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<H1> Expression Evaluator - Macro preprocessor and C Parser   </H1>

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<H2>Introduction</H2>
<P>          The Expression Evaluator module, <tt>ex.syn</tt>, exists only to
          evaluate the conditional expressions in <tt>#if</tt> and
	  <tt>#elif</tt>
          statements. The syntax for these expressions has been
          extracted from a full C syntax and reduced to that which is
          necessary for this purpose.
<P>
          The rules for the evaluation of constant expressions are
          given in Kernighan and Ritchie, sections A7.19 and A12.5
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<H2>Theory of Operation</H2>
          The only unusual feature of this grammar is the
          interposition of a token called "nonzero" to avoid division
          by zero. Otherwise the grammar is a conventional expression
          grammar.
<P>
          The parser is implemented as an event driven parser. The
          initializer is <tt>init_ex()</tt>. The parser itself is
          <tt>ex()</tt>. The
          parser has been interfaced to the rest of the program by a
          class definition, <tt>expression_evaluator</tt>.
<P>
          The class definition for <tt>expression_evaluator</tt> is given in
          <tt>mpp.h</tt>. Each instance of the class has its own parser control
          block. Therefore the parser makes all of its references
          indirectly through a pointer, <tt>ex_pcb</tt>. Whenever it is
          necessary to invoke <tt>init_ex()</tt> or <tt>ex()</tt>,
          therefore, it is
          necessary first to make sure <tt>ex_pcb</tt> is correctly set.
<P>
          Input to the parser consists of tokens as defined in
          <tt>token.h</tt>. Since the input does not consist of simple
          characters, the <b>input values</b> switch is set. The input
          procedures store the full token into the input_value field
          of the pcb and the id field into input_code.

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                  AnaGram parser generator - examples<BR>
                  Expression Evaluator - Macro preprocessor and C Parser <BR>
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author	David A. Holland
date	Mon, 13 Jun 2022 00:40:23 -0400
parents	13d2b8934445
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