Converting Roman Numerals to integers in python

Question

This is now my current code after what user2486 said.

def romanMap():
    map=((\"M\",  1000),(\"CM\", 900),(\"D\",  500),(\"CD\", 400),(\"C\",  100),(\"XC\"         


        

Answer 1

There is a very detailed description of the development of the Roman numeral converters in the Dive Into Python 3 by Mark Pilgrim. See the 5.3. Case Study: Roman Numerals that introduces the problem and details.

But that is not all. See the Chapter 9. Unit Testing where the analysis and the implementation of Roman numeral converters continues, including interesting optimization and exception throwing -- the (unit) test driven development.

It is directly related to the enginefree's reference to the code in the first comment below the question (the code was written by Mark Pilgrim).

Answer 2

I know this is an old post, but I would like to add 3 solutions to convert roman numerals to numbers.

Solution 1: (Approx Runtime = 52ms)

def romanToInt(self, s: str) -> int:

     roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }    
     num = 0

     for i in range(len(s)):

        if i!= len(s)-1 and roman[s[i]] < roman[s[i+1]]:
             num += roman[s[i]]*-1
        else:
             num += roman[s[i]]

      return num

Solution 2: (Approx Runtime = 60ms)

def romanToInt(self, s: str) -> int:

     roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }    
     num = 0

     s = s.replace("IV", "IIII").replace("IX", "VIIII")
     s = s.replace("XL", "XXXX").replace("XC", "LXXXX")
     s = s.replace("CD", "CCCC").replace("CM", "DCCCC")

     for x in s:
        num += roman[x]

     return num

Solution 3: (Approx Runtime = 48ms)

def romanToInt(self, s: str) -> int:

     roman = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000 }    
     num = 0

     for i in range(len(s)-1):
        if roman[s[i]] < roman[s[i+1]]:
            num += roman[s[i]]*-1
            continue

         num += roman[s[i]]

      num +=roman[s[-1]]

      return num

The simplest solution appears to be the best at times :)

Answer 3

Here is my solution:

numerals = [
        {'letter': 'M', 'value': 1000},
        {'letter': 'D', 'value': 500},
        {'letter': 'C', 'value': 100},
        {'letter': 'L', 'value': 50},
        {'letter': 'X', 'value': 10},
        {'letter': 'V', 'value': 5},
        {'letter': 'I', 'value': 1},
    ]

def arabic_to_roman(number):
    remainder = number
    result = ''
    for numeral_index in xrange(len(numerals)):
        numeral = numerals[numeral_index]
        next_numeral = numerals[numeral_index + 1] if numeral_index + 1 < len(numerals) else None

        factor = remainder / numeral['value']
        remainder -= factor * numeral['value']

        if next_numeral:
            numeral_difference = numeral['value'] - next_numeral['value']
            if (remainder - numeral_difference >= 0) and (numeral_difference > next_numeral['value']):
                result += next_numeral['letter'] + numeral['letter']
                remainder -= numeral_difference

        if factor > 0:
            result += numeral['letter'] * factor

    return result


def roman_to_arabic(number):
    index_by_letter = {}
    for index in xrange(len(numerals)):
        index_by_letter[numerals[index]['letter']] = index

    result = 0
    previous_value = None
    for letter in reversed(number):
        index = index_by_letter[letter]
        value = numerals[index]['value']
        if (previous_value is None) or (previous_value <= value):
            result += value
        else:
            result -= value
        previous_value = value

    return result

Answer 4

A nice compact version with no external libraries:

def rn_to_int(s):
  d = {'m': 1000, 'd': 500, 'c': 100, 'l': 50, 'x': 10, 'v': 5, 'i': 1}
  n = [d[i] for i in s.lower() if i in d]
  return sum([i if i>=n[min(j+1, len(n)-1)] else -i for j,i in enumerate(n)])


for numeral, expected in [['CLXIV', 164], ['MDCCLXXXIII', 1783], ['xiv', 14]]:
  assert rn_to_int(numeral) == expected

Answer 5

Work from right to left of the roman numeral to add or subtract values. Easy.

def rome(roman_num):
     d = {'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000}
     nl = list(roman_num)
     sum = d[nl[len(nl)-1]]
     for i in range(len(nl)-1,0,-1):
             if d[nl[i]]>d[nl[i-1]]:
                     sum -= d[nl[i-1]]
             else:
                     sum += d[nl[i-1]]       
     return sum