[Python-Dev] Error in PEP3118? (original) (raw)

Travis Oliphant oliphant.travis at ieee.org
Wed Jan 23 17:17:01 CET 2008

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.

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