[Numpy-discussion] ndrange, like range but multidimensiontal

[Eric Wieser]

If you use this as the dtype, you both set and get element as tuples.

Elements are not got as tuples, but they can be explicitly cast

What about it makes tuple coercion awkward?

This explicit cast

>>> dt_ind2d = np.dtype([('i0', np.intp), ('i1', np.intp)])
>>> ind = np.zeros((), dt_ind2d)[0]
>>> ind, type(ind)
((0, 0), <class 'numpy.void'>)
>>> m[ind]
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    m[inds[0]]
IndexError: only integers, slices (`:`), ellipsis (`...`),
numpy.newaxis (`None`) and integer or boolean arrays are valid indices
>>> m[tuple(ind)]
1.0

On Wed, 10 Oct 2018 at 09:08 Stephan Hoyer shoyer at gmail.com
<http://mailto:shoyer@gmail.com> wrote:

On Tue, Oct 9, 2018 at 9:34 PM Eric Wieser <wieser.eric+numpy at gmail.com>
> wrote:
>
>> One thing that worries me here - in python, range(...) in essence
>> generates a lazy list - so I’d expect ndrange to generate a lazy ndarray.
>> In practice, that means it would be a duck-type defining an __array__
>> method to evaluate it, and only implement methods already present in numpy.
>>
>> It’s not clear to me what the datatype of such an array-like would be.
>> Candidates I can think of are:
>>
>>    1. [('i0', intp), ('i1', intp), ...], but this makes tuple coercion a
>>    little awkward
>>
>> I think this would be the appropriate choice. What about it makes tuple
> coercion awkward? If you use this as the dtype, you both set and get
> element as tuples.
>
> In particular, I would say that ndrange() should be a lazy equivalent to
> the following explicit constructor:
>
> def ndrange(shape):
> dtype = [('i' + str(i), np.intp) for i in range(len(shape))]
> array = np.empty(shape, dtype)
> for indices in np.ndindex(*shape):
> array[indices] = indices
> return array
>
> >>> ndrange((2,)
> array([(0,), (1,)], dtype=[('i0', '<i8')])
>
> >>> ndrange((2, 3))
> array([[(0, 0), (0, 1), (0, 2)], [(1, 0), (1, 1), (1, 2)]], dtype=[('i0',
> '<i8'), ('i1', '<i8')])
>
> The one deviation in behavior would be that ndrange() iterates over
> flattened elements rather than the first axes.
>
> It is indeed a little awkward to have field names, but given that NumPy
> creates those automatically when you supply a dtype like 'i8,i8' this is
> probably a reasonable choice.
>
>
>>    1. (intp, (N,)) - which collapses into a shape + (3,) array
>>    2. object_.
>>    3. Some new np.tuple_ dtype, a heterogenous tuple, which is like the
>>    structured np.void but without field names. I’m not sure how
>>    vectorized element indexing would be spelt though.
>>
>> Eric
>> ​
>>
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