CODEWITHSUNDEEP
pythonpython-3.x
durander
durander

Reputation: 1

Python-Indexing- I do not understand how this bit of code is working

What I do not understand is how indexing a string s = "azcbobobegghakl" would correctly give me the answer 'beggh' if I was looking for the longest substring in alphabetical order. How does this code tell the computer that 'beggh' is in alphabetical order? I thought indexing just gave each letter in the string a number that increased by one until the string ended? I tweaked the code a bit and it worked, but I do not actually understand how the code works in the first place. 

s = "azcbobobegghakl"
longest = s[0]
current = s[0]
for c in s[1:]:
    if c >= current[-1]:
        current += c
    else:
        if len(current) > len(longest):
            longest = current
        current = c
print "Longest substring in alphabetical order is:", longest

Upvotes: 0

Views: 78

Answers (3)

sbru
sbru

Reputation: 887

s = "azcbobobegghakl"

# initialize vars to first letter in string
longest = s[0]
current = s[0]

# start iterating through rest of string
for c in s[1:]:

    # c is the letter of current iteration and current[-1]
    # is the last letter of the current string. Every character 
    # has an ASCII value so this is checking the character's 
    # numerical values to see if they are in alphabetical order (a<b<c etc.)
    if c >= current[-1]:

        # so if c is bigger than current[-1] (meaning it comes
        # after in the alphabet, then we want to add it to current
        current += c
    else:
        # if c was smaller then we want to check if the alphabetic
        # string we just read is longer than the previous longest
        if len(current) > len(longest):
            longest = current

        # start the current string over beginning with the letter in c
        current = c
print "Longest substring in alphabetical order is:", longest

Upvotes: 1

mhawke
mhawke

Reputation: 87074

The expression c >= current[-1] lexicographically compares each character with its predecessor in the string. More simply:

>>> 'a' >= 'a'
True
>>> 'b' >= 'a'
True
>>> 'b' >= 'c'
False
>>> 'z' >= 'a'
True

So all the code is doing is to accumulate runs of characters that have each character >= the last one, and to keep the longest one seen.

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1121904

>= compares two characters by their lexicographical order; the expression

if c >= current[-1]:

tests if c (a single character), is the same or is 'greater' than current[-1] (another single character. Strings are ordered lexicographically by their codepoint value; a is smaller than b because the character a comes before b in the Unicode standard:

>>> 'a' > 'b'
False
>>> 'a' < 'b'
True

As such, characters are only added to current if they come 'later' in the alphabet than the last character of current. If they do not, then the sequence so far is tested for length, and a new substring is started from c.

Upvotes: 1

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