Implement C++ program for expression conversion as infix to postfix and its evaluation using stack based on given conditions i. Operands and operator, both must be single character. ii. Input Postfix expression must be in a desired format. iii. Only '+', '-', '*' and '/ ' operators are expected.

#include<iostream>
#include<string.h>
using namespace std;
class stackop
{  char  st[20],st1[20];  int top,top1,ss[10],e1,e2,e3,flag;
   public:
    void input();
    void push(char a);
    void pop();
    int pri(char b);
    void eval();
    void push1(int d);
    void pop1();
 };
 int stackop::pri(char b)
 {   if(b=='-')
       return 1;
        if(b=='+')
       return 2;
        if(b=='/')
       return 3;
        if(b=='*')
       return 4;
 }
       
void stackop::input()
{   char ch[20];   top=-1;   int f=1,l,i=0,j=0;   flag=0;
     cout<<"\n enter the expression\n";
     cin>>ch;
     l=strlen(ch);
    
    
  
     while(i<l)
     {     f=1;
         if(isalpha(ch[i]))
           {    cout<<ch[i]; st1[j]=ch[i]; j++;  flag=1;   }
           if(isdigit(ch[i]))
           {  cout<<ch[i];  st1[j]=ch[i]; j++;   }  
         if(ch[i]=='(')
           {    push(ch[i]); }
          if(ch[i]==')')
           {
             while(st[top]!='(')
              {   cout<<st[top]; st1[j]=st[top]; j++; pop();} 
                    pop();
           }          
           if((ch[i]=='+')||(ch[i]=='-')||(ch[i]=='*')||(ch[i]=='/'))
           {    while(f==1)
                {
                  if(top==-1)
                  {   push(ch[i]); f=0;   } 
                  else
                  {   if(st[top]=='(')
                      {   push(ch[i]);  f=0; }
                      else
                      {  
                             if((pri(ch[i]))>(pri(st[top])))
                             {         push(ch[i]); f=0;       }
                             else
                             {      cout<<st[top]; st1[j]=st[top]; j++;  pop();       }
                         
                      }      
                  }      
                }
           }
      i++;
     }
    while(top!=-1)
     { cout<<st[top]; st1[j]=st[top]; j++; pop();   }  cout<<"\n";
     cout<<st1;
}

void stackop::eval()
{  int j=0;  top1=-1;
  if(flag==0)
{
 while(j<strlen(st1))
{
    if(st1[j]=='1')
      push1(1);
if(st1[j]=='2')
      push1(2);
if(st1[j]=='3')
      push1(3);
if(st1[j]=='4')
      push1(4);
if(st1[j]=='5')
      push1(5);
if(st1[j]=='6')
      push1(6);
if(st1[j]=='7')
      push1(7);
if(st1[j]=='8')
      push1(8);
if(st1[j]=='9')
      push1(9);
if(st1[j]=='0')
      push1(0);
if(st1[j]=='+')
{      e1=ss[top1]; pop1();
       e2=ss[top1]; pop1();
       e3=e2+e1;
       push1(e3);
}
if(st1[j]=='-')
 {      e1=ss[top1]; pop1();
       e2=ss[top1]; pop1();
       e3=e2-e1;
       push1(e3);
}
   
if(st1[j]=='*')
{      e1=ss[top1]; pop1();
       e2=ss[top1]; pop1();
       e3=e2*e1;
       push1(e3);
}

if(st1[j]=='/')
{      e1=ss[top1]; pop1();
       e2=ss[top1]; pop1();
       e3=e2/e1;
       push1(e3);
} j++;
}
cout<<"\n evaluated value:";
cout<<ss[0];
}
else
{  cout<<"\n cannot evaluate given input";
}
}  

void stackop::push(char a)
{    top++; st[top]=a;   }
void stackop::pop()
{  top--;         }
void stackop::push1(int d)
{    top1++; ss[top1]=d;   }
void stackop::pop1()
{  top1--;         }

int main()
{  stackop s;
   s.input();
   s.eval();
   cout<<"\n";
   return 0;
}