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<div class="title">EVAL Evaluate a String </div> </div>
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<div class="textblock"><p>Section: <a class="el" href="sec_freemat.html">FreeMat Functions</a> </p>
<h1><a class="anchor" id="Usage"></a>
Usage</h1>
<p>The <code>eval</code> function evaluates a string. The general syntax for its use is </p>
<pre class="fragment"> eval(s)
</pre><p> where <code>s</code> is the string to evaluate. If <code>s</code> is an expression (instead of a set of statements), you can assign the output of the <code>eval</code> call to one or more variables, via </p>
<pre class="fragment"> x = eval(s)
[x,y,z] = eval(s)
</pre><p>Another form of <code>eval</code> allows you to specify an expression or set of statements to execute if an error occurs. In this form, the syntax for <code>eval</code> is </p>
<pre class="fragment"> eval(try_clause,catch_clause),
</pre><p> or with return values, </p>
<pre class="fragment"> x = eval(try_clause,catch_clause)
[x,y,z] = eval(try_clause,catch_clause)
</pre><p> These later forms are useful for specifying defaults. Note that both the <code>try_clause</code> and <code>catch_clause</code> must be expressions, as the equivalent code is </p>
<pre class="fragment"> try
[x,y,z] = try_clause
catch
[x,y,z] = catch_clause
end
</pre><p> so that the assignment must make sense in both cases. </p>
<h1><a class="anchor" id="Example"></a>
Example</h1>
<p>Here are some examples of <code>eval</code> being used.</p>
<pre class="fragment">--> eval('a = 32')
a =
32
--> b = eval('a')
b =
32
</pre><p>The primary use of the <code>eval</code> statement is to enable construction of expressions at run time.</p>
<pre class="fragment">--> s = ['b = a' ' + 2']
s =
b = a + 2
--> eval(s)
b =
34
</pre><p>Here we demonstrate the use of the catch-clause to provide a default value</p>
<pre class="fragment">--> a = 32
a =
32
--> b = eval('a','1')
b =
32
--> b = eval('z','a+1')
b =
33
</pre><p>Note that in the second case, <code>b</code> takes the value of 33, indicating that the evaluation of the first expression failed (because <code>z</code> is not defined). </p>
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