Thread: Roman Numbers

help here pls.. i'm having problem on how can i conver a roman number into a real number

heres what i've got so far

public class RomanNumbers {
 
    private static BufferedReader br = new BufferedReader(new InputStreamReader(
                                       System.in));
 
    public static void main(String[] args) throws IOException {
 
        char temp = 0;
 
        int rN = 0;
 
        int num = 0;
 
        System.out.print("Enter The Roman Number: ");
        String romanNumber = br.readLine();
 
        for (int count = 0; count <= romanNumber.length() - 1; count++) {
 
            temp = romanNumber.charAt(count);
 
            switch (temp) {
 
                case 'I':
 
                    rN = 1;
 
                    break;
 
                case 'V':
 
                    rN = 5;
 
                    break;
            }
 
            num = num + rN;
        }
 
        System.out.println(num);
    }
}

the problem is when i enetered a roman number for example:

IV = 4, because I is less than V which is one of the rules in converting a roman number

if the first number is less than the next one... it should be subtracted... otherwise it will be added...


my problem is how can i compare the first value of the roman number into the value next to it..


i dont have any more problems when the sequence is greatest to least.. because i only have to add the values,,
but im having a hard time thinking of how could i compare the values to get the answer if the first number is less than the next number

How abou tthis

import java.util.Scanner;
 
 
public class Roman {
	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
 
		System.out.print("Enter a Roman Number:> ");
		char[] roman = stdIn.nextLine().toCharArray();
 
		int total = 0;
		for(int i = roman.length-1; i > -1; i--){
			switch(roman[i]){
				case 'I':
					total += value(roman[i]); break;
				case 'V':
				case 'X':
				case 'L':
				case 'C':
				case 'D':
				case 'M':
					if(i != 0 && (value(roman[i-1]) < value(roman[i]))){
						total += value(roman[i]) - value(roman[i-1]);
						i--;
					}else{
						total += value(roman[i]);
					}
					break;
			}
		}
		System.out.println(total);
	}
 
	public static int value(char c){
		switch(c){
			case 'I':
				return 1;
			case 'V':
				return 5;
			case 'X':
				return 10;
			case 'L':
				return 50;
			case 'C':
				return 100;
			case 'D':
				return 500;
			case 'M':
				return 1000;
			default:
				return 0;
		}
	}
 
}

Chris

Basically what happens here is that for every character you translate you need to check the next character to see if its bigger than the current character and if it is you know that its a subtraction.

Read the section called "Subtractive principle" on Roman numerals - Wikipedia, the free encyclopedia

Generally, Roman numerals are written in descending order from left to right, and are added sequentially, for example MMVI (2006) is interpreted as 1000 + 1000 + 5 + 1.
Certain combinations employ a subtractive principle, which specifies that where a symbol of smaller value precedes a symbol of larger value, the smaller value is subtracted from the larger value, and the result is added to the total. For example, in MCMXLIV (1944), the symbols C, X and I each precede a symbol of higher value, and the result is interpreted as 1000 plus (1000 minus 100) plus (50 minus 10) plus (5 minus 1).
A numeral for 10n (I, X, or C) may not precede a numeral larger than 10n+1, where n is an integer.[citation needed] That is, I may precede V and X, but not L or C; X may precede L or C, but not D or M. The numerals 5×10n (V, L, or D) may not be followed by a numeral of greater or equal value.[citation needed] Any symbol that appears more than once consecutively may not be followed by a symbol of larger value.

// Json

chris.. i noticed something in the for statement.... the "length" method , i looks different..
in my IDE (netbeans) it is color green..


and color green, statnds for fields or objects... but i dont know where that "length" came from...

the variable roman is a char Array, in Java these have a variable known as length containing the size of the array.

Chris

ahh so thats it.. a method that returns a length of an array of characters.....

tnx!!

Not a method, an instance variable

aw .. sorry....

Just making sure you now

Chris

tnx tnx!

chris ... just want to ask something about this part...

			case 'V':
				case 'X':
				case 'L':
				case 'C':
				case 'D':
				case 'M':

ahmm...

if i pass the characer for example 'V' to the method .value(<char>),
its doest not return ALL the succeding values after the 'V' right?

the return value will just depend on the character that i will pass to that method...?

does my analyzation ryt?


coz im a bit confuse on my first look on your code...

i thought it's just an ordinary sequence of switch... (because i dont see a 'break' statement)

so i thought it will return all the next values after the character that i passed....


sorry for bad english......

Basically what happens here is that if you pass in 'V' it will fall through down to the 'M' case and run that code, the same thing happens if you pass in either 'X', 'L', 'C' or 'D'.

You need to take note that switch cases falls through to the next one unless the case has a call to break in it.

		switch(roman[i]){
				case 'I':
					total += value(roman[i]); break; // We call break here to break out of the switch
				case 'V': // No break so fall down to X
				case 'X': // No break so fall down to L
				case 'L': // No break so fall down to C
				case 'C': // No break so fall down to D
				case 'D': // No break so fall down to M
				case 'M':
					if(i != 0 && (value(roman[i-1]) < value(roman[i]))){
						total += value(roman[i]) - value(roman[i-1]);
						i--;
					}else{
						total += value(roman[i]);
					}
					break; // BREAK
			}

// Json

what do you mean by 'fall down' sir?

does it mean it also returns value ? ryt?

in this case... we are only asking for the value of 'V' ... so the value of the rest are not used....

am i ryt sir?