Comparing characters in a string sequentially in Python

Question

I am trying to figure out how to compare a character in a string to the next character in the string. For example, if I have a string:

s = \'vzcbotdebobeggg         


        

Answer 1

How about this:

def longest_ascending(s):
    matches = []
    current = [s[0]]
    for index, character in enumerate(s[1:]):
        if character >= s[index]:
            current.append(character)
        else:
            matches.append(current)
            current = [character]
    matches.append(current)
    return "".join(max(matches, key=len))

Explanation:

• matches is a list of all substrings with "ascending" characters.
• current is the substring of ascending characters being built as we iterate through the string. We start out with the first character of the string.
• We now iterate through the remaining string character by character. enumerate() helps us keep track of the index of the previous character (because the enumeration starts at 0 and we iterate the string from the second character onwards).
• If the current character is "greater or equal" to the previous one, we add it to the current substring and move on.
• If not, we add the current substring to the list of substrings, and seed the next substring with the current character.
• After the iteration is over, don't forget to add the current substring to the list as well.

Answer 2

This is less obvious, but seems to work:

seq = "vzcbotdebobeggglakyl"

import itertools
result = max(
    (
        list(next(sub)) + [b for a, b in sub]
        for ascending, sub in itertools.groupby(zip(seq,seq[1:]), lambda x: x[0] <= x[1])
        if ascending
    ),
    key=len
)

print ''.join(result)