[Numpy-discussion] Counting array elements

[Russell E Owen]

At 7:08 PM +0200 2004-10-25, Peter Verveer wrote:
>On 25 Oct 2004, at 18:51, Gary Strangman wrote:
>
>>
>>>  I'm not sure how feasible it is, but I'd much rather an 
>>>efficient, non-copying, 1-D view of an noncontiguous array (from 
>>>an enhanced version of flat or ravel or whatever) than a bunch of 
>>>extra methods. The former allows all of the standard methods to 
>>>just work efficiently using sum(ravel(A)) or sum(A.flat) [ and max 
>>>and min, etc]. Making special whole array methods for everything 
>>>just leads to method eplosion.
>>
>>  I completely agree with this ... an efficient flat/ravel would 
>>seem to solve many of the issues being raised. Forgive the 
>>potentially naive question here, but is there any reason such an 
>>efficient, enhanced view can't be implemented for the .flat method?
>
>I believe it is not possible without copying data. The strides 
>between elements of a noncontiguous array are not always the same, 
>so you cannot efficiently view it as a 1D array.

How about providing an iterator that counts through all the elements 
of an array (e.g. arr.itervalues()). So long as C extensions could 
efficiently make use of such an iterator, I think it'd do the job.

One could also imagine:
- arr.iteritems(), which returned (index, value) for each item
- a mask argument: a boolean array the same shape as the data array; 
True means elide the corresponding value from the data array
- general support for indexing

More generally, I agree that sum should work the same as a function 
and a method, and that an extra axis argument could be a good thing 
(it is so common elsewhere, e.g. size). I'd be tempted to break 
backwards compatibility to fix this, since numarray is still new and 
the current situation is very confusing.

-- Russell