[Numpy-discussion] storage order and concatenate

[Travis Oliphant]

David Cournapeau wrote:
> Hi,
>
>
> What are the rules concerning storage with numpy ? 
The rule is that a numpy array has "strides" which specify how many 
bytes to skip to get to the next element in the array.   That's the 
internal model.  There are no hard and fast rules about storage order.  
Internally, C-order is as good as Fortran-order (except the array 
iterator gives special preference to C-order and all functions for which 
the order can be specified (like zeros) default to C-order).

Thus, the storage order is whatever the strides say it is.  Now, there 
are flags that keep track of whether or not the strides agree with the 2 
recognized special cases of "Fortran-order" (first-index varies the 
fastest) or "C-order" (last-index varies the fastest).  But, this is 
only for convenience.   Very few functions actually require a 
specification of storage order.  Those that allow it default to "C-order".

You can't think of a NumPy array has having a particular storage order 
unless you explicitly request it.  One of the most common ways that 
Fortran-order arrays show up, for example is when a C-order array is 
transposed.  A transpose operation does nothing except flip the strides 
(and therefore the flags) of the array.  This is what is happening in 
concatenate (using axis=1) to give you a Fortran-order array.  
Bascially, code equivalent to the following is being run:  
concatenate([X1.T, X2.T]).T

In the second example, you explicitly create the array (and therefore 
the strides) as C-order and then fill it (so it doesn't change on you).  
The first example used array calculations which don't guarantee the 
storage order. 

This is all seamless to the user until you have to interface with 
extension code.  Ideally, you write compiled code that deals with 
strided arrays.  If you can't, then you request an array of the required 
storage-order. 

By the way, for interfacing with ctypes, check out the 
ctypeslib.ndpointer class-creation function for flag checking and the 
require function for automatic conversion of an array to specific 
requirements.

-Travis