Question

Write a program in c++ to convert an expression written in infix notation to be converted to postfix notation. The program must do the following:

a. Read a string of characters representing an expression in infix notation. The '$' is to be added at the end of the string to mark its ending. Each character is a letter, digit, +,-,*, or /. If a character is any other character an error must be signaled and the program is terminated

b. Use stacks to convert an expression in infix notation into postfix notation

c. Print out the postfix notation

iii. 6*w/5+z/8+9-x$

Answer 1

//###################### PGM START ###############################

#include<iostream>
#include<stack>

using namespace std;

//get the precedence of the character
int getPrecedence(char c){
   if(c=='*' || c=='/')
       return 2;
   else if(c=='+' || c=='-'){
       return 1;
   }
}

//function to convert an infix expression to postfix
void convertInfixToPostfix(string s){
  
   stack<char> stk;
   stk.push('$');
  
   int n=s.length();
   string result;
  
   //repeat untill length of the string
   for(int i=0;i<n;i++){
      
       //if the character is alphabet or digit add it to final result
       if( isalpha(s[i])||(isdigit(s[i])))
          result+=s[i];
         
       //if the character is operator check the predecne of the operator
       else if(s[i]=='+' ||s[i]=='-' || s[i]=='*' || s[i]=='/'){
          
           //if the precedence of operator is less than that of one in stack then pop operator in stack
           while(stk.top() != '$' && getPrecedence(s[i]) <= getPrecedence(stk.top()))
            {
                char ch = stk.top();
                stk.pop();
                result += ch;
            }
            //fincally push the current oprator to stack
            stk.push(s[i]);
       }
       //if the input character is $ then print the result
       else if(s[i]=='$'){
           break;
       }
       //for all other characters print ivlaid
       else{
           cout<<"Invalid input\n";
           return;
       }
   }
   while(stk.top() != '$')
    {
        char ch = stk.top();
        stk.pop();
        result += ch;
    }
    
   cout<<"Postfix: "<<result<<"\n";
      
}
int main(){
  
   //string exp = "3+4*5/6$";
   //string exp="A+B+C+D$";
  
   string exp;
   cout<<"Enter the infix expression: ";
   getline(cin,exp);
    convertInfixToPostfix(exp);
  
   return 0;
}

//######################## PGM END ###################################

OUTPUT
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