Discussion on: Convert Roman Numerals to Integers

Special casing all those checks is asking for problems though, and I'm pretty sure it's not that neat. If we assume that the input is a valid Roman numeral (which this code does do; "IIX" is not a valid Roman numeral, but your code will parse it as 10), then the rule is that "if a numeral is smaller than the next numeral, subtract its value". Using that rule gives a much neater function:

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

def roman_to_int(input_string: str) -> int:
    ints = [vals[x] for x in input_string] + [0]
    output = 0
    for current, next in zip(ints, ints[1:]):
        output += -current if current < next else current
    return output

(Apologies for using Python, but I don't trust my JS enough to write it error free) (Edited to add the vals dict so the code actually runs)