[Numpy-discussion] summarizing blocks of an array using a moving window
Thu Jul 22 12:49:05 CDT 2010
On Thu, Jul 22, 2010 at 10:35 AM, Warren Weckesser
> Keith Goodman wrote:
>> On Thu, Jul 22, 2010 at 7:48 AM, Warren Weckesser
>> <email@example.com> 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])
Thanks, Warren. It's very useful to me to know that reshape can return
a copy. Now I know how to slow down the other guy's function in a
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