[Numpy-discussion] Why do mgrid and meshgrid not return broadcast arrays?

[Per.Brodtkorb at ffi.no]

Hi, Juan.

Meshgrid can actually give what you want, but you must use the options: copy=False  and indexing=’ij’.

In [7]: %timeit np.meshgrid(np.arange(512), np.arange(512))
1000 loops, best of 3: 1.24 ms per loop

In [8]: %timeit np.meshgrid(np.arange(512), np.arange(512), copy=False)
10000 loops, best of 3: 27 µs per loop

In [9]: %timeit np.meshgrid(np.arange(512), np.arange(512), copy=False, indexing='ij')
10000 loops, best of 3: 23 µs per loop

Best regards
Per A. Brodtkorb

From: NumPy-Discussion [mailto:numpy-discussion-bounces at scipy.org] On Behalf Of Juan Nunez-Iglesias
Sent: 9. mars 2017 04:20
To: Discussion of Numerical Python
Subject: Re: [Numpy-discussion] Why do mgrid and meshgrid not return broadcast arrays?

Hi Warren,

ogrid doesn’t solve my problem. Note that my code returns arrays that would evaluate as equal to the mgrid output. It’s just that they are copied in mgrid into a giant array, instead of broadcast:


In [176]: a0, b0 = np.mgrid[:5, :5]

In [177]: a1, b1 = th.broadcast_mgrid((np.arange(5), np.arange(5)))

In [178]: a0
Out[178]:
array([[0, 0, 0, 0, 0],
       [1, 1, 1, 1, 1],
       [2, 2, 2, 2, 2],
       [3, 3, 3, 3, 3],
       [4, 4, 4, 4, 4]])

In [179]: a1
Out[179]:
array([[0, 0, 0, 0, 0],
       [1, 1, 1, 1, 1],
       [2, 2, 2, 2, 2],
       [3, 3, 3, 3, 3],
       [4, 4, 4, 4, 4]])

In [180]: a0.strides
Out[180]: (40, 8)

In [181]: a1.strides
Out[181]: (8, 0)



On 9 Mar 2017, 2:05 PM +1100, Warren Weckesser <warren.weckesser at gmail.com<mailto:warren.weckesser at gmail.com>>, wrote:



On Wed, Mar 8, 2017 at 9:48 PM, Juan Nunez-Iglesias <jni.soma at gmail.com<mailto:jni.soma at gmail.com>> wrote:
I was a bit surprised to discover that both meshgrid nor mgrid return fully instantiated arrays, when simple broadcasting (ie with stride=0 for other axes) is functionally identical and happens much, much faster.


Take a look at ogrid: https://docs.scipy.org/doc/numpy/reference/generated/numpy.ogrid.html
Warren

I wrote my own function to do this:


def broadcast_mgrid(arrays):
    shape = tuple(map(len, arrays))
    ndim = len(shape)
    result = []
    for i, arr in enumerate(arrays, start=1):
        reshaped = np.broadcast_to(arr[[...] + [np.newaxis] * (ndim - i)],
                                   shape)
        result.append(reshaped)
    return result


For even a modest-sized 512 x 512 grid, this version is close to 100x faster:


In [154]: %timeit th.broadcast_mgrid((np.arange(512), np.arange(512)))
10000 loops, best of 3: 25.9 µs per loop

In [156]: %timeit np.meshgrid(np.arange(512), np.arange(512))
100 loops, best of 3: 2.02 ms per loop

In [157]: %timeit np.mgrid[:512, :512]
100 loops, best of 3: 4.84 ms per loop


Is there a conscious design decision as to why this isn’t what meshgrid/mgrid do already? Or would a PR be welcome to do this?

Thanks,

Juan.

_______________________________________________
NumPy-Discussion mailing list
NumPy-Discussion at scipy.org<mailto:NumPy-Discussion at scipy.org>
https://mail.scipy.org/mailman/listinfo/numpy-discussion

_______________________________________________
NumPy-Discussion mailing list
NumPy-Discussion at scipy.org<mailto:NumPy-Discussion at scipy.org>
https://mail.scipy.org/mailman/listinfo/numpy-discussion
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20170309/4ffd6e61/attachment.html>