The NumPy Fortran-ordering quiz

Charles R Harris charlesr.harris at gmail.com
Wed Oct 18 14:10:16 CDT 2006


On 10/18/06, Tim Hochberg <tim.hochberg at ieee.org> wrote:
>
> Charles R Harris wrote:
> >
> >
> > On 10/18/06, *Tim Hochberg* <tim.hochberg at ieee.org
> > <mailto:tim.hochberg at ieee.org>> wrote:
> >
> >     Charles R Harris wrote:
> >
> >     [SNIP]
> >     >
> >     > I'm not talking about the keyword in the ravel call, I'm talking
> >     about
> >     > the flag in a. The question is: do we *need* a fortran flag. I am
> >     > argueing not, because the only need is for fortran contiguous
> >     arrays
> >     > to pass to fortran function, or translation from fortran
> contiguous
> >     > arrays to numpy arrays. What I am saying is that things are
> >     > unnecessarily complicated. None of the LaPack stuff seems to use
> >     the
> >     > Fortran stuff, they just transpose and copy. I don't even think
> >     I want
> >     > to change that, because it is *clear* what is going on.
> >     Interfacing to
> >     > fortran is all about memory layout, nothing more or less.
> >     >
> >
> >     Chuck,
> >
> >     There are two things here. One is the order keyword and one is the
> >     FORTRAN flag. The latter is mainly an optimization for use at the
> >     C-level so that one doesn't have to check whether a given array is
> in
> >     contiguous FORTRAN order by examining the strides, in the same way
> >     that
> >     the CONTIGUOUS flag allows you to skip examining the strides when
> you
> >     need a contiguous C-order matrix.
> >
> >
> > That sounds like the two flags should be named f-contiguous and
> > c-contiguous. Then they would be orthogonal and one could have all
> > four combinations. Is that the case now? Perhaps I am misunderstanding
> > the meaning of the flags.
> That is the case now. The flag names simply mirror their values in C.
> Why they have those names in something of a historical accident I
> believe. Take a look at this:


OK, that is good. I no longer have any objection to the flags, I just wish
the names were more descriptive of what they mean. In fact, it looks like
the following sort of construction will be useful in the linalg module.

In [17]:a = array([[1,2],[3,4]], dtype=int)

In [18]:b = array(a, dtype=double, order='f')

In [19]:b.flags
Out[19]:
  CONTIGUOUS : False
  FORTRAN : True
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False

I've been a pain in the a** because I really want to know what is going on
down in the boiler room.

Chuck
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