[Numpy-discussion] Vectorizing computation

Francesc Alted faltet at gmail.com
Fri Feb 13 07:32:57 EST 2015


2015-02-13 13:25 GMT+01:00 Julian Taylor <jtaylor.debian at googlemail.com>:

> On 02/13/2015 01:03 PM, Francesc Alted wrote:
> > 2015-02-13 12:51 GMT+01:00 Julian Taylor <jtaylor.debian at googlemail.com
> > <mailto:jtaylor.debian at googlemail.com>>:
> >
> >     On 02/13/2015 11:51 AM, Francesc Alted wrote:
> >     > Hi,
> >     >
> >     > I would like to vectorize the next computation:
> >     >
> >     > nx, ny, nz = 720, 180, 3
> >     > outheight = np.arange(nz) * 3
> >     > oro = np.arange(nx * ny).reshape((nx, ny))
> >     >
> >     > def compute1(outheight, oro):
> >     >     result = np.zeros((nx, ny, nz))
> >     >     for ix in range(nx):
> >     >         for iz in range(nz):
> >     >             result[ix, :, iz] = outheight[iz] + oro[ix, :]
> >     >     return result
> >     >
> >     > I think this should be possible by using an advanced use of
> >     broadcasting
> >     > in numpy.  Anyone willing to post a solution?
> >
> >
> >     result = outheight + oro.reshape(nx, ny, 1)
> >
> >
> > And 4x faster for my case.  Oh my, I am afraid that my mind will never
> > scratch all the amazing possibilities that broadcasting is offering :)
> >
> > Thank you very much for such an elegant solution!
> >
>
>
> if speed is a concern this is faster as it has a better data layout for
> numpy during the computation, but the result may be worse layed out for
> further processing
>
>     result = outheight.reshape(nz, 1, 1) + oro
>     return np.rollaxis(result, 0, 3)
>
>
Holly cow, this makes for another 4x speed improvement!  I don't think I
need that much in my scenario, so I will stick with the first one (more
readable and the expected data layout), but thanks a lot!

Francesc
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