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) )
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) )
Given a Postfix expression, convert it into a Prefix expression.
Conversion of Postfix expression directly to Prefix without going through the process of converting them first to Infix and then to Prefix is much better in terms of computation and better understanding the expression (Computers evaluate using Postfix expression).
Examples:
Input : Postfix : AB+CD-*
Output : Prefix : *+AB-CD
Explanation : Postfix to Infix : (A+B) * (C-D)
Infix to Prefix : *+AB-CD
Input : Postfix : ABC/-AK/L-*
Output : Prefix : *-A/BC-/AKL
Explanation : Postfix to Infix : ((A-(B/C))*((A/K)-L))
Infix to Prefix : *-A/BC-/AKL
Algorithm for Postfix to Prefix:
- Read the Postfix expression from left to right
- 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 before them.
string = operator + operand2 + operand1
And push the resultant string back to Stack- Repeat the above steps until end of Prefix expression.
C++
// CPP Program to convert postfix to prefix #include <iostream> #include <stack> using namespace std; // function to check if character is operator or not bool isOperator(char x) { switch (x) { case '+': case '-': case '/': case '*': return true; } return false; } // Convert postfix to Prefix expression string postToPre(string post_exp) { stack<string> s; // length of expression int length = post_exp.size(); // reading from right to left for (int i = 0; i < length; i++) { // check if symbol is operator if (isOperator(post_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 = post_exp[i] + op2 + op1; // 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, post_exp[i])); } } // stack[0] contains the Prefix expression return s.top(); } // Driver Code int main() { string post_exp = "ABC/-AK/L-*"; cout << "Prefix : " << postToPre(post_exp); return 0; } |
Java
// Java Program to convert postfix to prefix import java.util.*; class GFG { // function to check if character // is operator or not static boolean isOperator(char x) { switch (x) { case '+': case '-': case '/': case '*': return true; } return false; } // Convert postfix to Prefix expression static String postToPre(String post_exp) { Stack<String> s = new Stack<String>(); // length of expression int length = post_exp.length(); // reading from right to left for (int i = 0; i < length; i++) { // check if symbol is operator if (isOperator(post_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 = post_exp.charAt(i) + op2 + op1; // Push String temp back to stack s.push(temp); } // if symbol is an operand else { // push the operand to the stack s.push(post_exp.charAt(i) + ""); } } // stack[0] contains the Prefix expression return s.peek(); } // Driver Code public static void main(String args[]) { String post_exp = "ABC/-AK/L-*"; System.out.println("Prefix : " + postToPre(post_exp)); } } // This code is contributed by Arnab Kundu |
C#
// C# Program to convert postfix to prefix using System; using System.Collections; class GFG { // function to check if character // is operator or not static Boolean isOperator(char x) { switch (x) { case '+': case '-': case '/': case '*': return true; } return false; } // Convert postfix to Prefix expression static String postToPre(String post_exp) { Stack s = new Stack(); // length of expression int length = post_exp.Length; // reading from right to left for (int i = 0; i < length; i++) { // check if symbol is operator if (isOperator(post_exp[i])) { // Pop two operands from stack String op1 = (String)s.Peek(); s.Pop(); String op2 = (String)s.Peek(); s.Pop(); // concat the operands and operator String temp = post_exp[i] + op2 + op1; // Push String temp back to stack s.Push(temp); } // if symbol is an operand else { // Push the operand to the stack s.Push(post_exp[i] + ""); } } // stack[0] contains the Prefix expression return (String)s.Peek(); } // Driver Code public static void Main(String[] args) { String post_exp = "ABC/-AK/L-*"; Console.WriteLine("Prefix : " + postToPre(post_exp)); } } // This code is contributed by Arnab Kundu |
Output:
Prefix : *-A/BC-/AKL
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