Question

JAVA PROGRAM

Question : Design and implement two classes called InfixToPostfix and PostFixCalculator. The InfixToPrefix class converts an infix expression to a postfix expression. The PostFixCalculator class evaluates a postfix expression. This means that the expressions will have already been converted into correct postfix form. Write a main method that prompts the user to enter an expression in the infix form, converts it into postfix, displays the postfix expression as well as it's evaluation. For simplicity, use only these operators, + , - , * , / and %.

I need help answering the programming question, not sure how this really functions. If you can please explain how it works with comments. Thank you I greatly appreciate the help.

Answer 1

The below code illustartes two methods.

infixToPostfix : converts infix expression to postfix expression

postFixCalculator : calculates the value of postfix expression.

The code is commented for understanding each logic.

import java.util.*;
  
class Main
{ 
   // this method returns the precedence of operators
    static int Preced(char c) 
    { 
        switch (c)    // define some integer value as per the order of precedence
        { 
        case '+': 
        case '-': 
            return 1; 
       
        case '*': 
        case '/': 
        case '%': 
            return 2; 
       
        case '^': 
            return 3; 
        } 
        return 0; 
    } 
       
    // this method converts infix expression to postfix expression 
    static String infixToPostfix(String infix) 
    { 
        // initialize one  empty stack 
        Stack<Character> st = new Stack<>(); 
        
        // initialize one empty string 
        String postfix = new String(""); 
          
         
        for (int i = 0; i<infix.length(); i++) 
        { 
            char c = infix.charAt(i); 
              
             // if the character is operand, add it to output 
            if (Character.isLetterOrDigit(c))   // this built in function returns true if the char is letter or digit
                postfix += c; 
               
            // if the character is '(', push it to stack
            else if (c == '(') 
                st.push(c); 
              
            //  if the character is ')', pop and add it to output string until '(' is found
            else if (c == ')') 
            { 
                while (!st.isEmpty() && st.peek() != '(') 
                    postfix = postfix+ st.pop(); 
                
                // if '(' is not found at the end then its not an infix expression
                if (!st.isEmpty() && st.peek() != '(') 
                    return "Wrong expression, not an infix expression"; // invalid expression                 
                else
                    st.pop(); 
            } 
            else // if an operator is found
            { 
                // check the precedence, while the precedence of the current operator is less than the precedence of operators in stack,
                //remove the operators from stack and add them to the result
                while (!st.isEmpty() && Preced(c) <= Preced(st.peek())){ 
                    if(st.peek() == '(') 
                        return "Wrong expression, not an infix expression"; 
                    postfix = postfix +  st.pop(); 
             } 
                st.push(c);  // at the end push the current operator into the stack
            } 
       
        } 
        // at the end pop all the operators from stack and add them to the result
        while (!st.isEmpty()){ 
            if(st.peek() == '(') 
                return "Wrong expression, not an infix expression"; 
            postfix = postfix + st.pop(); 
         } 
        return postfix; 
    } 
    
    static int postfixCalculator(String postfix) 
    { 
        
        Stack<Integer> st=new Stack<>(); 
          
        // loop through all the characters
        for(int i=0;i<postfix.length();i++) 
        { 
            char c=postfix.charAt(i); 
              
            // if operand (digit) push into stack 
            if(Character.isDigit(c)) 
            st.push(c - '0'); 
              
            // if operator, pop two elements from top and do the calculation
            // push the result back to the stack
            else
            { 
                int a= st.pop(); 
                int b = st.pop(); 
                  
                switch(c) 
                { 
                    case '+': 
                    st.push(b+a); 
                    break; 
                      
                    case '-': 
                    st.push(b- a); 
                    break; 
                      
                    case '/':
                    st.push(b/a); 
                    break; 
                      
                    case '*': 
                    st.push(b*a); 
                    break; 
              } 
            } 
        } 
        return st.pop();     // at the end return the single element from the stack which is the result
    } 
    
     
    public static void main(String[] args)  
    { 
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the infix expression: ");
        String infix = sc.nextLine();
        String postfix = infixToPostfix(infix); 
        System.out.println("The postfix expression is: "+postfix );
        int res = postfixCalculator(postfix);
        System.out.println("The value of the postfix expression is: "+ res);
    } 
} 

Output sample screenshot: