Variable scoping of “for i in range(XX)” in Python

Question

This is a exercise for Python, and I am confused about the variable scoping in Python.

"Return True if the given string contains an appearance of "xyz" where the xyz is not directly preceeded by a period (.). So "xxyz" counts but "x.xyz" does not.

xyz_there('abcxyz') → True
xyz_there('abc.xyz')→ False xyz_there('xyz.abc') → True"

This is my answer:

def xyz_there(str):
   for i in range(len(str)-2):
        if str[i]=='.':
           i+=1
           continue
        elif str[i:i+3]=='xyz':
           return True
   return False

And it is wrong. xyz_there('abc.xyz') → False will always return True. Because the variable i will always be 0,1,2.... And the i+=1 doesn't mean anything. Why???

Answer 1

It's not that you can't change the value of i. You can. The trick here is that you are iterating over the contents of the return value of range. Each time the loop resets you get the next value from that iterable, it does not increment the value of i to progress the loop.

Answer 2

Are you required to use a for loop? This would be a great place for a regular expression. I've modified the example below so I'm not just giving you the answer... Instead of looking for "xyz", I'm looking for "hello". And I'm also trying to avoid seeing a "?" before that "hello".

import re
def hello_there(str):
    if re.match("(.*[^\?])?hello.*", str):
        return True
    return False

The regex pattern consists of:

• .* - any number of non-newline characters at the beginning of the string;
• [^\?] - disallowing ^ a(n escaped) question mark \? before everything that follows;
• hello - ensuring that hello will make an appearance;
• .* - and ending in any number of other characters.

re.match will return a MatchObject that resolves to a boolean value of True, so it can be used as the condition in the if statement.

>>> hello_there("foohellobar")
True
>>> hello_there("foo?hellobar")
False
>>> hello_there("foohello?bar")
True

Answer 3

simplest answer is:

def xyz_there(str):
    for i in range(len(str)-2):
        if str[i:i+3] == "xyz" and str[i-1] != ".":
            return True
    return False