[Numpy-discussion] question about array slicing and element assignment
Stephan Hoyer
shoyer at gmail.com
Tue Jun 19 17:09:24 EDT 2018
You will need to convert "a[(2,3,5),][mask]" into a single indexing
expression, e.g, by using utility functions like np.nonzero() on mask.
NumPy can't support assignment in chained indexing.
On Tue, Jun 19, 2018 at 1:25 PM Emil Sidky <sidky at uchicago.edu> wrote:
> Hello,
> The following is an example where an array element assignment didn't work
> as I expected.
> Create a 6 x 3 matrix:
>
> In [70]: a = randn(6,3)
>
> In [71]: a
> Out[71]:
> array([[ 1.73266816, 0.948849 , 0.69188222],
> [-0.61840161, -0.03449826, 0.15032552],
> [ 0.4963306 , 0.77028209, -0.63076396],
> [-1.92273602, -1.03146536, 0.27744612],
> [ 0.70736325, 1.54687964, -0.75573888],
> [ 0.16316043, -0.34814532, 0.3683143 ]])
>
> Create a 3x3 boolean array:
> In [72]: mask = randn(3,3)>0.
>
> In [73]: mask
> Out[73]:
> array([[ True, True, True],
> [False, True, True],
> [ True, False, True]], dtype=bool)
>
> Try to modify elements of "a" with the following line:
> In [74]: a[(2,3,5),][mask] = 1.
> No elements are changed in "a":
> In [75]: a
> Out[75]:
> array([[ 1.73266816, 0.948849 , 0.69188222],
> [-0.61840161, -0.03449826, 0.15032552],
> [ 0.4963306 , 0.77028209, -0.63076396],
> [-1.92273602, -1.03146536, 0.27744612],
> [ 0.70736325, 1.54687964, -0.75573888],
> [ 0.16316043, -0.34814532, 0.3683143 ]])
>
> Instead try to modify elements of "a" with this line:
> In [76]: a[::2,][mask] = 1.
>
> This time it works:
> In [77]: a
> Out[77]:
> array([[ 1. , 1. , 1. ],
> [-0.61840161, -0.03449826, 0.15032552],
> [ 0.4963306 , 1. , 1. ],
> [-1.92273602, -1.03146536, 0.27744612],
> [ 1. , 1.54687964, 1. ],
> [ 0.16316043, -0.34814532, 0.3683143 ]])
>
>
> Is there a way where I can modify the elements of "a" selected by an
> expression like "a[(2,3,5),][mask]" ?
>
> Thanks , Emil
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