[Numpy-discussion] Need help for implementing a fast clip in numpy (was slow clip)

Charles R Harris charlesr.harris at gmail.com
Fri Jan 12 01:21:42 CST 2007


On 1/11/07, David Cournapeau <david at ar.media.kyoto-u.ac.jp> wrote:
>
> Travis Oliphant wrote:
> >
> > This is one thing I've exposed (and made use of in more than one place)
> > with NumPy.  In Numeric, the magic was in a few lines of the ufuncobject
> > file).  Now, it is exposed in the concept of an array iterator.  Anybody
> > can take advantage of it as it there is a C-API call to get an array
> > iterator from the array (it's actually the object returned by the .flat
> > method).   You can even get an iterator that iterates over one-less
> > dimension than the array has (with the dimension using the smallest
> > strides left "un-iterated" so that you can call an inner loop with it).
> The thing which confuses me is whether this is useful when you only one
> item of one array at a time. When I was implementing some functions for
> LPC, I took a look at your examples for array iterators and explanations
> in the numpy ebook, and it looked really helpful, indeed. For this kind
> of code, I needed to operate on several contiguous elements at a time.
>
> But here, for cliping with scalar min and max, I only need to access to
> one item at a time from the input array, and that's it; in particular, I
> don't care about the order of iteration. So the question really boils
> down to:
>
> "for a numpy array a of eg float32, am I guaranteed that
> a->data[sizeof(float32) * i] for 0 <= i < a.size gives me all the items
> of a, even for non contiguous arrays ?"


No. That is what the array iterator is for.

Chuck
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