[Numpy-discussion] Complex slicing and take

Eric Emsellem eemselle@eso....
Wed Dec 30 02:08:01 CST 2009


Hi

thanks for the tips. Unfortunately this is not what I am after.

>> > ? import numpy as num
>> > ? startarray = random((1000,100))
>> > ? take_sample = [1,2,5,6,1,2]
>> > ? temp = num.take(startarray,take_sample,axis=1)
> Would it help to make temp a 1000x4 array instead of 1000x6? Could you
> do that by changing take_sample to [1,2,5,6] and multiplying columns 1
> and 2 by a factor of 2? That would slow down the construction of temp
> but speed up the addition (and slicing?) in the loop below.

No it wouldn't help unfortunately, because the second instance of "1,2" would 
have different shifts. So I cannot just count the number of occurrence of each line.

 From the initial 2D array, 1D lines could be extracted several times, with each 
time a different shift.

>> > ? shift = [10,20,34,-10,22,-20]
>> > ? result = num.zeros(900) ?# shorter than initial because of the shift
>> > ? for i in range(len(shift)) :
>> > ? ? ?result += temp[100+shift[i]:-100+shift[1]]
> This looks fast to me. The slicing doesn't make a copy nor does the
> addition. I've read that cython does fast indexing but I don't know if
> that applies to slicing as well. I assume that shift[1] is a typo and
> should be shift[i].

(yes of course the shift[1] should be shift[i])
Well this may be fast, but not fast enough. And also, starting from my 2D 
startarray again, it looks odd that I cannot do something like:

startarray = random((1000,100))
take_sample = [1,2,5,6,1,2]
shift = [10,20,34,-10,22,-20]
result = num.sum(num.take(startarray,take_sample,axis=1)[100+shift:100-shift])

but of course this is nonsense because I cannot address the data this way (with 
"shift").

In fact I realise now that my question is simpler: how do I extract and sum 1d 
lines from a 2D array if I want first each line to be "shifted". So starting 
again now, I want a quick way to write:

startarray = random((1000,6))
shift = [10,20,34,-10,22,-20]
result = num.zeros(1000, dtype=float)
for i in len(shift) :
    result += startarray[100+shift[i]:900+shift[i]]


Can I write this directly with some numpy indexing without the loop in python?

thanks for any tip.

Eric




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