Question

Write a java program that converts a prefix expression to a postfix expression.

Input

The input file "Prefix.in" contains a series of error-free, simple arithmetic expressions in prefix notation. There is one expression per line. The expressions are made up of two types of symbols: (1) single-letter operands and (2) the operators +, -, *, and /. There may be zero, one, or more blanks between symbols.

Output

All output should be written to the output file, "Postfix.out". Each prefix expression from the input should be echo-printed, with an appropriate label. After your program converts the input expression to its postfix equivalent, the postfix expression should be printed to the output file, with an appropriate label.

Sample:

Prefix:         + * A B / C D

Postfix:        A B * C D / +

Answer 1

Program:

import java.io.*;
import java.util.Stack;
import java.util.Scanner;

//class PrefixToPostfix
public class PrefixToPostfix
{
   //method to check operator
   public static boolean isOperator(char c)
   {
       switch(c)
       {
           case '+': case '-': case '/': case '*':
               return true;
       }
       return false;
   }
  
   //method to convert from prefix to postfix expression
   public static String conversion(String s)
   {
       //create a stack
       Stack<String> stk = new Stack<String>();
      
       for(int i = s.length()-1; i>=0; i--)
       {
           //extract ith character
           char ch = s.charAt(i);
          
           //continue is blank space
           if(ch==' ') continue;
          
           //check for operator
           if(isOperator(ch))
           {
               //pop two operands from the stack
               String opr1 = stk.pop();
               String opr2 = stk.pop();
              
               String t = opr1 + " " + opr2 + " " + ch;
              
               //push to the stack
               stk.push(t);
           }
           //for operand
           else
           {
               stk.push("" + ch);
           }
       }
      
       String post = stk.pop();
      
       //return postfix expression
       return post;
   }
  
   //main method
   public static void main (String[] args) throws IOException
   {
       //create instance of Scanner class
       Scanner sc = new Scanner(new File("Prefix.in"));
      
       //create instance of FileWriter class
       FileWriter fw = new FileWriter(new File("Postfix.out"));
      
       //read the file until end of file
       while(sc.hasNext())
       {
           //read a line from the file
           String pre = sc.nextLine();
          
           System.out.println("Prefix expression: " + pre);
          
           //convert from prefix to postfix
           String post = conversion(pre);
          
           System.out.println("Postfix expression: " + post);
          
           //write to the file
           fw.write(post + "\n");
       }
      
       //close the files
       sc.close();
       fw.close();
   }
}

Output: