<html><head><META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>5.4.2.&nbsp;Undefined Value</title><link href="style.css" rel="stylesheet" type="text/css"><meta content="DocBook XSL Stylesheets V1.72.0" name="generator"><link rel="start" href="index.html" title="The VIATRA2 Model Transformation Framework"><link rel="up" href="sec_ASMConstants.html" title="5.4.&nbsp;Constants"><link rel="prev" href="def_AsmConstant.html" title="5.4.1.&nbsp;ASM Constant (Value)"><link rel="next" href="def_BooleanConstant.html" title="5.4.3.&nbsp;Boolean Constant"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table summary="Navigation header" width="100%"><tr><th align="center" colspan="3">5.4.2.&nbsp;Undefined Value</th></tr><tr><td align="left" width="20%"><a accesskey="p" href="def_AsmConstant.html">Prev</a>&nbsp;</td><th align="center" width="60%">5.4.&nbsp;Constants</th><td align="right" width="20%">&nbsp;<a accesskey="n" href="def_BooleanConstant.html">Next</a></td></tr></table><hr></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="def_UndefinedValue"></a>5.4.2.&nbsp;Undefined Value</h4></div></div></div><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip: Description"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Tip]" src="images/tip.png"></td><th align="left"><a name="def_UndefinedValue_Description"></a>Description</th></tr><tr><td valign="top" align="left"><p>
<span class="strong"><strong>Undefined value</strong></span> 
<a name="N1285F" class="indexterm"></a>
<a name="N12866" class="indexterm"></a>
is a constant, which is used when the value of a term (e.g. a variable)
is not known. 
</p></td></tr></table></div><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Important: Syntax"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Important]" src="images/important.png"></td><th align="left"><a name="def_UndefinedValue_Syntax"></a>Syntax</th></tr><tr><td valign="top" align="left"><pre class="programlisting">
UndefValueAST ::= undef
</pre><p>
The undefined value is identified by the <code class="computeroutput">undef</code> keyword. 
</p></td></tr></table></div><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note: Semantics"><tr><td valign="top" align="center" rowspan="2" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left"><a name="def_UndefinedValue_Semantics"></a>Semantics</th></tr><tr><td valign="top" align="left"><p>
The undefined value is assigned to a term, if no value has been assigned to it.  
</p></td></tr></table></div><div class="highlights"><a name="def_UndefinedValue_Remark"></a><p><b>Remark.&nbsp;</b>
Undefined values are frequently used in the <a href="def_LetRule.html" title="4.3.5.&nbsp;Let Rule">let rule</a>,
when the initialization of a variable is irrelevant at a certain stage.
</p></div><div class="highlights"><a name="def_UndefinedValue_DefinedIn"></a><p><b>Defined In:&nbsp;</b>
<a href="def_AsmConstant.html" title="5.4.1.&nbsp;ASM Constant (Value)">ASM constant</a>
</p></div></div><div class="navfooter"><hr><table summary="Navigation footer" width="100%"><tr><td align="left" width="40%"><a accesskey="p" href="def_AsmConstant.html">Prev</a>&nbsp;</td><td align="center" width="20%"><a accesskey="u" href="sec_ASMConstants.html">Up</a></td><td align="right" width="40%">&nbsp;<a accesskey="n" href="def_BooleanConstant.html">Next</a></td></tr><tr><td valign="top" align="left" width="40%">5.4.1.&nbsp;ASM Constant (Value)&nbsp;</td><td align="center" width="20%"><a accesskey="h" href="index.html">Home</a></td><td valign="top" align="right" width="40%">&nbsp;5.4.3.&nbsp;Boolean Constant</td></tr></table></div></body></html>