[SciPy-User] Best method to pick-up every N-th sample of an array

Matthieu Rigal rigal@rapideye...
Tue Aug 10 09:11:27 CDT 2010


Hi folks,

I could not find an appropriate function to pick-up every N-th value of an 
array into another one... for example :
[0,1,2,3,4,5] could return [0,2,4], for N = 2
Neither I could find help browsing the dev-lists

The functions numpy.choose and numpy.take may give correct results, but after 
having created the appropriate mask.. which may not be the best when you 
handle arrays containing hundred million of values, and I wouldn't be sure how 
to build it.

Therefore I wrote the following subfunction, but it is not very powerful:

def ReduceArray(x,y):
    x0=[]
    j=0
    for i in x:
        j=j+1
        if j==y:
            x0.append(i)
            j=0
    return numpy.array(x0)
    
Yes, using a simple Python list in between ... :-(((

So if you had some hints on which way to follow to have this computed quickly, 
on a flat or a ndarray, you will make someone happy :-)

Best regards,
Matthieu

PS : I apologize it this should have go to the numpy list, I was unsure...

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