Java Program Reads an Infix Expression, Converts the Expression to Postfix form and then Evaluates the Postfix Expression

import java.util.Scanner;

class CharacterStack {

    int size, top;

    char stack[];

   

    CharacterStack (int num) {

        size = num;

        top = -1;

        stack = new char[num];

    }

   

    public void push(char n) {

        stack[++top] = n;

    }

   

    public char pop() {

        return stack[top--];

    }

   

    public char peek() {

        return stack[top];

    }

   

    public boolean isEmpty() {

        return top == -1;

    }

   

    public boolean isFull() {

        return top == size;

    }

}

public class InfixToPostfix {  

   

    public static String infixToPostfix(String infix) {

       

        char symbol;

        String postfix = "";

        CharacterStack operator = new CharacterStack(infix.length());

       

        for(int i=0; i < infix.length(); ++i) {

           

            symbol = infix.charAt(i);

           

            if(Character.isLetter(symbol))

                postfix += symbol;

           

            else if( symbol ==  '(' || symbol == '{' || symbol == '[' )

                operator.push(symbol);

           

            else if( symbol ==  ')' || symbol == '}' || symbol == ']' ) {

                while( !( operator.peek() == '(' ||operator.peek() == '{' ||operator.peek() == '[') )

                    postfix += operator.pop();

                operator.pop();

            } else {

                while(!operator.isEmpty() && !(operator.peek()=='(') && isPref(symbol) <= isPref(operator.peek()))

                    postfix += operator.pop();

                operator.push(symbol);

            }

        }

        while(!operator.isEmpty())

            postfix += operator.pop();

        return postfix;

    }

   

    private static int isPref(char c) {

        if(c == '+' || c == '-') 

            return 1;

        if(c == '*' || c == '/' || c == '%')

            return 2;

        if(c == '^')

            return 3;

        return 0;

    }

   

    private static int postfixEvaluation(String infixToPostfix) {

        String alphabet = "abcdefghijklmnopqrstuvwxyz";

        int[] counts = new int[26];

        Scanner input = new Scanner(System.in);

       

          for(int i=0; i < infixToPostfix.length(); i++) {

              char ch = Character.toLowerCase(infixToPostfix.charAt(i));

              int index = alphabet.indexOf(ch);

              if(index != -1)

                   counts[index] += 1;

        }

       

        for(int i=0; i<26; i++) {

            if(counts[i] > 0) {

                System.out.print(" Enter value of " + alphabet.charAt(i) + " : ");

                counts[i] = input.nextInt();

            }  

        }

       

        CharacterStack token = new CharacterStack(infixToPostfix.length());

       

        for(int i=0; i < infixToPostfix.length(); i++) {           

            char c = infixToPostfix.charAt(i);

           

            if(Character.isLetter(c)) {               

                int index = alphabet.indexOf(c);

                token.push((char)counts[index]);

            }

            else {

                int x = token.pop();

                int y = token.pop();

                int calculation = calculation(y, x, c);

                token.push((char)calculation);

            }

        }

        return (int)token.pop();

    }

   

    private static int calculation(int a, int b, char c) {

        switch(c) {

            case '+' : return a+b;

            case '-' : return a-b;

            case '*' : return a*b;

            case '/' : return a/b;

            case '%' : return a%b;

            case '^' : return (int) Math.pow(a, b);

        }

        return 1;

    }

   

    public static void main(String[] ar) {

       

        Scanner input = new Scanner(System.in);

       

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

        String str = input.nextLine();

       

        String infixToPostfix = infixToPostfix(str);

        System.out.println(" Postfix Expression : " + infixToPostfix);

       

        int postfixEvaluation = postfixEvaluation(infixToPostfix);

        System.out.println("\n Evaluation of Postfix: " + postfixEvaluation);

    }

}