How to convert a roman number in decimal in Java

This article will present how to convert a Roman number into a decimal using Java.

Theory

The numbers between 1 and 10 are composed by a combination between I, V and X. Note that number 4 and 9 are obtained by using the subtractive notation.

The same logic is applied fur values between 10 and 100.

Note that 40 (XL) and 90 (XC) follow the same subtractive pattern as 4 and 9.

Now, applying the logic for numbers between 100 and 1000 will get:

Again, note that 400 (CD) and 900 (CM) follow the same subtractive pattern as 4 and 9.

Examples

MCMLXXXII = 1982

MMXVII = 2017

Code

package com.admfactory;

public class RomanNumberToDecimal {

    public static int toDecimal(String str) {
	int len = str.length();

	/**
	 * adding an random char just to be used by the next char to eliminated
	 * unnecessary out of index checks
	 */
	str = str + " ";
	int result = 0;
	for (int i = 0; i < len; i++) {
	    char ch = str.charAt(i);
	    /** the next char is needed in case the number is using subtractive pattern */
	    char nextChar = str.charAt(i + 1);

	    /** if ch is M add 1000 - definitely not part of subtractive pattern */
	    if (ch == 'M') {
		result += 1000;
	    /** if ch is C - possible part of subtractive pattern */
	    } else if (ch == 'C') {
		if (nextChar == 'M') {
		    result += 900;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else if (nextChar == 'D') {
		    result += 400;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else {
		    result += 100;
		}
	    /** if ch is D add 500 - definitely not part of subtractive pattern */
	    } else if (ch == 'D') {
		result += 500;
	    /** if ch is X - possible part of subtractive pattern */
	    } else if (ch == 'X') {
		if (nextChar == 'C') {
		    result += 90;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else if (nextChar == 'L') {
		    result += 40;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else {
		    result += 10;
		}
	    /** if ch is L add 50 - definitely not part of subtractive pattern */
	    } else if (ch == 'L') {
		result += 50;
	    /** if ch is V add 5 - definitely not part of subtractive pattern */		
	    } else if (ch == 'V') {
		result += 5;
            /** if ch is I - possible part of subtractive pattern */
	    } else if (ch == 'I') {
		if (nextChar == 'X') {
		    result += 9;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else if (nextChar == 'V') {
		    result += 4;
		    /** Additionally increasing the index by 1 as the subtractive pattern was used */
		    i++;
		} else {
		    result += 1;
		}	    
	    }
	}
	return result;
    }

    public static void main(String[] args) {
	System.out.println("Roman number to decimal conversion example");
	System.out.println();
	String rn1 = "MCMLXXXII";
	int dec1 = toDecimal(rn1);
	System.out.println(rn1 + " converts to " + dec1);

	String rn2 = "MMXVII";
	int dec2 = toDecimal(rn2);
	System.out.println(rn2 + " converts to " + dec2);
    }
}

Output

Roman number to decimal example

MCMLXXXII converts to 1982
MMXVII converts to 2017

References

• Roman numbers