Convert roman numeral to integer

Question

The roman numeral to integer converter I am following:

https://www.selftaughtjs.com/algorithm-sundays-converting-roman-numerals/

My attempt at converting the Javascript function to Java:

public class RomanToDecimal {
public static void main (String[] args) {

    int result = 0;
    int[] decimal = {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
    String[] roman = {"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"};

    // Test string, the number 895
    String test = "DCCCXCV";

    for (int i = 0; i < decimal.length; i++ ) {
        while (test.indexOf(roman[i]) == 0) {
            result += decimal[i];
            test = test.replace(roman[i], "");
        }
    }
    System.out.println(result);
}

}

The output is 615, which is incorrect.

Please help me understand where I went wrong.

Answer 1

Your test = test.replace(roman[i], ""); replaces all occurrences of "C" with "", so after you find the first "C" and add 100 to the total, you eliminate all the remaining "C"s, and never count them. Therefore you actually compute the value of "DCXV", which is 615.

You should only replace the occurrence of roman[i] whose start index 0, which you can achieve by replacing:

test = test.replace(roman[i], "");

with:

test = test.substring(roman[i].length()); // this will remove the first 1 or 2 characters
                                          // of test, depending on the length of roman[i]

The following:

int result = 0;
int[] decimal = {1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1};
String[] roman = {"M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"};

// Test string, the number 895
String test = "DCCCXCV";

for (int i = 0; i < decimal.length; i++ ) {
    while (test.indexOf(roman[i]) == 0) {
        result += decimal[i];
        test = test.substring(roman[i].length());
    }
}
System.out.println(result);

prints:

895

Answer 2

test = test.replace(roman[i], "");

This will replace every occurrence. Instead, you should only truncate off the occurrence at the beginning of the string (position 0).

Try using substring instead of replace, and pass as an argument the length of roman[i]