ch3-04.y

%{
#include "ch3hdr.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
extern FILE *yyin;
int yyerror(char *);
int yylex();
%}

%union {
  double dval;
  struct symtab *symp;
}
%token <symp> NAME
%token <dval> NUMBER
%left '-' '+'
%left '*' '/'
%nonassoc UMINUS

%type <dval> expression
%%
statement_list:  statement '\n'
  |  statement_list statement '\n'
  ;

statement:  NAME '=' expression  { $1->value = $3; }
  |  expression    { printf("= %g\n", $1); }
  ;

expression:  expression '+' expression { $$ = $1 + $3; }
  |  expression '-' expression { $$ = $1 - $3; }
  |  expression '*' expression { $$ = $1 * $3; }
  |  expression '/' expression
        {  if($3 == 0.0)
            yyerror("divide by zero");
          else
            $$ = $1 / $3;
        }
  |  '-' expression %prec UMINUS  { $$ = -$2; }
  |  '(' expression ')'  { $$ = $2; }
  |  NUMBER
  |  NAME      { $$ = $1->value; }
  ;
%%
/* look up a symbol table entry, add if not present */
struct symtab *symlook(char *s)
{
  char *p;
  struct symtab *sp;
  
  for(sp = symtab; sp < &symtab[NSYMS]; sp++) {
    /* is it already here? */
    if(sp->name && !strcmp(sp->name, s))
      return sp;
    
    /* is it free */
    if(!sp->name) {
      sp->name = strdup(s);
      return sp;
    }
    /* otherwise continue to next */
  }
  yyerror("Too many symbols");
  exit(1);  /* cannot continue */
} /* symlook */

int main()
{
  do {
    yyparse();
  } while (!feof(yyin));
  return 0;
}

int yyerror(char *s)
{
  fprintf(stderr, "error: %s\n", s);
  return 0;
}