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@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 _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion

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@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 _______________________________________________ NumPy-Discussion mailing list NumPy-Discussion@python.org https://mail.python.org/mailman/listinfo/numpy-discussion