[Numpy-discussion] reduce array by computing min/max every n samples

Brad Buran bburan@cns.nyu....
Mon Jun 21 14:09:26 CDT 2010


This would certainly be useful in my case as well.  I originally tried doing
something similar:

fun = lambda x: (x.min(), x,max())
apply_along_axis(fun, -1, val_pts)

It turned out to be much slower, which I guess isn't too surprising.

Brad

On Sat, Jun 19, 2010 at 4:45 PM, Warren Weckesser <
warren.weckesser@enthought.com> wrote:

> Benjamin Root wrote:
> > Brad, I think you are doing it the right way, but I think what is
> > happening is that the reshape() call on the sliced array is forcing a
> > copy to be made first.  The fact that the copy has to be made twice
> > just worsens the issue.  I would save a copy of the reshape result (it
> > is usually a view of the original data, unless a copy is forced), and
> > then perform a min/max call on that with the appropriate axis.
> >
> > On that note, would it be a bad idea to have a function that returns a
> > min/max tuple?
>
> +1.  More than once I've wanted exactly such a function.
>
> Warren
>
>
> >   Performing two iterations to gather the min and the max information
> > versus a single iteration to gather both at the same time would be
> > useful.  I should note that there is a numpy.ptp() function that
> > returns the difference between the min and the max, but I don't see
> > anything that returns the actual values.
> >
> > Ben Root
> >
> > On Thu, Jun 17, 2010 at 4:50 PM, Brad Buran <bburan@cns.nyu.edu
> > <mailto:bburan@cns.nyu.edu>> wrote:
> >
> >     I have a 1D array with >100k samples that I would like to reduce by
> >     computing the min/max of each "chunk" of n samples.  Right now, my
> >     code is as follows:
> >
> >     n = 100
> >     offset = array.size % downsample
> >     array_min = array[offset:].reshape((-1, n)).min(-1)
> >     array_max = array[offset:].reshape((-1, n)).max(-1)
> >
> >     However, this appears to be running pretty slowly.  The array is data
> >     streamed in real-time from external hardware devices and I need to
> >     downsample this and compute the min/max for plotting.  I'd like to
> >     speed this up so that I can plot updates to the data as quickly as
> new
> >     data comes in.
> >
> >     Are there recommendations for faster ways to perform the
> downsampling?
> >
> >     Thanks,
> >     Brad
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> >
> >
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