Skip to main content

How to convert a number to a Roman numeral in Python

How to convert a number to a Roman numeral in Python.

Here's a step-by-step tutorial on how to convert a number to a Roman numeral in Python.

Step 1: Initialize a dictionary​

First, we need to create a dictionary that maps each decimal number to its corresponding Roman numeral. This will make the conversion process easier. We'll use this dictionary throughout the tutorial.

roman_numerals = {
    1000: 'M',
    900: 'CM',
    500: 'D',
    400: 'CD',
    100: 'C',
    90: 'XC',
    50: 'L',
    40: 'XL',
    10: 'X',
    9: 'IX',
    5: 'V',
    4: 'IV',
    1: 'I'
}

Step 2: Define the function​

Next, let's define a function called convert_to_roman that takes an integer as input and converts it to a Roman numeral.

def convert_to_roman(num):
    roman_numeral = ''
    for value, numeral in roman_numerals.items():
        while num >= value:
            roman_numeral += numeral
            num -= value
    return roman_numeral

Step 3: Test the function​

Now, let's test our function with some examples to see if it's working correctly.

print(convert_to_roman(9))  # Output: IX
print(convert_to_roman(27))  # Output: XXVII
print(convert_to_roman(3999))  # Output: MMMCMXCIX

Step 4: Understand the code​

Let's go through the code and understand how it works.

  • The convert_to_roman function takes a number num as input.
  • It initializes an empty string roman_numeral to store the final Roman numeral.
  • It then loops through each key-value pair in the roman_numerals dictionary.
  • Inside the loop, it checks if the current value is less than or equal to num.
  • If it is, it appends the corresponding numeral to the roman_numeral string and subtracts the value from num.
  • This process continues until num becomes zero, indicating that the conversion is complete.
  • Finally, it returns the roman_numeral string.

Step 5: Additional considerations​

Our current implementation assumes that the input number is in the range of 1 to 3999, as Roman numerals are not commonly used for larger numbers. However, you can extend the dictionary and modify the code to handle larger numbers if needed.

That's it! Now you have a working function to convert decimal numbers to Roman numerals in Python. Feel free to use this code as a reference or modify it as per your requirements.