Error in Python program for converting roman numeral to integer

Question

    def romanToInt(self, s: str) -> int:
        num = 0
        lst = ["I","V","X","L","C","D","M"]
        dict = {"I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000}
        for i in range(len(s)):
            if lst.index(s[i]) >= lst.index(s[i+1]) or i == len(s)-1:
                num = num + dict[s[i]]
            else:
                num = num - dict[s[i]]
        return num
        

this is my code for converting roman numeral to integers

the program is triggering this error

IndexError: string index out of range

for line 6

Answer 1

    def romanToInt(self, s: str) -> int:
        num = 0
        lst = ["I", "V", "X", "L", "C", "D", "M"]

 ## I changed the var name dict to dictionary because using dict as a variable name 
 ## can lead to confusion and potential issues, as it may override the
 ## built-in dict type within the scope of your function.
        dictionary = {"I": 1, "V": 5, "X": 10, "L": 50, "C": 100, "D": 500, "M": 1000}

        for i in range(len(s) - 1):
            if lst.index(s[i]) >= lst.index(s[i + 1]):
                num += dictionary[s[i]]
            else:
                num -= dictionary[s[i]]

        num += dictionary[s[-1]]

        return num

The error is because you are trying to access s[i+1] in the loop when i is equal to len(s)-1. At that point, s[i+1] goes out of the string's bounds, resulting in an IndexError.

Answer 2

A simple fix to your solution would be to change:

if lst.index(s[i]) >= lst.index(s[i+1]) or i == len(s)-1:
                                           ^^^^^^^^^^^^^

to

if i == len(s)-1 or lst.index(s[i]) >= lst.index(s[i + 1]):
   ^^^^^^^^^^^^^

Because you first check i == len(s) - 1, if it's True, it won't execute the other part and you won't get index error.

---

As an alternative you can have those special numbers in dictionary (e.g 'IV','IX','XL','XC','CD','CM') and re-write it like:

roman_map = {
    "I": 1,
    "V": 5,
    "X": 10,
    "L": 50,
    "C": 100,
    "D": 500,
    "M": 1000,
    "IV": 4,
    "IX": 9,
    "XL": 40,
    "XC": 90,
    "CD": 400,
    "CM": 900,
}


def roman_to_int(s: str) -> int:
    i = 0
    result = 0
    while i < len(s):
        if i + 1 < len(s) and s[i : i + 2] in roman_map:
            result += roman_map[s[i : i + 2]]
            i += 2
        else:
            result += roman_map[s[i]]
            i += 1
    return result


print(roman_to_int("III"))     # 3
print(roman_to_int("CDXLIII")) # 443

• Try following the Python's naming convention(romanToInt).
• Do not shadow builtin names(dict).