Convert numbers to roman numerals using Python

  • Roman Numerals are a number system used in ancient Rome. Numbers here are represented using a combination of letters. Modern style uses 7 symbols, each with different values. The largest number that can be written using these symbols are 3,999.

    Symbol Integer Value
    I 1
    V 5
    X 10
    L 50
    C 100
    D 500
    M 1000

    Numerals can be combined in different order to form numbers. See below roman numerals for numbers from 1 to 10 and some other 2 letter numerals for specific numbers.

    I = 1
    II = 2
    III = 3
    IV = 4
    V = 5
    VI = 6
    VII = 7
    VIII = 8
    IX = 9
    X = 10
    XL = 40
    XC = 90
    CD = 40
    CM = 900

    Some other examples:
    209 = CC + IX = CCIX
    93 = XC + III = XCIII
    1005 = M + V = MV
    1045 = M + CD + V = MCDV

  • Convert integer to roman numerals

    As seen above, we represent certain numbers using a single letter. And for some numbers, we require a combination of 2 letters.

    So we can use a simple logic to find the corresponding roman numeral of a number. We will keep the specific values of each of these characters in descending order in a tuple. Then loop through this tuple and find the matching characters. And reduce the number till reaches 0.

    Consider an example for better understanding.
    Input number = 1025
    In the for loop, 1025 > first option (1000, 'M')
    So, romannum = M, num = num-1000 = 25
    Loop continues. (10, 'X') is the first option that satisfies the condition.
    romannum = MX, num = num-10 = 15
    Again 15 > 10. So, (10, 'X')
    Now, romannum = MXX, num = num-10 = 5
    Now last loop and (5, 'V') matches
    So romannum = MXXV, num = num-5 = 0

    Copied 
    def findRoman(num):
    valuesTuple = ( (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')  )
    romannum = ''

    for i in valuesTuple:
        while num >= i[0]:
            romannum = romannum + i[1]
            num -= i[0]
        
    print(romannum)


findRoman(3999)
findRoman(2949)
    Output:
      MMMCMXCIX
      MMCMXLIX
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