval">eval</a>, <a href="#XREFassignin">assignin</a>. </p></dd></dl> <a name="XREFassignin"></a><dl> <dt><a name="index-assignin"></a><em></em> <strong>assignin</strong> <em>(<var>context</var>, <var>varname</var>, <var>value</var>)</em></dt> <dd><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> <p><strong>See also:</strong> <a href="#XREFevalin">evalin</a>. </p></dd></dl> <hr> <div class="header"> <p> Previous: <a href="Calling-a-Function-by-its-Name.html#Calling-a-Function-by-its-Name" accesskey="p" rel="prev">Calling a Function by its Name</a>, Up: <a href="Evaluation.html#Evaluation" accesskey="u" rel="up">Evaluation</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>