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

Call romantoint after the for loop

 def checkIfRomanNumeral(numeral):
     """Controls that the userinput only contains valid roman numerals"""
     numeral = numeral.upper()
     validRomanNumerals = ["M", "D", "C", "L", "X", "V", "I"]
     for letters in numeral:
        if letters not in validRomanNumerals:
            print("Sorry that is not a valid roman numeral")
            return False
     romanToInt(numeral)

Answer 2

Apart from the design problems that have already been pointed out, I'd like to just answer the question why your for-loop doesn't go through all the numerals

If the entries are considered valid by your code, then the loop goes into the elif clause where it calls romanToInt(numeral)and then break. There's your problem: break take that out.

Illustration: As soon as condition is met in this example, the loop will stop going through i in list

for i in list:
   # do something
   if condition:
       break # "Stop the innermost loop now!"

Answer 3

Instead of looping, you can convert both input and valid literals to sets and then substract them:

def checkIfRomanNumeral(numeral):
    numeral = {c for c in numeral.upper()}
    validRomanNumerals = {c for c in "MDCLXVI()"}
    return not numeral - validRomanNumerals

Returns True if numeral is valid, False otherwise. (Assuming that the empty string is valid)

Answer 4

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

Answer 5

def checkIfRomanNumeral(numeral):
"""Controls that the userinput only contains valid roman numerals"""
    numeral = numeral.upper()
    validRomanNumerals = ["M", "D", "C", "L", "X", "V", "I", "(", ")"]
    valid = True
    for letters in numeral:
        if letters not in validRomanNumerals:
            print("Sorry that is not a valid roman numeral")
            valid = False
            break
    return valid

Returns a boolean whether the given 'numeral' is roman numeral or not.