finding the longest substring of letters in alphabetical order in a given string

Question

When executing my code for the given task, I keep getting the longest string plus the next letter in the iteration. For example, if I use

s = \'azcbobobeggh         


        

Answer 1

The problem here is that result and final point to the same list. You are probably thinking that += will create a new list when you issue result += letters, but it won't:

>>> x = [1,2]
>>> y = x
>>> x += [3]
>>> x
[1, 2, 3]
>>> y
[1, 2, 3]
>>> x is y
True

However, when you use x = x + [3]:

>>> x = [1,2]
>>> y = x
>>> x = x + [3]
>>> x
[1, 2, 3]
>>> y
[1, 2]
>>> x is y
False

For an explanation of this behavior, see this question. This is what's happening in your for loop (edit: of your original code) when letters is the last a character in your string:

1. at the beginning, final and result both point to ['b', 'e', 'g', 'g', 'h'].
2. after result += 'a' final and result both point to ['b', 'e', 'g', 'g', 'h', 'a'].
3. now the elif block is entered and result will point to a new list ['a'], while final still points to ['b', 'e', 'g', 'g', 'h', 'a'].
4. final will never be updated again after this

Hence, your original code (before you edited it) can be fixed by changing

result += letters

to

result = result + [letters]:

s = 'azcbobobegghakl'
result = []
final = []
for letters in s:
    result = result + [letters]        
    if result == sorted(result) and len(result) >= len(final):
        final=result            
    elif result != sorted(result):
        result = [result[len(result)-1]]        

print(final)