Roman Numeral Conversion in Python

Published on 27 October 2019 (Updated: 02 May 2020)

In this article, we’ll be talking about Conversion of Roman Numeral to Decimal Number in Python.

How to Implement the Solution

Solution

#!/usr/bin/env python
import sys


def value(r):
    return {
        'I': 1,
        'V': 5,
        'X': 10,
        'L': 50,
        'C': 100,
        'D': 500,
        'M': 1000
    }[r]


def roman_to_decimal(string):
    res = 0
    i = 0
    while i < len(string):
        s1 = value(string[i])
        if i + 1 < len(string):
            s2 = value(string[i + 1])
            if s1 >= s2:
                res += s1
                i += 1
            else:
                res = res + s2 - s1
                i += 2
        else:
            res += s1
            i += 1
    return res


def exit_with_error(msg):
    print(msg)
    sys.exit(1)


def main(args):
    if len(args) < 1:
        exit_with_error('Usage: please provide a string of roman numerals')
    try:
        roman_numeral = sys.argv[1]
        print(roman_to_decimal(roman_numeral))
    except KeyError:
        exit_with_error('Error: invalid string of roman numerals')


if __name__ == '__main__':
    main(sys.argv[1:])

The Main Function

Breaking down this solution bottom up,

if __name__ == "__main__":
    main(sys.argv[1:])

This bit of code checks to see if this is the main module run. If it is then it calls the main function and passes user input to it. In this case the user input would be a string "XV" (to convert from roman numeral to decimal number).

def main(args):
    if len(args) < 1:
        exit_with_error('Usage: please provide a string of roman numerals')
    try:
        roman_numeral = sys.argv[1]
        print(roman_to_decimal(roman_numeral))
    except KeyError:
        exit_with_error('Error: invalid string of roman numerals')

This is the main function of this file. It parses the input, then calls our roman to decimal function (and it also prints the results). It also deals with any errors raised.

Throw Errors

def exit_with_error(msg):
    print(msg)
    sys.exit(1)

This function prints a message and then exits the script with an error, sys.exit(1). If any non-zero value is returned then the program didn’t complete properly. This function is called if the user input isn’t correct.

Roman Numeral to Decimal number in Python

def roman_to_decimal(string):
    res = 0
    i = 0
    while i < len(string):
        s1 = value(string[i])
        if i + 1 < len(string):
            s2 = value(string[i + 1])
            if s1 >= s2:
                res += s1
                i += 1
            else:
                res = res + s2 - s1
                i += 2
        else:
            res += s1
            i += 1
    return res

Finally we’re at the brain of the entire program. This function takes an roman numeral as string and returns in form of Decimal number(Integer type). This function roman_to_decimal has another function inside of it value. The nested function can map the each digit of roman numeral to decimal equivalent.

For example, if XV is the input:

• First take res as zero(res =0) and i(useful for accesing characters of input string) as zero (i=0)

• While i less than length of input in roman numeral we keep on iterating loop.

• We store respective value of first character of roman numeral(with value function) in s1.

• Then we check that i+1 is less than that of length of string or not. We use this because we need to get value of next character of roman numeral into s2. If i+1 is greater than length of string then we take the res as res + s1 (res += s1) and increment i by 1 and rerurns the result.

• If the value of s1 is greater than or equal to s2 then we update res as res+s1(res += s1) and increment i by 1.

• else we update the value of res equal to res + s2 - s1(res = res + s2 - s1) and increment i by 2 (i+=2).

• Finally we return the result in variable res.

How to Run Solution

If we want to run this program, we should probably download a copy of roman_numeral_to_decimal in Python After that, we should make sure we have the latest Python interpreter. From there, we can run the following command in the terminal:

python roman_numeral_to_decimal.py "XIV"

Alternatively, we can copy the solution into an online Python interpreter and hit run then u need to modify sys.argv[i] to your inputs.

Further Reading

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