File: bison_5.html

package info (click to toggle)
bisonc%2B%2B 6.09.02-1
links: PTS, VCS
area: main
in suites: forky, sid
size: 5,984 kB
sloc: cpp: 9,375; ansic: 1,505; fortran: 1,134; makefile: 1,062; sh: 526; yacc: 84; lex: 60
file content (1353 lines) | stat: -rw-r--r-- 59,444 bytes
parent folder | download | duplicates (11)
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
                      "http://www.w3.org/TR/html40/loose.dtd">
<HTML>
<!-- Created on January, 28 2005 by texi2html 1.66 -->
<!--
Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
            Karl Berry  <karl@freefriends.org>
            Olaf Bachmann <obachman@mathematik.uni-kl.de>
            and many others.
Maintained by: Many creative people <dev@texi2html.cvshome.org>
Send bugs and suggestions to <users@texi2html.cvshome.org>

-->
<HEAD>
<TITLE>Bison 2.21.5: Examples</TITLE>

<META NAME="description" CONTENT="Bison 2.21.5: Examples">
<META NAME="keywords" CONTENT="Bison 2.21.5: Examples">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">
<META NAME="Generator" CONTENT="texi2html 1.66">

</HEAD>

<BODY LANG="en" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000">

<A NAME="SEC14"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_4.html#SEC13"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_4.html#SEC6"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H1> 2. Examples </H1>
<!--docid::SEC14::-->
<P>

Now we show and explain three sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator, and a multi-function calculator.  All three have been tested
under BSD Unix 4.3; each produces a usable, though limited, interactive
desk-top calculator.
</P>
<P>

These examples are simple, but Bison grammars for real programming
languages are written the same way.
You can copy these examples out of the Info file and into a source file
to try them.
</P>
<P>

<TABLE BORDER="0" CELLSPACING="0">
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC15">2.1 Reverse Polish Notation Calculator</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Reverse polish notation calculator;
                        a first example with no operator precedence.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC26">2.2 Infix Notation Calculator: <CODE>calc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Infix (algebraic) notation calculator.
                        Operator precedence is introduced.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC27">2.3 Simple Error Recovery</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Continuing after syntax errors.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC28">2.4 Multi-Function Calculator: <CODE>mfcalc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Calculator with memory and trig functions.
                           It uses multiple data-types for semantic values.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC32">2.5 Exercises</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Ideas for improving the multi-function calculator.</TD></TR>
</TABLE>
<P>

<A NAME="RPN Calc"></A>
<HR SIZE="6">
<A NAME="SEC15"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC16"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H2> 2.1 Reverse Polish Notation Calculator </H2>
<!--docid::SEC15::-->
<P>

The first example is that of a simple double-precision <EM>reverse polish
notation</EM> calculator (a calculator using postfix operators).  This example
provides a good starting point, since operator precedence is not an issue.
The second example will illustrate how operator precedence is handled.
</P>
<P>

The source code for this calculator is named `<TT>rpcalc.y</TT>'.  The
`<SAMP>.y</SAMP>' extension is a convention used for Bison input files.
</P>
<P>

<TABLE BORDER="0" CELLSPACING="0">
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC16">2.1.1 Declarations for <CODE>rpcalc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Bison and C declarations for rpcalc.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC17">2.1.2 Grammar Rules for <CODE>rpcalc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Grammar Rules for rpcalc, with explanation.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC21">2.1.3 The <CODE>rpcalc</CODE> Lexical Analyzer</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The lexical analyzer.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC22">2.1.4 The Controlling Function</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The controlling function.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC23">2.1.5 The Error Reporting Routine</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">The error reporting function.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC24">2.1.6 Running Bison to Make the Parser</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Running Bison on the grammar file.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC25">2.1.7 Compiling the Parser File</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Run the C compiler on the output code.</TD></TR>
</TABLE>
<P>

<A NAME="Rpcalc Decls"></A>
<HR SIZE="6">
<A NAME="SEC16"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC17"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.1 Declarations for <CODE>rpcalc</CODE> </H3>
<!--docid::SEC16::-->
<P>

Here are the C and Bison declarations for the reverse polish notation
calculator.  As in C, comments are placed between `<SAMP>/*<small>...</small>*/</SAMP>'.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>/* Reverse polish notation calculator. */

%{
#define YYSTYPE double
#include &lt;math.h&gt;
%}

%token NUM

%% /* Grammar rules and actions follow */
</pre></td></tr></table><P>

The C declarations section (see section <A HREF="bison_6.html#SEC35">The C Declarations Section</A>) contains two
preprocessor directives.
</P>
<P>

The <CODE>#define</CODE> directive defines the macro <CODE>YYSTYPE</CODE>, thus
specifying the C data type for semantic values of both tokens and groupings
(see section <A HREF="bison_6.html#SEC43">Data Types of Semantic Values</A>).  The Bison parser will use whatever type
<CODE>YYSTYPE</CODE> is defined as; if you don't define it, <CODE>int</CODE> is the
default.  Because we specify <CODE>double</CODE>, each token and each expression
has an associated value, which is a floating point number.
</P>
<P>

The <CODE>#include</CODE> directive is used to declare the exponentiation
function <CODE>pow</CODE>.
</P>
<P>

The second section, Bison declarations, provides information to Bison about
the token types (see section <A HREF="bison_6.html#SEC36">The Bison Declarations Section</A>).  Each terminal symbol that is
not a single-character literal must be declared here.  (Single-character
literals normally don't need to be declared.)  In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is <CODE>NUM</CODE>, the token
type for numeric constants.
</P>
<P>

<A NAME="Rpcalc Rules"></A>
<HR SIZE="6">
<A NAME="SEC17"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC16"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC18"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.2 Grammar Rules for <CODE>rpcalc</CODE> </H3>
<!--docid::SEC17::-->
<P>

Here are the grammar rules for the reverse polish notation calculator.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>input:    /* empty */
        | input line
;

line:     '\n'
        | exp '\n'  { printf (&quot;\t%.10g\n&quot;, $1); }
;

exp:      NUM             { $$ = $1;         }
        | exp exp '+'     { $$ = $1 + $2;    }
        | exp exp '-'     { $$ = $1 - $2;    }
        | exp exp '*'     { $$ = $1 * $2;    }
        | exp exp '/'     { $$ = $1 / $2;    }
      /* Exponentiation */
        | exp exp '^'     { $$ = pow ($1, $2); }
      /* Unary minus    */
        | exp 'n'         { $$ = -$1;        }
;
%%
</pre></td></tr></table><P>

The groupings of the rpcalc &quot;language&quot; defined here are the expression
(given the name <CODE>exp</CODE>), the line of input (<CODE>line</CODE>), and the
complete input transcript (<CODE>input</CODE>).  Each of these nonterminal
symbols has several alternate rules, joined by the `<SAMP>|</SAMP>' punctuator
which is read as &quot;or&quot;.  The following sections explain what these rules
mean.
</P>
<P>

The semantics of the language is determined by the actions taken when a
grouping is recognized.  The actions are the C code that appears inside
braces.  See section <A HREF="bison_6.html#SEC45">3.5.3 Actions</A>.
</P>
<P>

You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules.  In each action, the
pseudo-variable <CODE>$$</CODE> stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to <CODE>$$</CODE> is the
main job of most actions.  The semantic values of the components of the
rule are referred to as <CODE>$1</CODE>, <CODE>$2</CODE>, and so on.
</P>
<P>

<TABLE BORDER="0" CELLSPACING="0">
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC18">2.1.2.1 Explanation of <CODE>input</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP"></TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC19">2.1.2.2 Explanation of <CODE>line</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP"></TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC20">2.1.2.3 Explanation of <CODE>expr</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP"></TD></TR>
</TABLE>
<P>

<A NAME="Rpcalc Input"></A>
<HR SIZE="6">
<A NAME="SEC18"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC17"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC19"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC17"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H4> 2.1.2.1 Explanation of <CODE>input</CODE> </H4>
<!--docid::SEC18::-->
<P>

Consider the definition of <CODE>input</CODE>:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>input:    /* empty */
        | input line
;
</pre></td></tr></table><P>

This definition reads as follows: &quot;A complete input is either an empty
string, or a complete input followed by an input line&quot;.  Notice that
&quot;complete input&quot; is defined in terms of itself.  This definition is said
to be <EM>left recursive</EM> since <CODE>input</CODE> appears always as the
leftmost symbol in the sequence.  See section <A HREF="bison_6.html#SEC41">Recursive Rules</A>.
</P>
<P>

The first alternative is empty because there are no symbols between the
colon and the first `<SAMP>|</SAMP>'; this means that <CODE>input</CODE> can match an
empty string of input (no tokens).  We write the rules this way because it
is legitimate to type <KBD>Ctrl-d</KBD> right after you start the calculator.
It's conventional to put an empty alternative first and write the comment
`<SAMP>/* empty */</SAMP>' in it.
</P>
<P>

The second alternate rule (<CODE>input line</CODE>) handles all nontrivial input.
It means, &quot;After reading any number of lines, read one more line if
possible.&quot;  The left recursion makes this rule into a loop.  Since the
first alternative matches empty input, the loop can be executed zero or
more times.
</P>
<P>

The parser function <CODE>yyparse</CODE> continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end of file.
</P>
<P>

<A NAME="Rpcalc Line"></A>
<HR SIZE="6">
<A NAME="SEC19"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC18"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC20"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC17"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H4> 2.1.2.2 Explanation of <CODE>line</CODE> </H4>
<!--docid::SEC19::-->
<P>

Now consider the definition of <CODE>line</CODE>:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>line:     '\n'
        | exp '\n'  { printf (&quot;\t%.10g\n&quot;, $1); }
;
</pre></td></tr></table><P>

The first alternative is a token which is a newline character; this means
that rpcalc accepts a blank line (and ignores it, since there is no
action).  The second alternative is an expression followed by a newline.
This is the alternative that makes rpcalc useful.  The semantic value of
the <CODE>exp</CODE> grouping is the value of <CODE>$1</CODE> because the <CODE>exp</CODE> in
question is the first symbol in the alternative.  The action prints this
value, which is the result of the computation the user asked for.
</P>
<P>

This action is unusual because it does not assign a value to <CODE>$$</CODE>.  As
a consequence, the semantic value associated with the <CODE>line</CODE> is
uninitialized (its value will be unpredictable).  This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.
</P>
<P>

<A NAME="Rpcalc Expr"></A>
<HR SIZE="6">
<A NAME="SEC20"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC19"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC21"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC17"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H4> 2.1.2.3 Explanation of <CODE>expr</CODE> </H4>
<!--docid::SEC20::-->
<P>

The <CODE>exp</CODE> grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just numbers.
The second handles an addition-expression, which looks like two expressions
followed by a plus-sign.  The third handles subtraction, and so on.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>exp:      NUM
        | exp exp '+'     { $$ = $1 + $2;    }
        | exp exp '-'     { $$ = $1 - $2;    }
        <small>...</small>
        ;
</pre></td></tr></table><P>

We have used `<SAMP>|</SAMP>' to join all the rules for <CODE>exp</CODE>, but we could
equally well have written them separately:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>exp:      NUM ;
exp:      exp exp '+'     { $$ = $1 + $2;    } ;
exp:      exp exp '-'     { $$ = $1 - $2;    } ;
        <small>...</small>
</pre></td></tr></table><P>

Most of the rules have actions that compute the value of the expression in
terms of the value of its parts.  For example, in the rule for addition,
<CODE>$1</CODE> refers to the first component <CODE>exp</CODE> and <CODE>$2</CODE> refers to
the second one.  The third component, <CODE>'+'</CODE>, has no meaningful
associated semantic value, but if it had one you could refer to it as
<CODE>$3</CODE>.  When <CODE>yyparse</CODE> recognizes a sum expression using this
rule, the sum of the two subexpressions' values is produced as the value of
the entire expression.  See section <A HREF="bison_6.html#SEC45">3.5.3 Actions</A>.
</P>
<P>

You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of <CODE>$1</CODE> into <CODE>$$</CODE>.
This is what happens in the first rule (the one that uses <CODE>NUM</CODE>).
</P>
<P>

The formatting shown here is the recommended convention, but Bison does
not require it.  You can add or change whitespace as much as you wish.
For example, this:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>exp   : NUM | exp exp '+' {$$ = $1 + $2; } | <small>...</small>
</pre></td></tr></table><P>

means the same thing as this:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>exp:      NUM
        | exp exp '+'    { $$ = $1 + $2; }
        | <small>...</small>
</pre></td></tr></table><P>

The latter, however, is much more readable.
</P>
<P>

<A NAME="Rpcalc Lexer"></A>
<HR SIZE="6">
<A NAME="SEC21"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC20"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC22"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.3 The <CODE>rpcalc</CODE> Lexical Analyzer </H3>
<!--docid::SEC21::-->
<P>

The lexical analyzer's job is low-level parsing: converting characters or
sequences of characters into tokens.  The Bison parser gets its tokens by
calling the lexical analyzer.  See section <A HREF="bison_7.html#SEC60">The Lexical Analyzer Function <CODE>yylex</CODE></A>.
</P>
<P>

Only a simple lexical analyzer is needed for the RPN calculator.  This
lexical analyzer skips blanks and tabs, then reads in numbers as
<CODE>double</CODE> and returns them as <CODE>NUM</CODE> tokens.  Any other character
that isn't part of a number is a separate token.  Note that the token-code
for such a single-character token is the character itself.
</P>
<P>

The return value of the lexical analyzer function is a numeric code which
represents a token type.  The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways.  If the token type is a character literal, then its
numeric code is the ASCII code for that character; you can use the same
character literal in the lexical analyzer to express the number.  If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number.  In this example,
therefore, <CODE>NUM</CODE> becomes a macro for <CODE>yylex</CODE> to use.
</P>
<P>

The semantic value of the token (if it has one) is stored into the global
variable <CODE>yylval</CODE>, which is where the Bison parser will look for it.
(The C data type of <CODE>yylval</CODE> is <CODE>YYSTYPE</CODE>, which was defined
at the beginning of the grammar; see section <A HREF="bison_5.html#SEC16">Declarations for <CODE>rpcalc</CODE></A>.)
</P>
<P>

A token type code of zero is returned if the end-of-file is encountered.
(Bison recognizes any nonpositive value as indicating the end of the
input.)
</P>
<P>

Here is the code for the lexical analyzer:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>/* Lexical analyzer returns a double floating point 
   number on the stack and the token NUM, or the ASCII
   character read if not a number.  Skips all blanks
   and tabs, returns 0 for EOF. */

#include &lt;ctype.h&gt;

yylex ()
{
  int c;

  /* skip white space  */
  while ((c = getchar ()) == ' ' || c == '\t')  
    ;
  /* process numbers   */
  if (c == '.' || isdigit (c))                
    {
      ungetc (c, stdin);
      scanf (&quot;%lf&quot;, &amp;yylval);
      return NUM;
    }
  /* return end-of-file  */
  if (c == EOF)                            
    return 0;
  /* return single chars */
  return c;                                
}
</pre></td></tr></table><P>

<A NAME="Rpcalc Main"></A>
<HR SIZE="6">
<A NAME="SEC22"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC21"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC23"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.4 The Controlling Function </H3>
<!--docid::SEC22::-->
<P>

In keeping with the spirit of this example, the controlling function is
kept to the bare minimum.  The only requirement is that it call
<CODE>yyparse</CODE> to start the process of parsing.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>main ()
{
  yyparse ();
}
</pre></td></tr></table><P>

<A NAME="Rpcalc Error"></A>
<HR SIZE="6">
<A NAME="SEC23"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC22"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC24"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.5 The Error Reporting Routine </H3>
<!--docid::SEC23::-->
<P>

When <CODE>yyparse</CODE> detects a syntax error, it calls the error reporting
function <CODE>yyerror</CODE> to print an error message (usually but not always
<CODE>&quot;parse error&quot;</CODE>).  It is up to the programmer to supply <CODE>yyerror</CODE>
(see section <A HREF="bison_7.html#SEC58">Parser C-Language Interface</A>), so here is the definition we will use:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>#include &lt;stdio.h&gt;

yyerror (s)  /* Called by yyparse on error */
     char *s;
{
  printf (&quot;%s\n&quot;, s);
}
</pre></td></tr></table><P>

After <CODE>yyerror</CODE> returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(see section <A HREF="bison_9.html#SEC80">6. Error Recovery</A>).  Otherwise, <CODE>yyparse</CODE> returns nonzero.  We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit.  This is not clean behavior for a
real calculator, but it is adequate in the first example.
</P>
<P>

<A NAME="Rpcalc Gen"></A>
<HR SIZE="6">
<A NAME="SEC24"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC23"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC25"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.6 Running Bison to Make the Parser </H3>
<!--docid::SEC24::-->
<P>

Before running Bison to produce a parser, we need to decide how to arrange
all the source code in one or more source files.  For such a simple example,
the easiest thing is to put everything in one file.  The definitions of
<CODE>yylex</CODE>, <CODE>yyerror</CODE> and <CODE>main</CODE> go at the end, in the
&quot;additional C code&quot; section of the file (see section <A HREF="bison_4.html#SEC13">The Overall Layout of a Bison Grammar</A>).
</P>
<P>

For a large project, you would probably have several source files, and use
<CODE>make</CODE> to arrange to recompile them.
</P>
<P>

With all the source in a single file, you use the following command to
convert it into a parser file:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>bison <VAR>file_name</VAR>.y
</pre></td></tr></table><P>

In this example the file was called `<TT>rpcalc.y</TT>' (for &quot;Reverse Polish
CALCulator&quot;).  Bison produces a file named `<TT><VAR>file_name</VAR>.tab.c</TT>',
removing the `<SAMP>.y</SAMP>' from the original file name. The file output by
Bison contains the source code for <CODE>yyparse</CODE>.  The additional
functions in the input file (<CODE>yylex</CODE>, <CODE>yyerror</CODE> and <CODE>main</CODE>)
are copied verbatim to the output.
</P>
<P>

<A NAME="Rpcalc Compile"></A>
<HR SIZE="6">
<A NAME="SEC25"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC24"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC26"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC15"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.1.7 Compiling the Parser File </H3>
<!--docid::SEC25::-->
<P>

Here is how to compile and run the parser file:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre># List files in current directory.
% ls
rpcalc.tab.c  rpcalc.y

# Compile the Bison parser.
# `<SAMP>-lm</SAMP>' tells compiler to search math library for <CODE>pow</CODE>.
% cc rpcalc.tab.c -lm -o rpcalc

# List files again.
% ls
rpcalc  rpcalc.tab.c  rpcalc.y
</pre></td></tr></table><P>

The file `<TT>rpcalc</TT>' now contains the executable code.  Here is an
example session using <CODE>rpcalc</CODE>.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>% rpcalc
4 9 +
13
3 7 + 3 4 5 *+-
-13
3 7 + 3 4 5 * + - n              Note the unary minus, `<SAMP>n</SAMP>'
13
5 6 / 4 n +
-3.166666667
3 4 ^                            Exponentiation
81
^D                               End-of-file indicator
%
</pre></td></tr></table><P>

<A NAME="Infix Calc"></A>
<HR SIZE="6">
<A NAME="SEC26"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC25"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC27"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H2> 2.2 Infix Notation Calculator: <CODE>calc</CODE> </H2>
<!--docid::SEC26::-->
<P>

We now modify rpcalc to handle infix operators instead of postfix.  Infix
notation involves the concept of operator precedence and the need for
parentheses nested to arbitrary depth.  Here is the Bison code for
`<TT>calc.y</TT>', an infix desk-top calculator.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>/* Infix notation calculator--calc */

%{
#define YYSTYPE double
#include &lt;math.h&gt;
%}

/* BISON Declarations */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG     /* negation--unary minus */
%right '^'    /* exponentiation        */

/* Grammar follows */
%%
input:    /* empty string */
        | input line
;

line:     '\n'
        | exp '\n'  { printf (&quot;\t%.10g\n&quot;, $1); }
;

exp:      NUM                { $$ = $1;         }
        | exp '+' exp        { $$ = $1 + $3;    }
        | exp '-' exp        { $$ = $1 - $3;    }
        | exp '*' exp        { $$ = $1 * $3;    }
        | exp '/' exp        { $$ = $1 / $3;    }
        | '-' exp  %prec NEG { $$ = -$2;        }
        | exp '^' exp        { $$ = pow ($1, $3); }
        | '(' exp ')'        { $$ = $2;         }
;
%%
</pre></td></tr></table><P>

The functions <CODE>yylex</CODE>, <CODE>yyerror</CODE> and <CODE>main</CODE> can be the same
as before.
</P>
<P>

There are two important new features shown in this code.
</P>
<P>

In the second section (Bison declarations), <CODE>%left</CODE> declares token
types and says they are left-associative operators.  The declarations
<CODE>%left</CODE> and <CODE>%right</CODE> (right associativity) take the place of
<CODE>%token</CODE> which is used to declare a token type name without
associativity.  (These tokens are single-character literals, which
ordinarily don't need to be declared.  We declare them here to specify
the associativity.)
</P>
<P>

Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence.  Hence, exponentiation
has the highest precedence, unary minus (<CODE>NEG</CODE>) is next, followed
by `<SAMP>*</SAMP>' and `<SAMP>/</SAMP>', and so on.  See section <A HREF="bison_8.html#SEC70">Operator Precedence</A>.
</P>
<P>

The other important new feature is the <CODE>%prec</CODE> in the grammar section
for the unary minus operator.  The <CODE>%prec</CODE> simply instructs Bison that
the rule `<SAMP>| '-' exp</SAMP>' has the same precedence as <CODE>NEG</CODE>---in this
case the next-to-highest.  See section <A HREF="bison_8.html#SEC75">Context-Dependent Precedence</A>.
</P>
<P>

Here is a sample run of `<TT>calc.y</TT>':
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>% calc
4 + 4.5 - (34/(8*3+-3))
6.880952381
-56 + 2
-54
3 ^ 2
9
</pre></td></tr></table><P>

<A NAME="Simple Error Recovery"></A>
<HR SIZE="6">
<A NAME="SEC27"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC26"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC28"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H2> 2.3 Simple Error Recovery </H2>
<!--docid::SEC27::-->
<P>

Up to this point, this manual has not addressed the issue of <EM>error
recovery</EM>---how to continue parsing after the parser detects a syntax
error.  All we have handled is error reporting with <CODE>yyerror</CODE>.  Recall
that by default <CODE>yyparse</CODE> returns after calling <CODE>yyerror</CODE>.  This
means that an erroneous input line causes the calculator program to exit.
Now we show how to rectify this deficiency.
</P>
<P>

The Bison language itself includes the reserved word <CODE>error</CODE>, which
may be included in the grammar rules.  In the example below it has
been added to one of the alternatives for <CODE>line</CODE>:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>line:     '\n'
        | exp '\n'   { printf (&quot;\t%.10g\n&quot;, $1); }
        | error '\n' { yyerrok;                  }
;
</pre></td></tr></table><P>

This addition to the grammar allows for simple error recovery in the event
of a parse error.  If an expression that cannot be evaluated is read, the
error will be recognized by the third rule for <CODE>line</CODE>, and parsing
will continue.  (The <CODE>yyerror</CODE> function is still called upon to print
its message as well.)  The action executes the statement <CODE>yyerrok</CODE>, a
macro defined automatically by Bison; its meaning is that error recovery is
complete (see section <A HREF="bison_9.html#SEC80">6. Error Recovery</A>).  Note the difference between
<CODE>yyerrok</CODE> and <CODE>yyerror</CODE>; neither one is a misprint.</P>
<P>

This form of error recovery deals with syntax errors.  There are other
kinds of errors; for example, division by zero, which raises an exception
signal that is normally fatal.  A real calculator program must handle this
signal and use <CODE>longjmp</CODE> to return to <CODE>main</CODE> and resume parsing
input lines; it would also have to discard the rest of the current line of
input.  We won't discuss this issue further because it is not specific to
Bison programs.
</P>
<P>

<A NAME="Multi-function Calc"></A>
<HR SIZE="6">
<A NAME="SEC28"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC27"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC29"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H2> 2.4 Multi-Function Calculator: <CODE>mfcalc</CODE> </H2>
<!--docid::SEC28::-->
<P>

Now that the basics of Bison have been discussed, it is time to move on to
a more advanced problem.  The above calculators provided only five
functions, `<SAMP>+</SAMP>', `<SAMP>-</SAMP>', `<SAMP>*</SAMP>', `<SAMP>/</SAMP>' and `<SAMP>^</SAMP>'.  It would
be nice to have a calculator that provides other mathematical functions such
as <CODE>sin</CODE>, <CODE>cos</CODE>, etc.
</P>
<P>

It is easy to add new operators to the infix calculator as long as they are
only single-character literals.  The lexical analyzer <CODE>yylex</CODE> passes
back all non-number characters as tokens, so new grammar rules suffice for
adding a new operator.  But we want something more flexible: built-in
functions whose syntax has this form:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre><VAR>function_name</VAR> (<VAR>argument</VAR>)
</pre></td></tr></table><P>

At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=example><pre>% mfcalc
pi = 3.141592653589
3.1415926536
sin(pi)
0.0000000000
alpha = beta1 = 2.3
2.3000000000
alpha
2.3000000000
ln(alpha)
0.8329091229
exp(ln(beta1))
2.3000000000
%
</pre></td></tr></table><P>

Note that multiple assignment and nested function calls are permitted.
</P>
<P>

<TABLE BORDER="0" CELLSPACING="0">
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC29">2.4.1 Declarations for <CODE>mfcalc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Bison declarations for multi-function calculator.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC30">2.4.2 Grammar Rules for <CODE>mfcalc</CODE></A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Grammar rules for the calculator.</TD></TR>
<TR><TD ALIGN="left" VALIGN="TOP"><A HREF="bison_5.html#SEC31">2.4.3 The <CODE>mfcalc</CODE> Symbol Table</A></TD><TD>&nbsp;&nbsp;</TD><TD ALIGN="left" VALIGN="TOP">Symbol table management subroutines.</TD></TR>
</TABLE>
<P>

<A NAME="Mfcalc Decl"></A>
<HR SIZE="6">
<A NAME="SEC29"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC28"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC30"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC28"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.4.1 Declarations for <CODE>mfcalc</CODE> </H3>
<!--docid::SEC29::-->
<P>

Here are the C and Bison declarations for the multi-function calculator.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>%{
#include &lt;math.h&gt;  /* For math functions, cos(), sin(), etc. */
#include &quot;calc.h&quot;  /* Contains definition of `symrec'        */
%}
%union {
double     val;  /* For returning numbers.                   */
symrec  *tptr;   /* For returning symbol-table pointers      */
}

%token &lt;val&gt;  NUM        /* Simple double precision number   */
%token &lt;tptr&gt; VAR FNCT   /* Variable and Function            */
%type  &lt;val&gt;  exp

%right '='
%left '-' '+'
%left '*' '/'
%left NEG     /* Negation--unary minus */
%right '^'    /* Exponentiation        */

/* Grammar follows */

%%
</FONT></pre></td></tr></table><P>

The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types
(see section <A HREF="bison_6.html#SEC44">More Than One Value Type</A>).
</P>
<P>

The <CODE>%union</CODE> declaration specifies the entire list of possible types;
this is instead of defining <CODE>YYSTYPE</CODE>.  The allowable types are now
double-floats (for <CODE>exp</CODE> and <CODE>NUM</CODE>) and pointers to entries in
the symbol table.  See section <A HREF="bison_6.html#SEC51">The Collection of Value Types</A>.
</P>
<P>

Since values can now have various types, it is necessary to associate a
type with each grammar symbol whose semantic value is used.  These symbols
are <CODE>NUM</CODE>, <CODE>VAR</CODE>, <CODE>FNCT</CODE>, and <CODE>exp</CODE>.  Their
declarations are augmented with information about their data type (placed
between angle brackets).
</P>
<P>

The Bison construct <CODE>%type</CODE> is used for declaring nonterminal symbols,
just as <CODE>%token</CODE> is used for declaring token types.  We have not used
<CODE>%type</CODE> before because nonterminal symbols are normally declared
implicitly by the rules that define them.  But <CODE>exp</CODE> must be declared
explicitly so we can specify its value type.  See section <A HREF="bison_6.html#SEC52">Nonterminal Symbols</A>.
</P>
<P>

<A NAME="Mfcalc Rules"></A>
<HR SIZE="6">
<A NAME="SEC30"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC29"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC31"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC28"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.4.2 Grammar Rules for <CODE>mfcalc</CODE> </H3>
<!--docid::SEC30::-->
<P>

Here are the grammar rules for the multi-function calculator.
Most of them are copied directly from <CODE>calc</CODE>; three rules,
those which mention <CODE>VAR</CODE> or <CODE>FNCT</CODE>, are new.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>input:   /* empty */
        | input line
;

line:
          '\n'
        | exp '\n'   { printf (&quot;\t%.10g\n&quot;, $1); }
        | error '\n' { yyerrok;                  }
;

exp:      NUM                { $$ = $1;                         }
        | VAR                { $$ = $1-&gt;value.var;              }
        | VAR '=' exp        { $$ = $3; $1-&gt;value.var = $3;     }
        | FNCT '(' exp ')'   { $$ = (*($1-&gt;value.fnctptr))($3); }
        | exp '+' exp        { $$ = $1 + $3;                    }
        | exp '-' exp        { $$ = $1 - $3;                    }
        | exp '*' exp        { $$ = $1 * $3;                    }
        | exp '/' exp        { $$ = $1 / $3;                    }
        | '-' exp  %prec NEG { $$ = -$2;                        }
        | exp '^' exp        { $$ = pow ($1, $3);               }
        | '(' exp ')'        { $$ = $2;                         }
;
/* End of grammar */
%%
</FONT></pre></td></tr></table><P>

<A NAME="Mfcalc Symtab"></A>
<HR SIZE="6">
<A NAME="SEC31"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC30"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC32"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC28"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H3> 2.4.3 The <CODE>mfcalc</CODE> Symbol Table </H3>
<!--docid::SEC31::-->
<P>

The multi-function calculator requires a symbol table to keep track of the
names and meanings of variables and functions.  This doesn't affect the
grammar rules (except for the actions) or the Bison declarations, but it
requires some additional C functions for support.
</P>
<P>

The symbol table itself consists of a linked list of records.  Its
definition, which is kept in the header `<TT>calc.h</TT>', is as follows.  It
provides for either functions or variables to be placed in the table.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>/* Data type for links in the chain of symbols.      */
struct symrec
{
  char *name;  /* name of symbol                     */
  int type;    /* type of symbol: either VAR or FNCT */
  union {
    double var;           /* value of a VAR          */
    double (*fnctptr)();  /* value of a FNCT         */
  } value;
  struct symrec *next;    /* link field              */
};

typedef struct symrec symrec;

/* The symbol table: a chain of `struct symrec'.     */
extern symrec *sym_table;

symrec *putsym ();
symrec *getsym ();
</FONT></pre></td></tr></table><P>

The new version of <CODE>main</CODE> includes a call to <CODE>init_table</CODE>, a
function that initializes the symbol table.  Here it is, and
<CODE>init_table</CODE> as well:
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>#include &lt;stdio.h&gt;

main ()
{
  init_table ();
  yyparse ();
}

yyerror (s)  /* Called by yyparse on error */
     char *s;
{
  printf (&quot;%s\n&quot;, s);
}

struct init
{
  char *fname;
  double (*fnct)();
};

struct init arith_fncts[]
  = {
      &quot;sin&quot;, sin,
      &quot;cos&quot;, cos,
      &quot;atan&quot;, atan,
      &quot;ln&quot;, log,
      &quot;exp&quot;, exp,
      &quot;sqrt&quot;, sqrt,
      0, 0
    };

/* The symbol table: a chain of `struct symrec'.  */
symrec *sym_table = (symrec *)0;

init_table ()  /* puts arithmetic functions in table. */
{
  int i;
  symrec *ptr;
  for (i = 0; arith_fncts[i].fname != 0; i++)
    {
      ptr = putsym (arith_fncts[i].fname, FNCT);
      ptr-&gt;value.fnctptr = arith_fncts[i].fnct;
    }
}
</FONT></pre></td></tr></table><P>

By simply editing the initialization list and adding the necessary include
files, you can add additional functions to the calculator.
</P>
<P>

Two important functions allow look-up and installation of symbols in the
symbol table.  The function <CODE>putsym</CODE> is passed a name and the type
(<CODE>VAR</CODE> or <CODE>FNCT</CODE>) of the object to be installed.  The object is
linked to the front of the list, and a pointer to the object is returned.
The function <CODE>getsym</CODE> is passed the name of the symbol to look up.  If
found, a pointer to that symbol is returned; otherwise zero is returned.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>symrec *
putsym (sym_name,sym_type)
     char *sym_name;
     int sym_type;
{
  symrec *ptr;
  ptr = (symrec *) malloc (sizeof (symrec));
  ptr-&gt;name = (char *) malloc (strlen (sym_name) + 1);
  strcpy (ptr-&gt;name,sym_name);
  ptr-&gt;type = sym_type;
  ptr-&gt;value.var = 0; /* set value to 0 even if fctn.  */
  ptr-&gt;next = (struct symrec *)sym_table;
  sym_table = ptr;
  return ptr;
}

symrec *
getsym (sym_name)
     char *sym_name;
{
  symrec *ptr;
  for (ptr = sym_table; ptr != (symrec *) 0;
       ptr = (symrec *)ptr-&gt;next)
    if (strcmp (ptr-&gt;name,sym_name) == 0)
      return ptr;
  return 0;
}
</FONT></pre></td></tr></table><P>

The function <CODE>yylex</CODE> must now recognize variables, numeric values, and
the single-character arithmetic operators.  Strings of alphanumeric
characters with a leading nondigit are recognized as either variables or
functions depending on what the symbol table says about them.
</P>
<P>

The string is passed to <CODE>getsym</CODE> for look up in the symbol table.  If
the name appears in the table, a pointer to its location and its type
(<CODE>VAR</CODE> or <CODE>FNCT</CODE>) is returned to <CODE>yyparse</CODE>.  If it is not
already in the table, then it is installed as a <CODE>VAR</CODE> using
<CODE>putsym</CODE>.  Again, a pointer and its type (which must be <CODE>VAR</CODE>) is
returned to <CODE>yyparse</CODE>.</P>
<P>

No change is needed in the handling of numeric values and arithmetic
operators in <CODE>yylex</CODE>.
</P>
<P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><pre><FONT SIZE=-1>#include &lt;ctype.h&gt;
yylex ()
{
  int c;

  /* Ignore whitespace, get first nonwhite character.  */
  while ((c = getchar ()) == ' ' || c == '\t');

  if (c == EOF)
    return 0;

  /* Char starts a number =&gt; parse the number.         */
  if (c == '.' || isdigit (c))
    {
      ungetc (c, stdin);
      scanf (&quot;%lf&quot;, &amp;yylval.val);
      return NUM;
    }

  /* Char starts an identifier =&gt; read the name.       */
  if (isalpha (c))
    {
      symrec *s;
      static char *symbuf = 0;
      static int length = 0;
      int i;

      /* Initially make the buffer long enough
         for a 40-character symbol name.  */
      if (length == 0)
        length = 40, symbuf = (char *)malloc (length + 1);

      i = 0;
      do
        {
          /* If buffer is full, make it bigger.        */
          if (i == length)
            {
              length *= 2;
              symbuf = (char *)realloc (symbuf, length + 1);
            }
          /* Add this character to the buffer.         */
          symbuf[i++] = c;
          /* Get another character.                    */
          c = getchar ();
        }
      while (c != EOF &amp;&amp; isalnum (c));

      ungetc (c, stdin);
      symbuf[i] = '\0';

      s = getsym (symbuf);
      if (s == 0)
        s = putsym (symbuf, VAR);
      yylval.tptr = s;
      return s-&gt;type;
    }

  /* Any other character is a token by itself.        */
  return c;
}
</FONT></pre></td></tr></table><P>

This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install predefined
variables such as <CODE>pi</CODE> or <CODE>e</CODE> as well.
</P>
<P>

<A NAME="Exercises"></A>
<HR SIZE="6">
<A NAME="SEC32"></A>
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC31"> &lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> Up </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H2> 2.5 Exercises </H2>
<!--docid::SEC32::-->
<P>

<OL>
<LI>
Add some new functions from `<TT>math.h</TT>' to the initialization list.
<P>

</P>
<LI>
Add another array that contains constants and their values.  Then
modify <CODE>init_table</CODE> to add these constants to the symbol table.
It will be easiest to give the constants type <CODE>VAR</CODE>.
<P>

</P>
<LI>
Make the program report an error if the user refers to an
uninitialized variable in any way except to store a value in it.
</OL>
<P>

<A NAME="Grammar File"></A>
<HR SIZE="6">
<TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0>
<TR><TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_5.html#SEC14"> &lt;&lt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_6.html#SEC33"> &gt;&gt; </A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT"> &nbsp; <TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison.html#SEC_Top">Top</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_toc.html#SEC_Contents">Contents</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_15.html#SEC92">Index</A>]</TD>
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bison_abt.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<BR>
<FONT SIZE="-1">
This document was generated
by <I>Frank B. Brokken</I> on <I>January, 28 2005</I>
using <A HREF="http://texi2html.cvshome.org"><I>texi2html</I></A>
</FONT>

</BODY>
</HTML>