calc.y

含有BACKTARACKING的編譯軟體

%{
# include <stdio.h>
# include <ctype.h>

int regs[26];
int base;

%}

%start list

%token DIGIT LETTER

%left '|'
%left '&'
%left '+' '-'
%left '*' '/' '%'
%left UMINUS   /* supplies precedence for unary minus */

%% /* beginning of rules section */

list  :  /* empty */
      |  list stat '\n'
      |  list error '\n'
            {  yyerrok ; }
      ;

stat  :  expr
            {  printf("%d\n",$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
         {  $$ = $1; base = ($1==0) ? 8 : 10; }
      |  number DIGIT
         {  $$ = base * $1 + $2; }
      ;

%% /* start of programs */

main ()

{
    while(!feof(stdin)) {
      yyparse();
   }
}

yyerror(char *s)
{  
   fprintf(stderr, "%s\n", s);
}


yylex() {   /* lexical 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()) == ' ' )   { /* skip blanks */ }

      /* c is now nonblank */

      if( islower( c )) {
         yylval = c - 'a';
         return ( LETTER );
         }
      if( isdigit( c )) {
         yylval = c - '0';
         return ( DIGIT );
         }
      return( c );
      }