: 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) )

: 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.
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 Prefix to Postfix:

  • 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.
// 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;
}
Output:
Prefix : *-A/BC-/AKL

- 1ny9qmuwb862pamtbm2j - Postfix to Prefix Conversion &#8211; GeeksforGeeks

If you like and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.




Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.




Source link

LEAVE A REPLY

Please enter your comment!
Please enter your name here