Counting bits in large string / bit vector
__peter__ at web.de
Mon Sep 26 11:11:07 CEST 2011
bmacinnis at comcast.net wrote:
> In Perl I can create a large bit vector as follows:
> vec($bitmap,10000000,1) = 0; # this will create a bit string of all
> To set bits I may using commands like:
> vec($bitmap,1000, 1) = 1 # turn on bit 1000
> vec($bitmap,10000, 1) = 1 # turn on bit 10000
> vec($bitmap,1000000, 1) = 1 # turn on bit 10000000
> which would set three bits in the string $bitmap - Note: in perl I don't
> even have to declare $bitmap until I use it in the vec command.
> Anyway there is a very cool perl command which return the number of bits
> set in a large bitmap string instantly.
> The command is:
> $setbits = unpack("%32b*", $bitmap);
> Which in this case would store the number 3 in $setbits
> Is there an equivalent command in python that would immediately provide
> the number of set bits in a large bit vector/string
> I know I could loop through but when you have 10,000,000 or 100,000,000
> bits, the looping takes quite a while the perl unpack is instant.
>>> b = numpy.zeros(10**7, dtype=bool)
>>> for x in 3, 4, 6: b[10**x] = True
I don't know how it's implemented, but it seems to be quite fast.
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