Using the algorithm evaluatePostfix, given in the photo below, Write a class in

Question

Using the algorithm evaluatePostfix, given in the photo below, Write a class in java that evaluate each of thefollowing postfix expressions. Assume that a = 2, b = 3, c = 4, d = 5, and e = 6.

a.) a b + c * d -

5.18 The evaluation algorithm follows direci Algorithm evaluatePostfix(postfix) siO valueStack a new empty stack while (postfix has characters left to parse) nextCharacter- nex nonblank character of postfix switch (nextCharacter) case variable: valueStack.push(value of the variable nextCharacter) break + case operandTwo - valueStack.pop(O operandOne - valueStack.popO case case case ' the result ofthe operation in nextchar acter and its operands operandone and operandTwo result valueStack.push(result) break default: break // Ignore unexpected characters return valueStack.peekO

Explanation / Answer

Program to Evalute the postfix expression

PostfixEvaluation.java

package org.students;

import java.io.DataInputStream;
import java.io.IOException;
import java.util.Scanner;

class PostfixEvaluation
{
public static void main(String[] args)throws IOException
{
String input;
//Always this loop will execute.
while(true)
{
System.out.print("Enter the postfix expresion (Without spaces):");
input=getString();
// if the user didn't entered any string,program will exit.
if(input.equals(""))
{ System.out.println(":: Program Exit::");
break;
}
// Create the Evaluation class object.
Evaluation e=new Evaluation();
// call the calculate method by passing the input string as
// argument.

System.out.println("Result:: "+e.calculate(input));

System.out.print("Do you want to continue(Y/N):");
       Scanner sc1 = new Scanner(System.in);
       // read the character entered by the user as input.
       char c = sc1.next(".").charAt(0);
       // check whether the character is not equal to 'Y' or 'y'
       if (c == 'y' || c == 'Y')
       {
           continue;
       }
       else
       {
           System.out.println("* Program Exit *");
           break;
       }
          
       }
}
public static String getString()throws IOException
{
DataInputStream inp=new DataInputStream(System.in);
String s=inp.readLine();
return s;
}
}

----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Evaluation.java

package org.students;
class Evaluation{
   public int calculate(String s)
   {
   int n,r=0;
   //find the length of the string.
   n=s.length();
   //create a stack
   Stack a=new Stack(n);
   //take each character from string using charAt() method .
   for(int i=0;i<n;i++)
   {
   char ch=s.charAt(i);
   //whether the character is number between 0-9(inclusive).
   if(ch>='0'&&ch<='9')
       //if it is number store it into the stack.
   a.push((int)(ch-'0'));
   else
   {
   //if the character is an operator symbol then get the operands from the stack.
   //pop() method will delete the top most operand from the stack and return it.
   //delete the top most operand from the stack and the returned value stored it in a variable x.

   int x=a.pop();
   //delete the top most operand from the stack
   //and the returned value stored it in a variable y.
   int y=a.pop();
   //execute the switch block if the character is operator.
   switch(ch)
   {
   case '+':r=x+y;
   break;
   case '-':r=y-x;
   break;
   case '*':r=x*y;
   break;
   case '/':r=y/x;
   break;
   default:r=0;
   }
   a.push(r);
   }
   }
   r=a.pop();
   return(r);
   }
   }

----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Stack.java

package org.students;

import java.io.*;
class Stack
{
private int[] a;
private int top,m;
public Stack(int max)
{
m=max;
a=new int[m];
top=-1;
}
public void push(int key)
{
a[++top]=key;
}
public int pop()
{
return(a[top--]);
}
}

----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Input: 23+4*5- (this is the input)

Enter the postfix expresion :23+4*5-
Result:: 15
Do you want to continue(Y/N):n
* Program Exit *

----------------------------------------------------------------------------------------------------------------------------------------------------------------------

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