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; } |
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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 |
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C#
// C# Program to convert prefix to postfix using System; using System.Collections.Generic; class GFG { // funtion to check if character // is operator or not static bool 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[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[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[i]+""); } } // stack contains only the Postfix expression return s.Peek(); } // Driver Code public static void Main(String []args) { String pre_exp = "*-A/BC-/AKL"; Console.WriteLine("Postfix : " + preToPost(pre_exp)); } } /* This code contributed by PrinciRaj1992 */ |
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Python 3
# Write Python3 code here # -*- coding: utf-8 -*- # Example Input s = "*-A/BC-/AKL" # Stack for storing operands stack = [] operators = set(['+', '-', '*', '/', '^']) # Reversing the order s = s[::-1] # iterating through individual tokens for i in s: # if token is operator if i in operators: # pop 2 elements from stack a = stack.pop() b = stack.pop() # concatenate them as operand1 + # operand2 + operator temp = a+b+i stack.append(temp) # else if operand else: stack.append(i) # printing final output print(*stack) |
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Output:
Postfix : ABC/-AK/L-*
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