Letters have an order

Letters line up in the order of the alphabet, as in a dictionary. So you can ask which of two letters comes first. The letter b comes after a, and c comes after b. You can compare letters with less than and greater than, just like numbers.

A word is in alphabetical order when no letter comes before the one in front of it. "abbey" is in order, but "abca" is not, because the last a comes before the c.

Letters compare: in the order of the alphabet.

In code

"a" < "b"      # True
"c" < "b"      # False

Check each pair of neighbours

To check a whole word, look at each letter and the one just before it. Start from the second letter, because the first has nobody in front of it. If a letter comes before its neighbour, the word is out of order, and you can say no right away.

If you get to the end without finding a problem, the answer is yes. A word of one letter, or no letters, is in order, because there is nothing to compare.

Start at the second letter, so there is always one before it.

In code

for i in range(1, len(s)):
    if s[i] < s[i - 1]:
        return False
return True

Is "abca" in order?

beforenow

now: b
before: a

Compare b with the letter before it, a. b doesn't come before a, so that's fine.

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