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Prefix to Postfix Conversion

Prefix : An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2).
Example : *+AB-CD (Infix : (A+B) * (C-D) )

Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator).
Example : AB+CD-* (Infix : (A+B * (C-D) )

Given a Prefix expression, convert it into a Postfix expression.
Conversion of Prefix expression directly to Postfix without going through the process of converting them first to Infix and then to Postfix is much better in terms of computation and better understanding the expression (Computers evaluate using Postfix expression).

Examples:

Input :  Prefix :  *+AB-CD
Output : Postfix : AB+CD-*
Explanation : Prefix to Infix :  (A+B) * (C-D)
              Infix to Postfix :  AB+CD-*

Input :  Prefix :  *-A/BC-/AKL
Output : Postfix : ABC/-AK/L-*
Explanation : Prefix to Infix :  A-(B/C)*(A/K)-L
              Infix to Postfix : ABC/-AK/L-* 

Algorithm for Prefix to Postfix:

  • Read the Prefix expression in reverse order (from right to left)
  • If the symbol is an operand, then push it onto the Stack
  • If the symbol is an operator, then pop two operands from the Stack
    Create a string by concatenating the two operands and the operator after them.
    string = operand1 + operand2 + operator
    And push the resultant string back to Stack
  • Repeat the above steps until end of Prefix expression.

C++

// CPP Program to convert prefix to postfix
#include <iostream>
#include <stack>
using namespace std;
  
// funtion to check if character is operator or not
bool isOperator(char x) {
  switch (x) {
  case '+':
  case '-':
  case '/':
  case '*':
    return true;
  }
  return false;
}
  
// Convert prefix to Postfix expression
string preToPost(string pre_exp) {
  
  stack<string> s;
  
  // length of expression
  int length = pre_exp.size();
  
  // reading from right to left
  for (int i = length - 1; i >= 0; i--) {
  
    // check if symbol is operator
    if (isOperator(pre_exp[i])) {
  
      // pop two operands from stack
      string op1 = s.top(); s.pop();
      string op2 = s.top(); s.pop();
  
      // concat the operands and operator
      string temp = op1 + op2 + pre_exp[i];
  
      // Push string temp back to stack
      s.push(temp);
    }
  
    // if symbol is an operand
    else {
  
      // push the operand to the stack
      s.push(string(1, pre_exp[i]));
    }
  }
  
  // stack contains only the Postfix expression
  return s.top();
}
  
// Driver Code
int main() {
  string pre_exp = "*-A/BC-/AKL";
  cout << "Postfix : " << preToPost(pre_exp);
  return 0;
}

Java

// JavaProgram to convert prefix to postfix
import java.util.*;
  
class GFG
{
  
// funtion to check if character 
// is operator or not
static boolean isOperator(char x) 
{
    switch (x) 
    {
        case '+':
        case '-':
        case '/':
        case '*':
        return true;
    }
    return false;
}
  
// Convert prefix to Postfix expression
static String preToPost(String pre_exp)
{
  
    Stack<String> s= new Stack<String>();
  
    // length of expression
    int length = pre_exp.length();
  
    // reading from right to left
    for (int i = length - 1; i >= 0; i--) 
    {
  
        // check if symbol is operator
        if (isOperator(pre_exp.charAt(i))) 
        {
  
            // pop two operands from stack
            String op1 = s.peek(); s.pop();
            String op2 = s.peek(); s.pop();
  
            // concat the operands and operator
            String temp = op1 + op2 + pre_exp.charAt(i);
  
            // Push String temp back to stack
            s.push(temp);
        }
  
        // if symbol is an operand
        else
        {
            // push the operand to the stack
            s.push( pre_exp.charAt(i)+"");
        }
    }
  
    // stack contains only the Postfix expression
    return s.peek();
}
  
// Driver Code
public static void main(String args[]) 
{
    String pre_exp = "*-A/BC-/AKL";
    System.out.println("Postfix : " + preToPost(pre_exp));
}
}
  
// This code is contributed by Arnab Kundu

Output:

Postfix : ABC/-AK/L-*


This article is attributed to GeeksforGeeks.org

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