[Numpy-discussion] Documentation question.

Travis Oliphant travis@continuum...
Wed Feb 1 20:14:48 CST 2012


On Feb 1, 2012, at 7:04 PM, Mark Wiebe wrote:

> On Wed, Feb 1, 2012 at 3:29 PM, Charles R Harris <charlesr.harris@gmail.com> wrote:
> The macro PyArray_RemoveLargest has been replaced by PyArray_RemoveSmallest (which seems strange), but I wonder if this documentation still makes sense.
> 
> My impression about this code is that it went through a number of rounds trying to choose an iteration order heuristic that has improved performance over C-order. The change of Largest to Smallest probably reflects one of these heuristic changes. I think it's safe to say that the nditer introduced in 1.6 completely removes the need for this functionality. I did a grep for this function in the master branch, and it is no longer used by NumPy internally.

There is a common need to iterate over all but one dimension of a NumPy array.   The final dimension is iterated over in an "internal" loop.   This is the essence of how ufuncs work and avoid the possibly expensive overhead of a C-call during each iteration. 

Initially, it seemed prudent to remove the dimension that had the largest size (so that the final inner-iteration was the largest number).   Later, timings revealed that that the 'inner' dimension should be the one with the smallest *striding*.   I have not looked at nditer in detail, but would appreciate seeing an explanation of how the nditer approach removes the need for this macro.   When that is clear, then this macro can and should be deprecated.   

-Travis




> 
> -Mark
>  
> diff --git a/doc/source/user/c-info.beyond-basics.rst b/doc/source/user/c-info.beyond-basics.rs
> index 9ed2ab3..3437985 100644
> --- a/doc/source/user/c-info.beyond-basics.rst
> +++ b/doc/source/user/c-info.beyond-basics.rst
> @@ -189,7 +189,7 @@ feature follows.
>          PyArray_MultiIter_NEXT(mobj);
>      }
>  
> -The function :cfunc:`PyArray_RemoveLargest` ( ``multi`` ) can be used to
> +The function :cfunc:`PyArray_RemoveSmallest` ( ``multi`` ) can be used to
>  take a multi-iterator object and adjust all the iterators so that
>  iteration does not take place over the largest dimension (it makes
>  that dimension of size 1). The code being looped over that makes use
> 
> Chuck
> 
> 
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