Splitting string based on list values into a list in Python

Question

At the moment I'm trying to create a new list from a string, based on matching values from another list. For example my list of values is:

['IX', 'C', 'D', 'XL', 'I', 'XC', 'M', 'L', 'CD', 'X', 'IV', 'CM']

Which are all Roman numerals. How would I go about taking a Roman numeral:

MMIX

and splitting it to:

['M', 'M', 'IX']

based on the previous list.

Any help would be great!

Answer 1

I'm not sure this approach will work in general to parse roman numerals. For example, this code fails to properly parse VIII but that's because V isn't in the list of tokens. But here's a simple recursive function that looks for one of the tokens at the beginning of the input string and assembles a list:

tokens = ['IX', 'C', 'D', 'XL', 'I', 'XC', 'M', 'L', 'CD', 'X', 'IV', 'CM']

def rn_split(numeral, results_so_far=[]):
    if len(numeral)==0:
        return results_so_far # Break the recursion
    for token in tokens:
        if numeral.startswith(token):
            results_so_far.append(token)
            recurse_numeral = numeral[ (len(token)): ]
            return rn_split(recurse_numeral, results_so_far)
    # Remainder of numeral didn't match.  Bail out
    results_so_far.append(numeral)
    return results_so_far