bit count or bit set && Python3

[Charles Hixson]

On 10/25/2012 08:57 AM, Steven D'Aprano wrote:
> On Fri, 26 Oct 2012 02:31:53 +1100, Chris Angelico wrote:
>
>> On Fri, Oct 26, 2012 at 2:25 AM, Christian Heimes<christian at python.org>
>> wrote:
>>> Simple, easy, faster than a Python loop but not very elegant:
>>>
>>>     bin(number).count("1")
>> Unlikely to be fast.
> Oh I don't know about that. Here's some timing results using Python 2.7:
>
> py>  from timeit import Timer
> py>  t = Timer('bin(number).count("1")', setup='number=2**10001-1')
> py>  min(t.repeat(number=10000, repeat=7))
> 0.6819710731506348
>
> Compare to MRAB's suggestion:
>
> def count_set_bits(number):
>       count = 0
>       while number:
>           count += 1
>           number&= number - 1
>       return count
>
> py>  t = Timer('count_set_bits(number)',
> ...     setup='from __main__ import count_set_bits; number=2**10001-1')
> py>  min(t.repeat(number=100, repeat=7))
> 4.141788959503174
>
>
> That makes the "inelegant" solution using bin() and count() about 600
> times faster than the mathematically clever solution using bitwise
> operations.
>
> On the other hand, I'm guessing that PyPy would speed up MRAB's version
> significantly.
>
>
>
Really nice and good to know.  I had guessed the other way.   (As you 
point out this is compiler dependent, and I'll be using Python3, 
but...conversion from an int to a bit string must be a *lot* faster than 
I had thought.)

-- 
Charles Hixson