Calculator using YACC program

Here is the Yacc program implementing a simple calculator:

%{
#include<stdio.h>
#include<ctype.h>
%}
%token num
%left '+''-'
%left '*''/'
%right '^'
%%
s:e'\n'{printf("%d",$1);}
e:    e '+' e{$$=$1+$3;}
 |e '-' e{$$=$1-$3;}
 |e '*' e{$$=$1*$3;}
 |e '/' e{$$=$1/$3;}
 |e '^' e {
  int i,j=$1;
  for(i=1;i<$3;i++)
  {
  j=j*$1;
  $$=j;
  }
  }
 |'('e')'{$$=$2;}
 |num1;
num1:num1 num{$$ = $1*10 + $2;}
 |num
 ;
%%
yylex()
{
int c;
c=getchar();
if(isdigit(c))
{
yylval=c-'0';
return num;
}
return c;
}
int main()
{
yyparse();
return 1;
}
int yyerror()
{
return 1;
}
int yywrap()
{
return 1;
}



Illustration:

Let the Input be 2+5

Moving from first definition s:e'\n'
s  --> move to DEF(e)
e: e '+' e is selected. Evaluate first 'e' ie e-->num1
num1 extracts the token (i.e, number here 2 is extracted and stored in $1)
Then '+' is extracted and stored in $2.
Then move to e--> num1, thus 5 is  extracted and stored  in $3.
Now add them and store result in $$.

Go back to S: e'\n' and print result (which is 'e' here..!!)


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Posted in: Language processing programs , YACC