[Numpy-discussion] np.nonzero - order guarantees?

[Matthew Brett]

Hi,

`np.nonzero` for a 2D array `A` returns:

row_inds, col_inds = np.nonzero(A)

I notice that `row_inds` appears to be sorted by value, and `col_inds`
appears to be sorted by value, within each row.

Is this a guarantee of the `np.nonzero` function? If not, does this
function guarantee any property of the returned indices, other than
the correspondence of the row, column entries?

Cheers,

Matthew

[Sebastian Berg]

Yes, I think it is guaranteed C order in the results.

Post by Matthew Brett
Hi,
row_inds, col_inds = np.nonzero(A)
I notice that `row_inds` appears to be sorted by value, and `col_inds`
appears to be sorted by value, within each row.
Is this a guarantee of the `np.nonzero` function? If not, does this
function guarantee any property of the returned indices, other than
the correspondence of the row, column entries?
Cheers,
Matthew
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http://mail.scipy.org/mailman/listinfo/numpy-discussion

[Matthew Brett]

On Tue, Jul 28, 2015 at 4:45 PM, Sebastian Berg

Post by Sebastian Berg
Yes, I think it is guaranteed C order in the results.

Post by Matthew Brett
Hi,
row_inds, col_inds = np.nonzero(A)
I notice that `row_inds` appears to be sorted by value, and `col_inds`
appears to be sorted by value, within each row.
Is this a guarantee of the `np.nonzero` function? If not, does this
function guarantee any property of the returned indices, other than
the correspondence of the row, column entries?

Joscha Reimer just pointed out that this is not guaranteed for scipy
sparse arrays:

https://github.com/scipy/scipy/pull/4875#discussion_r35528827

Post by Sebastian Berg

Post by Matthew Brett

A = scipy.sparse.coo_matrix(([2, 1], ([1, 0], [0, 1])))
A.todense()

matrix([[0, 1],
[2, 0]])

Post by Sebastian Berg

Post by Matthew Brett

A.nonzero()

(array([1, 0], dtype=int32), array([0, 1], dtype=int32))

This seems rather dangerous - I mean a convention that is nearly
always observed. I guess at very least we should say what the
guarantee is (or isn't) in the nonzero docstring?

Cheers,

Matthew

[Sebastian Berg]

Post by Matthew Brett
On Tue, Jul 28, 2015 at 4:45 PM, Sebastian Berg

Post by Sebastian Berg
Yes, I think it is guaranteed C order in the results.

Post by Matthew Brett
Hi,
row_inds, col_inds = np.nonzero(A)
I notice that `row_inds` appears to be sorted by value, and `col_inds`
appears to be sorted by value, within each row.
Is this a guarantee of the `np.nonzero` function? If not, does this
function guarantee any property of the returned indices, other than
the correspondence of the row, column entries?

Joscha Reimer just pointed out that this is not guaranteed for scipy
https://github.com/scipy/scipy/pull/4875#discussion_r35528827

Post by Sebastian Berg

Post by Matthew Brett

A = scipy.sparse.coo_matrix(([2, 1], ([1, 0], [0, 1])))
A.todense()

matrix([[0, 1],
[2, 0]])

Post by Sebastian Berg

Post by Matthew Brett

A.nonzero()

(array([1, 0], dtype=int32), array([0, 1], dtype=int32))
This seems rather dangerous - I mean a convention that is nearly
always observed. I guess at very least we should say what the
guarantee is (or isn't) in the nonzero docstring?

Yes. We should double check and document it. But if it were to change, I would go for an order keyword anyway.

-Sebastian

Post by Matthew Brett
Cheers,
Matthew
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