Program for Recursive Descent Parser

#include<iostream>
#include<string.h>
#include<ctype.h>
#include<stdio.h>

using namespace std;


class recursive
{
public:
char in[20];
int j,error;
void E();
void F();

};

main()
{ recursive obj;
  obj.j=0;
  obj.error=0;
  cout<<"Enter an arithmetic expression   :  ";
                cin>>obj.in;
  obj.E();

if(obj.in[obj.j]=='$' && obj.in[obj.j+1]=='\0')
       cout<<"\n String Accepted. . !!\n";
 else
       cout<<"\n String Rejected. . !!\n";
}

void recursive::E()
{
  if(isalpha(in[j]))
 {  j++;
    F();
 }
}


void recursive::F()
{ if(in[j]=='+' && in[j+1]!='$')
 { j++;
   if(isalpha(in[j]))
   {
     j++;
     F();
   }
 }

}


Explaination :-

Grammar - E -> iF
                  F -> +iF / ∈ ∈ ∈


 i is any alphabet from a to z.
Accepted Strings - i+i$ , i+i+i$ , . .etc
Rejected Strings - i++, (i), ++i . . etc

In recursive descent parser, for every variable, we are gonna write a function. In function E(), if first symbol is alphabet then increment pointer to next location and call F().
       In function F(), if first symbol is +(plus) then increment pointer to next location and if next symbol is again alphabet then increment pointer and call F().