Check if an input is a valid roman numeral

Question

I got a program that converts Roman numerals to integers and vice versa. My problem is that I don´t really know how to create a function that checks if the user input is a v

Answer 1

Writing a converter from ints to Romans is a standard interview question. I once wrote the following bi-directional implementation (toString-- decimal to Roman; parse -- Roman to decimal). The implementaion saticifies a number of additional criteria on the representation of Roman numbers, which are not obligatory, but generally followed:

'''
Created on Feb 7, 2013

@author: olegs
'''

ROMAN_CONSTANTS = (
            ( "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" ),
            ( "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" ),
            ( "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" ),
            ( "", "M", "MM", "MMM", "",   "",  "-",  "",    "",     ""   ),
        )

ROMAN_SYMBOL_MAP = dict(I=1, V=5, X=10, L=50, C=100, D=500, M=1000)

CUTOFF = 4000
BIG_DEC = 2900
BIG_ROMAN = "MMCM"
ROMAN_NOUGHT = "nulla"

def digits(num):
    if num < 0:
        raise Exception('range error: negative numbers not supported')
    if num % 1 != 0.0:
        raise Exception('floating point numbers not supported')
    res = []
    while num > 0:
        res.append(num % 10)
        num //= 10
    return res

def toString(num, emptyZero=False):
    if num < CUTOFF:
        digitlist = digits(num)
        if digitlist:
            res = reversed([ ROMAN_CONSTANTS[order][digit] for order, digit in enumerate(digitlist) ])
            return "".join(res)
        else:
            return "" if emptyZero else ROMAN_NOUGHT 
    else:
        if num % 1 != 0.0:
            raise Exception('floating point numbers not supported')
        # For numbers over or equal the CUTOFF, the remainder of division by 2900
        # is represented as above, prepended with the multiples of MMCM (2900 in Roman),
        # which guarantees no more than 3 repetitive Ms.
        return BIG_ROMAN * (num // BIG_DEC) + toString(num % BIG_DEC, emptyZero=True)

def parse(numeral):
    numeral = numeral.upper()
    result = 0
    if numeral == ROMAN_NOUGHT.upper():
        return result
    lastVal = 0
    lastCount = 0
    subtraction = False
    for symbol in numeral[::-1]:
        value = ROMAN_SYMBOL_MAP.get(symbol)
        if not value:
            raise Exception('incorrect symbol')
        if lastVal == 0:
            lastCount = 1
            lastVal = value
        elif lastVal == value:
            lastCount += 1
            # exceptions
        else:
            result += (-1 if subtraction else 1) * lastVal * lastCount
            subtraction = lastVal > value
            lastCount = 1
            lastVal = value
    return result + (-1 if subtraction else 1) * lastVal * lastCount