[Python-Dev] Error in PEP3118?

[Travis Oliphant]

Thomas Heller wrote:
> Hi Travis,
> 
> The pep contains this sample:
> 
> """
> Nested array
>     ::
> 
>         struct {
>              int ival;
>              double data[16*4];
>         }
>         """i:ival: 
>            (16,4)d:data:
>         """
> """
> 
> I think it is wrong and must be changed to the following; is this correct?
> 
> """
> Nested array
>     ::
> 
>         struct {
>              int ival;
>              double data[16][4];
>         }
>         """i:ival: 
>            (16,4)d:data:
>         """
> """

I responded off list to this email and wanted to summarize my response 
for others to peruse.

Basically,  the answer is that the struct syntax proposed for 
multi-dimensional arrays is not intended to mimic how the C-compiler 
handles statically defined C-arrays (i.e. the pointer-to-pointers style 
of multi-dimensional arrays).  It is intended to handle the 
contiguous-block-of-data style of multi-dimensional arrays that NumPy uses.

I wanted to avoid 2-d static arrays in the examples because it gets 
confusing and AFAIK the layout of the memory for a double data[16][4] is 
the same as data[16*4].  The only difference is how the C-compiler 
translates data[4][3] and data[4].

The intent of the struct syntax is to handle describing memory.  The 
point is not to replicate how the C-compiler deals with statically 
defined N-D arrays.  Thus, even though the struct syntax allows 
*communicating* the intent of a contiguous block of memory inside a 
structure as an N-d array, the fundamental memory block is the 
equivalent of a 1-d array in C.

So, I think the example is correct (and intentional).

-Travis O.