Question

JAVA - PLEASE COMMENT CODE - THANK YOU:

Implement a program that can input an expression in postfix notation and output its value.

Answer 1

import java.util.*;

public class PostfixToValue

{

// Method to evaluate value of a postfix expression

static int evaluatePostfix(String exp)

{

//create a stack

Stack<Integer> stack = new Stack<>();

// Scan all the characters one by one

for(int i = 0; i < exp.length(); i++) //Iterate over each and check

{

char op = exp.charAt(i);

if(op == ' ')

continue;

// If the scanned character is an operand  

// (number here),extract the number

// Push it to the stack.

else if(Character.isDigit(op))

{

int n = 0;

//extract the characters and store it in num

while(Character.isDigit(op))

{

n = n*10 + (int)(op-'0');

i++;

op = exp.charAt(i);

}

i--;

//push the number in stack

stack.push(n);

}

// If the scanned character is an operator, pop two

// elements from stack apply the operator

else

{

int val1 = stack.pop();

int val2 = stack.pop();

switch(op)

{

case '+':

stack.push(val2+val1);

break;

case '-':

stack.push(val2- val1);

break;

case '/':

stack.push(val2/val1);

break;

case '*':

stack.push(val2*val1);

break;

}

}

}

return stack.pop();  

}

public static void main(String[] args)  

{  

System.out.print("Enter postfix expression: ");  

Scanner sc= new Scanner(System.in); //System.in is a standard input stream  

String postexp=sc.nextLine();

System.out.print(evaluatePostfix(postexp));//takes the input and evaluates the post fix expression.

}

}

This is how the output appears. It takes the postfix expression given by user and evaluates it to a value and prints that value.