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

Sebastian Berg sebastian@sipsolutions....
Sat Mar 30 13:55:23 CDT 2013


On Fri, 2013-03-29 at 19:08 -0700, Matthew Brett wrote:
> Hi,
> 
> We were teaching today, and found ourselves getting very confused
> about ravel and shape in numpy.
> 
> Summary
> --------------
> 
> There are two separate ideas needed to understand ordering in ravel and reshape:
> 
> Idea 1): ravel / reshape can proceed from the last axis to the first,
> or the first to the last.  This is "ravel index ordering"
> Idea 2) The physical layout of the array (on disk or in memory) can be
> "C" or "F" contiguous or neither.
> This is "memory ordering"
> 
> The index ordering is usually (but see below) orthogonal to the memory ordering.
> 
> The 'ravel' and 'reshape' commands use "C" and "F" in the sense of
> index ordering, and this mixes the two ideas and is confusing.
> 
> What the current situation looks like
> ----------------------------------------------------
> 
> Specifically, we've been rolling this around 4 experienced numpy users
> and we all predicted at least one of the results below wrongly.
> 
> This was what we knew, or should have known:
> 
> In [2]: import numpy as np
> 
> In [3]: arr = np.arange(10).reshape((2, 5))
> 
> In [5]: arr.ravel()
> Out[5]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> 
> So, the 'ravel' operation unravels over the last axis (1) first,
> followed by axis 0.
> 
> So far so good (even if the opposite to MATLAB, Octave).
> 
> Then we found the 'order' flag to ravel:
> 
> In [10]: arr.flags
> Out[10]:
>   C_CONTIGUOUS : True
>   F_CONTIGUOUS : False
>   OWNDATA : False
>   WRITEABLE : True
>   ALIGNED : True
>   UPDATEIFCOPY : False
> 
> In [11]: arr.ravel('C')
> Out[11]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> 
> But we soon got confused.  How about this?
> 
> In [12]: arr_F = np.array(arr, order='F')
> 
> In [13]: arr_F.flags
> Out[13]:
>   C_CONTIGUOUS : False
>   F_CONTIGUOUS : True
>   OWNDATA : True
>   WRITEABLE : True
>   ALIGNED : True
>   UPDATEIFCOPY : False
> 
> In [16]: arr_F
> Out[16]:
> array([[0, 1, 2, 3, 4],
>        [5, 6, 7, 8, 9]])
> 
> In [17]: arr_F.ravel('C')
> Out[17]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> 
> Right - so the flag 'C' to ravel, has got nothing to do with *memory*
> ordering, but is to do with *index* ordering.
> 
> And in fact, we can ask for memory ordering specifically:
> 
> In [22]: arr.ravel('K')
> Out[22]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> 
> In [23]: arr_F.ravel('K')
> Out[23]: array([0, 5, 1, 6, 2, 7, 3, 8, 4, 9])
> 
> In [24]: arr.ravel('A')
> Out[24]: array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> 
> In [25]: arr_F.ravel('A')
> Out[25]: array([0, 5, 1, 6, 2, 7, 3, 8, 4, 9])
> 
> There are some confusions to get into with the 'order' flag to reshape
> as well, of the same type.
> 
> Ravel and reshape use the tems 'C' and 'F" in the sense of index ordering.
> 
> This is very confusing.  We think the index ordering and memory
> ordering ideas need to be separated, and specifically, we should avoid
> using "C" and "F" to refer to index ordering.
> 
> Proposal
> -------------
> 
> * Deprecate the use of "C" and "F" meaning backwards and forwards
> index ordering for ravel, reshape
> * Prefer "Z" and "N", being graphical representations of unraveling in
> 2 dimensions, axis1 first and axis0 first respectively (excellent
> naming idea by Paul Ivanov)
> 
> What do y'all think?
> 

Personally I think it is clear enough and that "Z" and "N" would confuse
me just as much (though I am used to the other names). Also "Z" and "N"
would seem more like aliases, which would also make sense in the memory
order context.
If anything, I would prefer renaming the arguments iteration_order and
memory_order, but it seems overdoing it...
Maybe the documentation could just be checked if it is always clear
though. I.e. maybe it does not use "iteration" or "memory" order
consistently (though I somewhat feel it is usually clear that it must be
iteration order, since no numpy function cares about the input memory
order as they will just do a copy if necessary).

Regards,

Sebastian 

> Cheers,
> 
> Matthew
> Paul Ivanov
> JB Poline
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