[Numpy-discussion] convert strides/shape/offset into nd index?

James Bergstra bergstrj at iro.umontreal.ca
Mon Nov 30 22:04:44 EST 2009

Your question involves a few concepts:

- an integer vector describing the position of an element

- the logical shape (another int vector)

- the physical strides (another int vector)

Ignoring the case of negative offsets, a physical offset is the inner
product of the physical strides with the position vector.

In these terms, you are asking how to solve the inner-product equation
for the position vector.  There can be many possible solutions (like,
if there is a stride of 1, then you can make that dimension account
for the entire offset.  This is often not the solution you want.).
For valid ndarrays though, there is at most one solution though with
the property that every position element is less than the shape.

You will also need to take into account that for certain stride
vectors, there is no way to get certain offsets.  Imagine all the
strides were even, and you needed to get at an odd offset... it would
be impossible.  It would even be impossible if there were a dimension
with stride 1 but it had shape of 1 too.

I can't think of an algorithm off the top of my head that would do
this in a quick and elegant way.


On Sun, Nov 29, 2009 at 10:36 AM, Zachary Pincus
<zachary.pincus at yale.edu> wrote:
> Hi all,
> I'm curious as to what the most straightforward way is to convert an
> offset into a memory buffer representing an arbitrarily strided array
> into the nd index into that array. (Let's assume for simplicity that
> each element is one byte...)
> Does sorting the strides from largest to smallest and then using
> integer division and mod (in the obvious way) always work? (I can't
> seem to find a counterexample, but I am not used to thinking too
> deeply about bizarrely-strided configurations.) Is there a method that
> doesn't involve sorting?
> Thanks,
> Zach
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