[Numpy-discussion] question about array slicing and element assignment

[Stephan Hoyer]

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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