Question

Can you please complete it in java and add comments explaining the program so I can understand it.

PROBLEM: Evaluate a prefix expression. The operands in the expression are single digit whole numbers. The operators are binary addition (+), subtraction (), and multiplication(*), and a trinary operator "switcher" (a). The a operator of a, b, and c returns b when a is positive; otherwise, it returns Example 1: * + 4 53 1 simplifies to 9 2, which has a value of 18 Example 2: 8 97+4 2 simplifies to -1 7 6,which has a value of 6. INPUT: Five lines of data. Each line is a string, < 128 characters. The string is a valid prefix expression with single digit whole number operands, and uses the operators +,-, * and a. All operands and operators are separated by at least one space OUTPUT: Evaluate each prefix expression and print the answer SAMPLE INPUT *4 5-3 1 e8 97+42 e-3 5-2 410 *4-5 7 3 2 +1 9 * 4693 8172 SAMPLE OUTPUT: #1. 18 #2. 6 #3. 0 #4. 40 #5 26

Test cases:

Test case 2 input

Answer 1

//Java program

import java.util.*;
  
public class Prefix {
   public static void main(String[] args)
   {
   String expression = "@ - 8 9 7 + 4 2";
   System.out.println(evaluatePrefix(expression));
     
   expression = "* + @ 4 6 9 @ - 3 8 1 7 2";
   System.out.println(evaluatePrefix(expression));
   }
  

static double evaluatePrefix(String expression)
{
Stack<Double> Stack = new Stack<Double>();

for (int j = expression.length() - 1; j >= 0; j--) {

// Push operand to Stack
// To convert expression[j] to digit subtract
// '0' from expression[j].
if (isOperand(expression.charAt(j)))
Stack.push((double)(expression.charAt(j) - '0'));
else if(expression.charAt(j)==' ')continue;

else {

// Operator encountered
// Pop two elements from Stack
double operand1 = Stack.peek();
Stack.pop();
double operand2 = Stack.peek();
Stack.pop();

// Use switch case to operate on operand1
// and operand2 and perform operand1 Operator operand2.
switch (expression.charAt(j)) {
case '+':
Stack.push(operand1 + operand2);
break;
case '-':
Stack.push(operand1 - operand2);
break;
case '*':
Stack.push(operand1 * operand2);
break;
case '/':
Stack.push(operand1 / operand2);
break;
case '@':
   double operand3 = Stack.peek();
   Stack.pop();
   if(operand1>=0)Stack.push(operand2);
   else Stack.push(operand3);
}
}
}

return Stack.peek();
}
static Boolean isOperand(char c)
{
if(c >= '0' && c <= '9') return true;
return false;
}
  

}

//sample output

Prefix [Java Application] C:Program Files\Javaljdk1.8.0 151 6.0 26.0 94-