.SH
Appendix A:  A Simple Example
.PP
This example gives the complete Yacc specification for a small desk calculator;
the desk calculator has 26 registers, labeled a through z, and accepts
arithmetic expressions made up of the operators +, \-, *, /,
% (mod operator), & (bitwise and), | (bitwise or), and assignment.
If an expression is an assignment at the top level, the value is not
printed; otherwise it is.
As in C, an integer which begins with 0 (zero) is assumed to be octal;
otherwise, it is assumed to be decimal.
.PP
As an example of a Yacc specification, the desk calculator
does a reasonable job of showing the way that precedences and ambiguities
are used, as well as showing how simple error recovery operates.
The major oversimplifications are that the
lexical analysis phase is much simpler than for most applications, and the
output is produced immediately, line by line.
Note the way that decimal and octal integers are read in by the grammar rules;
frequently, this job is better done by the lexical analyzer.
.LD


%token DIGIT LETTER	/* these are token names */
%left \'|\'				/* declarations of operator precedences */
%left \'&\'
%left \'+\' \'\-\'
%left \'*\' \'/\' \'%\'
%left UMINUS			/* supplies precedence for unary minus */
%{					/* declarations used by the actions */
	int base;
	int regs[26];
%}

%%   /* beginning of rules section */

list :		/* list is the start symbol */
	|	/* empty */
	list stat \'\en\' |
	list error \'\en\' =
	{
		yyerrok ;
	} ;

stat :
	expr =
	{
		printf("%d\en", $1) ;
	} |
	LETTER \'=\' expr =
	{
		regs[$1] = $3 ;
	} ;

expr :
	\'(\' expr \')\' =
	{
		$$ = $2 ;
	} |
	expr \'+\' expr =
	{
		$$ = $1 + $3 ;
	} |
	expr \'\-\' expr =
	{
		$$ = $1 \- $3 ;
	} |
	expr \'*\' expr =
	{
		$$ = $1 * $3 ;
	} |
	expr \'/\' expr =
	{
		$$ = $1 / $3 ;
	} |
	expr \'%\' expr =
	{
		$$ = $1 % $3 ;
	} |
	expr \'&\' expr
	{
		$$ = $1 & $3 ;
	} |
	expr \'|\' expr
	{
		$$ = $1 | $3 ;
	} |
	\'\-\' expr  %prec UMINUS
	{
		$$ = \- $2 ;
	} |
	LETTER
	{
		$$ = regs[$1] ;
	} |
	number ;

number :
	DIGIT =
	{
		$$ = $1 ;
		base = 10 ;
		if( $1 == 0 )
			base = 8 ;
	} |
	number DIGIT =
	{
		$$ = base * $1 + $2 ;
	} ;

%%		/* start of programs */

yylex( ) /* lexaical analysis routine */
{
	/* returns LETTER for a lower case letter, yylval = 0 through 25 */
	/* return DIGIT for a digit, yylval = 0 through 9 */
	/* all other characters are returned immediately */

	int c ;

	while( (c=getchar( )) == \' \' )
		;
	if( c >= \'a\' && c <= \'z\' ) {
		yylval = c \- \'a\' ;
		return( LETTER ) ;
	}
	if( c >= \'0\' && c <= \'9\' ) {
		yylval = c \- \'0\' ;
		return( DIGIT ) ;
	}
	return( c ) ;
}
.DE
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