Java Beginner - Roman Numerals to Decimal WITHOUT switch

Question

I'm taking an AP Computer Science course, and I'm having a problem with this code. We're to make a method that converts Roman Numeral Strings into decimal values.

So far, we've only gotten to if/else statements and while loops. I've seen a few answers here using switch, but our class hasn't gotten there yet. I need a way to do this without using switch. I've only written code assuming that the Roman Numerals are in non-ascending order, but whenever a string like "MM" is used, the method returns "0" as a value. Anyone know why this happens here?

Any tips on subtracting the numerals would also be appreciated.

   public int decimalValue ( String roman ){
     //given a Roman Numeral
     int decimal =0;
     int a = 0; // a and b used as to check each letter in String roman
     int b = 1;

     // Assuming in non-ascending order. Need to make for ascending
     while ( (a <= (roman.length() - 1) ) && (b <= roman.length() ) ) {
        if ( roman.substring(0,1) == "M") {
           decimal = decimal + 1000;
        }
        else if ( roman.substring(0,1) == "D") {
           decimal = decimal + 500;
        }
        else if ( roman.substring(0,1) == "C") {
           decimal = decimal + 100;
        }
        else if ( roman.substring(0,1) == "L") {
           decimal = decimal + 50;
        }
        else if ( roman.substring(0,1) == "X") {
           decimal = decimal + 10;
        }
        else if (roman.substring(0,1) == "V") {
           decimal = decimal + 5;
        }
        else if (roman.substring(0,1) == "I") {
           decimal = decimal + 1;
        }
        a++;
        b++;
        }

  return decimal;
  }

Don't compare content of `Strings` using `==`. Use `equals()` instead. — Alexis C., Dec 01 '13 at 21:50
Will your teacher not allow you to use `switch`? You should be allowed to make your code look nicer :p — keyser, Dec 01 '13 at 21:52
Switch is nice ! We were taught the switch prior to the if and else's :O — Sameer Sawla, Dec 01 '13 at 21:55

Blub · Answer 1 · 2013-12-01T22:03:59.850

A few things:

You don't need two counting variables a and b. One is enough
You want to check characters of the String, so you should use roman.charAt(a) to check the character at position a in the string
chars (and other primitive types) you compare with ==, Strings you have to compare with .equals()
Right now with roman.substring(0,1) you are just checking the first character of the string, so you want to change that to roman.substring(a, a+1) or use characters as I wrote in 2. This way you will keep checking one character after the other.
You should also think about numbers like IX, these ones you don't recognize yet ;)

Java Beginner - Roman Numerals to Decimal WITHOUT switch

1 Answers1