bis1.y

一个flex和bison用法入门的很好的例子。 按照词法分析,语法分析,语义分析的顺序来熟悉flex和bison这两个有力的工具。最后给出了一个用flex做词法分析用bison做语法分析设计的科学计算

/* Reverse polish notation calculator. */
%{
#include "calc.h"  /* Contains definition of `symrec'        */
typedef union {
double     val;  /* For returning numbers.                   */
symrec  *tptr;   /* For returning symbol-table pointers      */
} YYSTYPE;
#include <math.h>
#include <ctype.h>
#include "lex.c"

%}

%token <val>  NUM        /* Simple double precision number   */
%token <tptr> VAR FNCT   /* Variable and Function            */
%type  <val>  exp
%token EQ
%token ADD
%token SUB
%token MUL
%token DIV
%token ENDLINE
%token LBRO
%token RBRO
%right EQ
%left SUB ADD
%left MUL DIV
%left NEG     /* negation--unary minus */
%right POW    /* exponentiation        */

%% /* Grammar rules and actions follow */
input:    /* empty */
        | input line
;
line:     ENDLINE
        | exp ENDLINE  { printf ("The result is:\t%.10g\n", $1); }
	| error ENDLINE { yyerrok;                  }

;
exp:      NUM                { $$ = $1;         }
	| VAR                { $$ = $1->value.var; if(strcmp($1->name,"quit")==0){printf("\n********Bye-bye************\n");exit(0);}             }
        | VAR EQ exp        { $$ = $3; $1->value.var = $3;     }
        | FNCT LBRO exp RBRO   { $$ = (*($1->value.fnctptr))($3); }

        | exp ADD exp        { $$ = $1 + $3;    }
        | exp SUB exp        { $$ = $1 - $3;    }
        | exp MUL exp        { $$ = $1 * $3;    }
        | exp DIV exp        { if($3==0){printf("The Zero flowover\n");$$=0;} else $$ = $1 / $3;    }
        | SUB exp  %prec NEG { $$ = -$2;        }
        | exp POW exp        { $$ = pow ($1, $3); }
        | LBRO exp RBRO        { $$ = $2;         }

%%



yyerror (s)  /* Called by yyparse on error */
     char *s;
{
  printf ("%s\n", s);
  printf ("Now word is: %s\n", yylval);
}


struct init
{
  char *fname;
  double (*fnct)();
};
struct init arith_fncts[]
  = {
      "sin", sin,
      "cos", cos,
      "atan", atan,
      "ln", log,
      "exp", exp,
      "sqrt", 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->value.fnctptr = arith_fncts[i].fnct;
    }
}


symrec *
putsym (sym_name,sym_type)
     char *sym_name;
     int sym_type;
{
  symrec *ptr;
  ptr = (symrec *) malloc (sizeof (symrec));
  ptr->name = (char *) malloc (strlen (sym_name) + 1);
  strcpy (ptr->name,sym_name);
  ptr->type = sym_type;
  ptr->value.var = 0; /* set value to 0 even if fctn.  */
  ptr->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->next)
    if (strcmp (ptr->name,sym_name) == 0)
      return ptr;
  return 0;
}



main ()
{
  init_table ();
  printf("\n******Begin   you can type 'quit' to exit anytime you want***********\n");
  yyparse ();
}