Question

write a java program that takes infix notation equations and changes it to postfix notation. you will have to solve the postfix equation after. (make sure you are making your own stack class)

Answer 1

//Java Program

import java.util.ArrayList;
import java.util.Scanner;

class SStack<T>{
   private ArrayList<T> list;
   private int top;
  
   public SStack() {
       list = new ArrayList<T>();
       top = -1;
   }
   public boolean isEmpty() {
       return top==-1;
   }
   public T peek() {
       return list.get(top);
   }
   public T pop() {
       T temp = list.get(top);
       top--;
       return temp;
   }
   public void push(T data) {
       top++;
       list.add(top, data);
   }
}
public class InfixtoPostfix {
     
public static void main(String[] args)
{
String expression;
Scanner in = new Scanner(System.in);
System.out.print("Please enter the infix expression to process:");
expression = in.nextLine();
String postfix = infixToPostfix(expression);
System.out.println("Postfix Expression : "+ postfix);

System.out.println("The final result after evaluating the expression is:"+evaluatePostfix(postfix));
in.close();
}
  

public static String infixToPostfix(String exp)
   {
   String result = new String("");
  
   SStack<Character> stack = new SStack<>();
  
   for (int i = 0; i<exp.length(); ++i)
   {
   char c = exp.charAt(i);
   if(c==' ')continue;
   if (Character.isLetterOrDigit(c))
   result += c;
  
   else if (c == '(')
   stack.push(c);
  
   else if (c == ')')
   {
   while (!stack.isEmpty() && stack.peek() != '(')
   result += stack.pop();
  
   if (!stack.isEmpty() && stack.peek() != '(')
   return "Invalid Expression"; // invalid expression   
   else
   stack.pop();
   }
   else // an operator is encountered
   {
   while (!stack.isEmpty() && Prefer(c) <= Prefer(stack.peek()))
   result += stack.pop();
   stack.push(c);
   }
     
   }
     
   // pop all the operators from the stack
   while (!stack.isEmpty())
   result += stack.pop();
     
   return result;
   }
public static int Prefer(char ch)
{
switch (ch)
{
case '+':
case '-':
return 1;
  
case '*':
case '/':
return 2;
  
case '^':
return 3;
}
return -1;
}   
  
     
   public static double evaluatePostfix(String expression)
   {
   SStack<Double> stack=new SStack<>();
  
   for(int i=0;i<expression.length();i++)
   {
   char c=expression.charAt(i);
  
   if(Character.isDigit(c)) {
         
   stack.push((double)(c-'0')); }
     
   else
   {
   double val1 = stack.pop();
   double val2 = stack.pop();
     
   switch(c)
   {
   case '+':
   stack.push(val2+val1);
   break;
  
   case '-':
   stack.push(val2- val1);
   break;
  
   case '/':
   stack.push(val2/val1);
   break;
  
   case '*':
   stack.push(val2*val1);
   break;
  
   case '^':
       stack.push(Math.pow(val2, val1));
       break;
   }
   }
   }
   return stack.pop();   
   }
}
//sample output