external/bsd/flex/dist/examples/manual/expr.y

   1 /*
   2  * expr.y : A simple yacc expression parser
   3  *          Based on the Bison manual example.
   4  */
   5
   6 %{
   7 #include <stdio.h>
   8 #include <math.h>
   9
  10 %}
  11
  12 %union {
  13    float val;
  14 }
  15
  16 %token NUMBER
  17 %token PLUS MINUS MULT DIV EXPON
  18 %token EOL
  19 %token LB RB
  20
  21 %left  MINUS PLUS
  22 %left  MULT DIV
  23 %right EXPON
  24
  25 %type  <val> exp NUMBER
  26
  27 %%
  28 input   :
  29         | input line
  30         ;
  31
  32 line    : EOL
  33         | exp EOL { printf("%g\n",$1);}
  34
  35 exp     : NUMBER                 { $$ = $1;        }
  36         | exp PLUS  exp          { $$ = $1 + $3;   }
  37         | exp MINUS exp          { $$ = $1 - $3;   }
  38         | exp MULT  exp          { $$ = $1 * $3;   }
  39         | exp DIV   exp          { $$ = $1 / $3;   }
  40         | MINUS  exp %prec MINUS { $$ = -$2;       }
  41         | exp EXPON exp          { $$ = pow($1,$3);}
  42         | LB exp RB                      { $$ = $2;        }
  43         ;
  44
  45 %%
  46
  47 yyerror(char *message)
  48 {
  49   printf("%s\n",message);
  50 }
  51
  52 int main(int argc, char *argv[])
  53 {
  54   yyparse();
  55   return(0);
  56 }
  57
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