Re: [Numpy-discussion] summarizing blocks of an array using a moving window

Keith Goodman wrote:

On Thu, Jul 22, 2010 at 7:48 AM, Warren Weckesser wrote:

...

Actually, because of the use of reshape(3,3,4), your second example does make a copy.

When does reshape return a view and when does it return a copy?

According to the numpy.reshape docstring, it returns a view when it can. In the previous example, it is not possible to configure the strides so that the four elements in each 2x2 block can be represented in a single axis using the original memory layout, so the data must be copied to achieve the shape (3,3,4).

Here's a simple example that returns a view:

...

...

x = np.array([1,2,3,4]) y = x.reshape(2,2) y[0,0] = 9 x

array([9, 2, 3, 4])

What's a simple example that returns a copy?

In [85]: x = np.array([[1,2],[3,4],[5,6]]).T # Note the transpose. In [86]: x Out[86]: array([[1, 3, 5], [2, 4, 6]]) In [87]: y = x.reshape(6) In [88]: x[0,1] = 99 In [89]: y Out[89]: array([1, 3, 5, 2, 4, 6]) Warren

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