On Tue, Jun 8, 2010 at 9:39 AM, David Goldsmith <d.l.goldsm...@gmail.com>wrote:
> On Tue, Jun 8, 2010 at 8:27 AM, Pavel Bazant <maxpla...@seznam.cz> wrote: > >> >> > > Correct me if I am wrong, but the paragraph >> > > >> > > Note to those used to IDL or Fortran memory order as it relates to >> > > indexing. Numpy uses C-order indexing. That means that the last index >> > > usually (see xxx for exceptions) represents the most rapidly changing >> memory >> > > location, unlike Fortran or IDL, where the first index represents the >> most >> > > rapidly changing location in memory. This difference represents a >> great >> > > potential for confusion. >> > > >> > > in >> > > >> > > http://docs.scipy.org/doc/numpy/user/basics.indexing.html >> > > >> > > is quite misleading, as C-order means that the last index changes >> rapidly, >> > > not the >> > > memory location. >> > > >> > > >> > Any index can change rapidly, depending on whether is in an inner loop >> or >> > not. The important distinction between C and Fortran order is how >> indices >> > translate to memory locations. The documentation seems correct to me, >> > although it might make more sense to say the last index addresses a >> > contiguous range of memory. Of course, with modern processors, actual >> > physical memory can be mapped all over the place. >> > >> > Chuck >> >> To me, saying that the last index represents the most rapidly changing >> memory >> location means that if I change the last index, the memory location >> changes >> a lot, which is not true for C-order. So for C-order, supposed one scans >> the memory >> linearly (the desired scenario), it is the last *index* that changes most >> rapidly. >> >> The inverted picture looks like this: For C-order, changing the first >> index >> leads to the most rapid jump in *memory*. >> >> Still have the feeling the doc is very misleading at this important issue. >> >> Pavel >> > > The distinction between your two perspectives is that one is using for-loop > traversal of indices, the other is using pointer-increment traversal of > memory; from each of your perspectives, your conclusions are "correct," but > my inclination is that the pointer-increment traversal of memory perspective > is closer to the "spirit" of the docstring, no? > > I think the confusion is in "most rapidly changing memory location", which is kind of ambiguous because a change in the indices is always a change in memory location if one hasn't used index tricks and such. So from a time perspective it means nothing, while from a memory perspective the largest address changes come from the leftmost indices. Chuck
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