13 operations creative solution for Binary Count by aureooms - python coding challenges

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13 operations solution in Creative category for Binary Count by aureooms

def checkio ( v ) :

    """
        https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
    """

    v = v - ( ( v >> 1 ) & 0x55555555 )
    v = ( v & 0x33333333 ) + ( ( v >> 2 ) & 0x33333333 )
    return ( ( ( v + ( v >> 4 ) & 0xF0F0F0F ) * 0x1010101 ) & 0xFFFFFFFF ) >> 24

April 7, 2015

Comments:

veky on Oct. 11, 2017, 2:22 a.m.
This is really wrong, and shouldn't be "Editor's choice", except as an example of how not to do it. <pre class='brush: python'>>>> checkio(int('1'*89, 2)) 32 </pre> And the link (which really shouldn't go into the docstring, but into a comment - I am appalled at how often people confuse docstrings and comments) nicely says in the first line <pre class='brush: python'>unsigned int v; // count bits set in this (32-bit value) ^^^^^^ </pre> Python has given up on that particular atavism about 8 years ago. (For a good reason. In C, `int` means "what you use to index an array" [and `char` means "smallest addressable memory unit"... and don't get me started on what `void` means:-P. C likes to redefine things for its own purposes]. In Python, `int` means "what you use to count things". If it cannot count the number of people currently alive, it is not an int.)

aureooms on Oct. 11, 2017, 9:27 a.m.
<blockquote> Precondition: 0 < number ≤ 2^32 </blockquote> Should probably add <pre class='brush: python'>if v == 2**32: return 1 </pre> as the first instruction

veky on Oct. 11, 2017, 1:26 p.m.
Ah, the precondition is below the fold. :-) (When you click "show less", it's not shown.) And I'd bet the precondition wasn't there when I solved this task. Anyway, yes, the wrong open-closedness of that interval (on both sides:) is really mind-boggling. Dijkstra would be very angry. :-D

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