JAVA program to convert Roman numerals to integer numbers

Roman Numerals - Based on the ancient Roman system, using symbols to represent numbers. These numbers are called Roman numerals. The symbols are I, V, X, L, C, D and M, representing 1, 5, 10, 50, 100, 500 and 1,000 respectively.

Integer - An integer is an integer consisting of positive, negative and zero values. Fractions are not whole numbers.

Here we set the symbol value based on the integer value. Whenever a Roman numeral is given as input, we divide it into units and then calculate the appropriate Roman numeral.

I - 1
II – 2
III – 3
IV – 4
V – 5
VI – 6
.
.
.
X – 10
XI – 11
.
.
XV - 15

In this article, we will learn how to convert Roman numerals to integers in Java.

Show you some examples -

Example 1

Input Roman number is XIV.
Converting it to Integer = 14.

Example 2

Input Roman number is CCXXXIV.
Converting it to Integer = 234.

Example 3

Input Roman number is MCCXXXI.
Converting it to Integer = 1231.

algorithm

Step-1 - Get input Roman numerals in string form via static input or user input.

Step-2 - In a user-defined method, we declare some conditions where there are Roman numerals with appropriate integer values.

Step-3 - In another user defined method, we calculate the Roman numeral value by using the index value of the given string.

Step-4 - After getting the integer, we print it as output.

Multiple methods

We provide solutions in different ways.

• Through user-defined methods with static input values.

• Through user-defined methods and user input values.

Let’s look at the program and its output one by one.

Method 1: Using a user-defined method with static input values

In this method, we declare a Roman input number through the static input method and pass the number as a parameter to the user-defined method, and then use an algorithm inside the method to convert the Roman number to an integer.

Example

import java.util.*;
import java.io.*;
import java.lang.Math;
public class Main {
   public static void main(String args[]) {
      Main obj = new Main();
      String inputRoman= "LXVII";
      System.out.println("The Integer value of given Roman number is: "+obj.romanToInt(inputRoman));
   } 
   int NumValue(char rom) {
      if (rom == 'I')
         return 1;
      if (rom == 'V')
         return 5;
      if (rom == 'X')
         return 10;
      if (rom == 'L')
         return 50;
      if (rom == 'C')
         return 100;
      if (rom == 'D')
         return 500;
      if (rom == 'M')
         return 1000;
      return -1;
   }
   int romanToInt(String str) {
      int sum = 0;
      for (int i=0; i<str.length(); i++) {
         int s1 = NumValue(str.charAt(i));
         if (i+1 <str.length()) {
           int s2 = NumValue(str.charAt(i+1));
           if (s1 >= s2) {
              sum = sum + s1;
           }
           else{
              sum = sum - s1;
           }
        }
        else {
           sum = sum + s1;
        } 
     }  
     return sum;
   }
} 

Output

The Integer value of given Roman number is: 67

Method 2: Using user input value

In this method, we declare a Roman input number through the user input method and pass the number as a parameter to the user-defined method, and then use an algorithm inside the method to convert the Roman number to an integer.

Example

import java.util.*;
import java.io.*;
import java.lang.Math;
public class Main {
   public static void main(String args[]) {
      Main obj = new Main();
      Scanner sc = new Scanner(System.in);
      System.out.print("Enter a Roman Number in capital letters: ");
      String inputRoman= sc.nextLine();
      System.out.println("The Integer value of given Roman number is:"+obj.romanToInt(inputRoman));
   }
   int NumValue(char rom){
      if (rom == 'I')
         return 1;
      if (rom == 'V')
         return 5;
      if (rom == 'X')
         return 10;
      if (rom == 'L')
         return 50;
      if (rom == 'C')
         return 100;
      if (rom == 'D')
         return 500;
      if (rom == 'M')
         return 1000;
      return -1;
   } 
   int romanToInt(String str) {
      int sum = 0;
      for (int i=0; i<str.length(); i++) {
         int s1 = NumValue(str.charAt(i));
         if (i+1 <str.length()) {
            int s2 = NumValue(str.charAt(i+1));
            if (s1 >= s2) {
               sum = sum + s1;
            }
            else {
               sum = sum - s1;
            } 
         }
         else {
            sum = sum + s1;
         } 
      }
      return sum;
   } 
} 

Output

Enter a Roman Number in capital letters: V
The Integer value of given Roman number is: 5

In this article, we explored how to convert Roman numerals to integers in Java using different methods.

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