[Numpy-discussion] summarizing blocks of an array using a moving window
Thu Jul 22 12:35:57 CDT 2010
Keith Goodman wrote:
> On Thu, Jul 22, 2010 at 7:48 AM, Warren Weckesser
> <firstname.lastname@example.org> 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
> 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
> array([9, 2, 3, 4])
> What's a simple example that returns a copy?
In : x = np.array([[1,2],[3,4],[5,6]]).T # Note the transpose.
In : x
array([[1, 3, 5],
[2, 4, 6]])
In : y = x.reshape(6)
In : x[0,1] = 99
In : y
Out: array([1, 3, 5, 2, 4, 6])
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