[SciPy-User] From 1-D boolean array to integer index

josef.pktd@gmai... josef.pktd@gmai...
Tue Oct 11 16:11:49 CDT 2011


On Tue, Oct 11, 2011 at 4:38 PM, Jacob Biesinger
<jake.biesinger@gmail.com> wrote:
> Thanks for the ideas everyone.  Some clarification:  the 1d bool array
> represents the bits of the integer I'm interested in.  I have  binary
> observations in a vector of length ~10, so the integer index will be the
> particular combination of 1's and 0's in the vector.
> In this case, the array will not be very long (~10 elements) but the
> conversion needs to be fast (part of an inner loop).
> Here's some timings on your candidate suggestions:
> # setup
> size = 10
> b_array = sp.rand(size) < .5
> # eat's suggestion
> place_values =  2**sp.arange(size-1, -1, -1)
> %timeit (b_array * place_values).sum()
>  100000 loops, best of 3: 4.83 us per loop
> # Alok's suggestion
> bit_shift = sp.arange(size-1, -1, -1)
> %timeit sp.sum((1<<bits)[data])
> 100000 loops, best of 3: 7.58 us per loop
> # Oleksandr's suggestion
> string_format = size * '%d'
> %timeit int( string_format % tuple(b_array) ,2)
> 100000 loops, best of 3: 7.24 us per loop
> # my original idea
> %timeit int(''.join(['1' if e else '0' for e in b_array]), 2)
> 1000000 loops, best of 3: 1.49 us per loop
>
> I'm kinda surprised by the timings-- building a new list, doing a string
> join, then converting the string a an integer is faster than bit-shift &
> summation or multiplication & summation.  And it seems that python's string
> formatting operator is pretty efficient as well!  Just as fast as some of
> the other ops.
> Thanks again for the suggestions-- Guess I'll stick with my current
> (fastest) implementation.

the difference between python and using numpy would be that with eat's
solution you can do 1000000 in one operation instead of a 1000000
python loops.

Josef

> --
> Jake Biesinger
> Graduate Student
> Xie Lab, UC Irvine
>
>
>
> On Tue, Oct 11, 2011 at 12:25 PM, Alan G Isaac <alan.isaac@gmail.com> wrote:
>>
>> On 10/11/2011 1:29 PM, Jacob Biesinger wrote:
>> > I have a 1-D bool array which I'd like to convert to an integer index.
>>
>> Sounds like you may be able to just use your bool array:
>>
>>     >>> idx = np.array([True, False, True])
>>     >>> a = np.random.random(3)
>>     >>> a
>>     array([ 0.04818879,  0.49417941,  0.70470834])
>>     >>> a[idx]
>>     array([ 0.04818879,  0.70470834])
>>
>> hth,
>> Alan Isaac
>>
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