Explicit is better than implicit clear solution for CheckSum by veky - python coding challenges

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Explicit is better than implicit solution in Clear category for CheckSum by veky

def checkio(data):
    import string
    checksum = position = 0
    for character in reversed(data):
        if character in string.digits + string.ascii_uppercase:
            value = ord(character) - 48
            checksum += value if position%2 else sum(map(int, str(value*2)))
            position += 1
    return [str(-checksum%10), checksum]

Aug. 28, 2013

Comments:

David_de_Rivaz on Aug. 30, 2013, 7:36 p.m.
I like the (-checksum%10) instead of using a special case for 0.

veky on Aug. 30, 2013, 8:48 p.m.
Yeah, lots of people are afraid of using % on negative numbers, I guess because C leaves that undefined. But Guido was extra careful to make it work correctly. <a href="http://python-history.blogspot.com/2010/08/why-pythons-integer-division-floors.html">http://python-history.blogspot.com/2010/08/why-pythons-integer-division-floors.html</a> It would be a shame to let this effort go to waste. ;-) BTW I'm more proud of sum of digits. :-)

shreekar on Dec. 27, 2013, 7:35 a.m.
-checksum%10 for the last digit is cool. For smaller data this solution is fine, but for huge data, doing 'sum(map(int, str(value*2)))' repeatedly for the same character might be costly. To handle such cases, I used a dict to store the char and its mapping.

veky on Dec. 28, 2013, 7:27 a.m.
If you use Python for this stuff with huge data, then you have bigger problems. :-P Memoization is pretty standard technique (see Wikipedia for example), but sorry, you're doing it wrong. You're constructing a complicated data structure every time from scratch, penalizing what I would think to be much more frequent use case - a lot of calls to checksum for not so huge data. Sorry about the rant, but I hate when people think they understand Python performance. Guido himself doesn't understand it every time. :-P

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