[Numpy-discussion] Raveling, reshape order keyword unnecessarily confuses index and memory ordering

josef.pktd@gmai... josef.pktd@gmai...
Thu Apr 4 14:54:22 CDT 2013


On Thu, Apr 4, 2013 at 3:40 PM, Matthew Brett <matthew.brett@gmail.com> wrote:
> Hi,
>
> On Thu, Apr 4, 2013 at 11:45 AM, Matthew Brett <matthew.brett@gmail.com> wrote:
>> Hi,
>>
>> On Thu, Apr 4, 2013 at 9:21 AM, Chris Barker - NOAA Federal
>> <chris.barker@noaa.gov> wrote:
>>> On Wed, Apr 3, 2013 at 6:13 PM, Matthew Brett <matthew.brett@gmail.com> wrote:
>>>>> We all agree that 'order' is used with two different and orthogonal
>>>>> meanings in numpy.
>>
>> Brief thank you for your helpful and thoughtful discussion.
>>
>>> well, not entirely orthogonal -- they are the some concept, used in
>>> different contexts,
>>
>> Here's a further clarification, in the hope that it is helpful:
>>
>> Input and output index orderings are orthogonal - I can read the data
>> with C index ordering and return an array that is index ordered
>> any-old-how.
>>
>> F and C are used in the sense of F contiguous and C contiguous - where
>> contiguous is not the same concept as index ordering.
>>
>> So I think it's hard to say these concepts are not orthogonal, simply
>> in the technical sense that order='F" could mean:
>>
>> * read my data using F-style index ordering
>> * return my data in an array using F-style index ordering
>> * (related to above) return my data in F-contiguous memory layout
>
> Sorry this is not well-put and should increase confusion rather than
> decrease it.  I'll try again if I may.
>
> What do we mean by 'Fortran' 'order'.
>
> Two things :
>
> * np.array(a, order='F') - Fortran contiguous : the array memory is
> contiguous, the strides vector is strictly increasing
> * np.ravel(a, order='F') - first-to-last index ordering used to
> recover values from the array
>
> They are related in the sense that Fortran contiguous layout in memory
> means that returning the elements as stored in memory gives the same
> answer as first to last index ordering.  They are different in the
> sense that first-to-last index ordering applies to any memory layout -
> is orthogonal to memory layout.   In particular 'contiguous' has no
> meaning for first-to-last or last-to-first index ordering.
>
> So - to restate in other words - this :
>
> np.reshape(a, (3, 4), order='F')
>
> could reasonably mean one of two orthogonal things
>
> 1) Retrieve data from the array using first-to-last indexing, return
> any memory layout you like
> 2) Retrieve data from the array using the default last-to-first index
> ordering, and return memory in F-contiguous layout

no to interpretation 2)
reshape and ravel (in contrast to flatten) just return a view (if possible)
(with possible some strange strides)

docstring:
"
numpy.reshape(a, newshape, order='C')
Gives a new shape to an array without changing its data
"

functions that return views versus functions that create new arrays

Josef

>
> Cheers,
>
> Matthew
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion


More information about the NumPy-Discussion mailing list