check whether a string contains a character in python

Question

I am trying to compare and see if a particular character exists in the string, but when i try to access the character which is in another string, its throws an error 'argument of type 'int' is not iterable'. How do i access the character from a string without causing the error?

    def lengthOfLongestSubstring(self, s: str) -> int:
        longStrLen = 0
        totalStrLen = len(s)

        holderString = ""
        holderString += s[0]
        longStrLen = 0

        for i in range(1,totalStrLen-1):

            if s[i] not in holderString:
                holderString += s[i]
            else:
                if longStrLen < len(holderString):
                    longStrLen = len(holderString)
                holderString = 0

        return longStrLen

TypeError: argument of type 'int' is not iterable at Line 
if s[i] not in holderString:

Answer 1

Your problem is on this line:

holderString = 0

You reassigned the holderString variable to the integer 0. While strings can be iterated over, integers cannot. You attempt to iterate over the new integer on this line:

if s[i] not in holderString:

which causes the error.

There is a much better way to approach a function that returns the first repeated character though. Simply use the index() method:

def findChar(char, string):
    for c in string:
        if c == char: 
             return string.index(c)

Answer 2

It appears you only need to count unique characters until you find the first duplicate. You can do that with a set.

def longest_substring(s: str) -> int:
    seen = set()
    for c in s:
        if c in seen:
            break
        seen.add(c)
    return len(seen)