06 Yacc Directives
Directives¶
Along with the tokens
The directives written later have higher precedence.
Question 1¶
Write a simple calculator which can gives result for an addition/subtraction expression (without ambiguity).
06_1.l¶
same as Basic Calculator
06_1.y¶
%{
int yylex(void);
void yyerror(char *);
#include <stdio.h>
#include <stdlib.h>
%}
%token INTEGER PLUS MINUS NL
%left PLUS MINUS /* 1 */
%%
program:
expr NL {printf("%d\n", $1); exit(0);}
;
expr:
INTEGER {$$=$1;}
| expr PLUS expr {$$=$1+$3;}
| expr MINUS expr {$$=$1-$3;}
;
%%
void yyerror(char *s)
{
printf("%s\n", s);
}
int main()
{
yyparse();
}
Output¶
Question 2¶
The program should keep going on until the user exits using Ctrl-D
06_2.l¶
same as Basic Calculator
06_2.y¶
%{
int yylex(void);
void yyerror(char *);
#include <stdio.h>
#include <stdlib.h>
%}
%token INTEGER PLUS MINUS NL
%left PLUS MINUS /* 1 */
%%
program:
program expr NL {printf("%d\n", $2);} /* 2 */
|
;
expr:
INTEGER {$$=$1;}
| expr PLUS expr {$$=$1+$3;}
| expr MINUS expr {$$=$1-$3;}
;
%%
void yyerror(char *s)
{
printf("%s\n", s);
}
int main()
{
yyparse();
}
Output¶
Question 3¶
Extend the calculator to incorporate some new functionality. New features include arithmetic operators * and / that can multiply and divide integers respectively. Parentheses may be used to over-ride operator precedence. Note * and / operators have higher precedence over + and – operators. Also note that * and / are left associative. Ensure this using directive in YACC.
06_3.l¶
%{
#include "y.tab.h"
extern int yylval;
%}
%%
[0-9]+ {
yylval = atoi(yytext);
return INTEGER;
}
"+" return PLUS;
"-" return MINUS;
"*" return MUL; /* 1 */
"/" return DIV; /* 2 */
"^" return POW; /* 3 */
"\n" return NL; /* 4 */
[ \t] ;
. printf("Invalid");
%%
06_3.y¶
%{
int yylex(void);
void yyerror(char *);
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
%}
%token INTEGER PLUS MINUS NL
%left PLUS MINUS
%left MUL DIV
%right POW /* 1 */
%%
program:
program expr NL {printf("%d\n", $2);}
|
;
expr:
INTEGER {$$=$1;}
| expr PLUS expr {$$=$1+$3;}
| expr MINUS expr {$$=$1-$3;}
| expr MUL expr {$$=$1*$3;} /* 2 */
| expr DIV expr {$$=$1/$3;} /* 3 */
| expr POW expr {$$=pow($1, $3);} /* 4 */
;
%%
void yyerror(char *s)
{
printf("%s\n", s);
}
int main()
{
yyparse();
}
Output¶
Question 4¶
Modify the calculator application so that it works for floating point values also.
Question 5¶
Modify the grammar to allow single-character variables to be specified in assignment statements. The following illustrates sample input and calculator output:
user: 3 * (4 + 5)
calc: 27
user: x = 3 * (4 + 5)
user: y = 5
user: x
calc: 27
user: y
calc: 5
user: x + 2*y
calc: 37
06_5.l¶
%{
#include "y.tab.h"
extern int yylval;
%}
%%
[a-z] {
yylval = *yytext - 'a';
return VARIABLE;
}
[0-9]+ {
yylval = atoi(yytext);
return INTEGER;
}
[-+()=*/\n] { return *yytext; }
[ \t] ;
. yyerror("invalid character");
%%
int yywrap(void) {
return 1;
}
06_5.y¶
%{
#include<stdio.h>
int flag=0;
int yylex(void);
int sym[26];
%}
%token INTEGER VARIABLE
%left '+' '-'
%left '*' '/'
%%
program:
program statement '\n'
|
;
statement:
expr { printf("%d\n", $1); }
| VARIABLE '=' expr { sym[$1] = $3; }
;
expr:
INTEGER
| VARIABLE { $$ = sym[$1]; }
| expr '+' expr { $$ = $1 + $3; }
| expr '-' expr { $$ = $1 - $3; }
| expr '*' expr { $$ = $1 * $3; }
| expr '/' expr { $$ = $1 / $3; }
| '(' expr ')' { $$ = $2; }
;
%%
void main()
{
printf("\nEnter Any Arithmetic Expression which can have operations Addition, Subtraction, Multiplication, Division, Modulus and Round brackets:\n");
yyparse();
if(flag==0)
printf("\nEntered arithmetic expression is Valid\n\n");
}
void yyerror()
{
printf("\nEntered arithmetic expression is Invalid\n\n");
flag=1;
}