Okay so I am building a simple PostFix Calculator in Java and I was asked to cre

Question

Okay so I am building a simple PostFix Calculator in Java and I was asked to create a couple functions for it, one that I am struggling with is the memory. I am required to use something called HashMap, but i don't quite understand how to implement it into my program correctly. The way the Program will work is the User will Start it and it will say its a postFix calculator and will be prompted for input like this:

1 import java. util 2 import java.io.*; 3 public class Programs ix public static void main (String args System. out.println Servando Hernandez System. out.println RPN command line calculator Scanner Scan new Scanner (System.in); System.out.print 10 while scan. hasNext Line 11 12 system. out.print 13 ext Line String a Scan, n 14 String b quit 15 String c 3 "mem" 16 String d clear 17 if(a. equals (b)) 18 19 System.exit(0); 20 21 else 22 23 System. out.println (compute (a)); 25 System. out.print 26 27 28 29 30 31 public static String compute (String input) 32 33 List

Explanation / Answer

Tested on Eclipse

/********************ProgramSix.java******************/

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
import java.util.Stack;
import java.util.StringTokenizer;

public class ProgramSix {
   /**
   * memory map for storing variable value
   */
   static Map<String, Integer> memory = new HashMap<String, Integer>();

   // main method start
   public static void main(String args[]) {
       System.out.println("Servando Hernandez");
       System.out.println("RPN command Line calculator");
       Scanner scan = new Scanner(System.in);
       System.out.println(">");
       /**
       * It will continue until we type exit
       */
       while (scan.hasNextLine()) {
           System.out.println("> ");
           String a = scan.nextLine();
           String b = "exit";
           String c = "mem";
           String d = "clear";
           if (a.equals(b)) {
               System.exit(1);
           } /**
               * if we typed mem then it will print all variable with values
               * present in Map
               *
               */
           else if (a.equals(c)) {
               // loop a Map
               for (Map.Entry<String, Integer> entry : memory.entrySet()) {
                   System.out.println(entry.getKey() + ": " + entry.getValue());
               }
           } else {
               System.out.println(compute(a));
           }

           System.out.println(">");
       }
   }

   /**
   * compute method implementation
   */
   private static String compute(String input) {
       List<String> processedList = new ArrayList<String>();
       boolean isequal = false;
       /**
       * checking input string contains = or not
       */
       if (input.contains("=")) {
           isequal = true;
       }
       /**
       * creating token from input string
       */
       if (!input.isEmpty()) {
           StringTokenizer st = new StringTokenizer(input);
           while (st.hasMoreTokens()) {
               processedList.add(st.nextToken());
           }

       } else {
           return "Error";
       }
       Stack<String> tempList = new Stack<String>();
       Iterator<String> iter = processedList.iterator();
       String key = "";
       boolean flag = false;
       /**
       * Iterating all tokens
       */
       while (iter.hasNext()) {
           String temp = iter.next();
           if (temp.matches("[0-9]*")) {
               tempList.push(temp);
           } else if (temp.matches("[*-/+%]")) {
               if (temp.equals("*")) {
                   int rs = Integer.parseInt(tempList.pop());
                   int ls = Integer.parseInt(tempList.pop());
                   int result = ls * rs;
                   tempList.push("" + result);
               } else if (temp.equals("-")) {
                   int rs = Integer.parseInt(tempList.pop());
                   int ls = Integer.parseInt(tempList.pop());
                   int result = ls - rs;
                   tempList.push("" + result);
               } else if (temp.equals("/")) {
                   int rs = Integer.parseInt(tempList.pop());
                   int ls = Integer.parseInt(tempList.pop());
                   int result = ls / rs;
                   tempList.push("" + result);
               } else if (temp.equals("+")) {
                   int rs = Integer.parseInt(tempList.pop());
                   int ls = Integer.parseInt(tempList.pop());
                   int result = ls + rs;
                   tempList.push("" + result);
               } else if (temp.equals("%")) {
                   int rs = Integer.parseInt(tempList.pop());
                   int ls = Integer.parseInt(tempList.pop());
                   int result = ls % rs;
                   tempList.push("" + result);
               }
           } else if (temp.matches("[a-zA-z]*")) {
               if (!flag) {
                   key = temp;
               }
               if (memory.get(temp) == null && isequal == false) {
                   System.out.println(temp + " not found");
                   return "";
               } else if (memory.containsKey(temp)) {
                   tempList.push(memory.get(temp) + "");
               }
           } else if (temp.equals("=")) {
               flag = true;
           } else {

               return "Error";
           }
       }
       String value = tempList.pop();
       if (isequal) {
           memory.put(key, Integer.parseInt(value));
       }
       return value;
   }
}

/*********************output**************************/

Servando Hernandez
RPN command Line calculator
>
a = 3 5 + 1 -
>
7
>
bee = a 3 *
>
21
>
a bee +
>
28
>
bee 3 %
>
0
>
a = 4
>
4
>
57
>
57
>
2 c +
>
c not found

>
mem
>
a: 4
bee: 21
>
exit
>

Thanks a lot

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