[SciPy-User] re[SciPy-user] moving for loops...

[josef.pktd at gmail.com]

On Fri, May 21, 2010 at 8:59 AM, mdekauwe <mdekauwe at gmail.com> wrote:
>
> Hi,
>
> I am trying to extract data from a 4D array and store it in a 2D array, but
> avoid my current usage of the for loops for speed, as in reality the arrays
> sizes are quite big. Could someone also try and explain the solution as well
> if they have a spare moment as I am still finding it quite difficult to get
> over the habit of using loops (C convert for my sins). I get that one could
> precompute the indices's i and j i.e.
>
> i = np.arange(tsteps)
> j = np.arange(numpts)
>
> but just can't get my head round how i then use them...
>
> Thanks,
> Martin
>
> import numpy as np
>
> numpts=10
> tsteps = 12
> vari = 22
>
> data = np.random.random((tsteps, vari, numpts, 1))
> new_data = np.zeros((tsteps, numpts), dtype=np.float32)
> index = np.arange(numpts)
>
> for i in xrange(tsteps):
>    for j in xrange(numpts):
>        new_data[i,j] = data[i,5,index[j],0]

The index arrays need to be broadcastable against each other.

I think this should do it

new_data = data[np.arange(tsteps)[:,None], 5, np.arange(numpts), 0]

Josef
>
>
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