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

### Theory

Symbol | I | V | X | L | C | D | M |
---|---|---|---|---|---|---|---|

Value | 1 | 5 | 10 | 50 | 100 | 500 | 1000 |

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.

Symbol | I | II | III | IV | V | VI | VII | VIII | IX | X |
---|---|---|---|---|---|---|---|---|---|---|

Value | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

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

Symbol | X | XX | XX | XL | L | LX | LXX | LXXX | XC | C |
---|---|---|---|---|---|---|---|---|---|---|

Value | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 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:

Symbol | C | CC | C | CD | D | DC | DCC | DCCC | CM | M |
---|---|---|---|---|---|---|---|---|---|---|

Value | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | 1000 |

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
```