b = np.arange(48).reshape(4, 12)
b
array([[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11],
[12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23],
[24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35],
[36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]])rows_on = np.array([True, False, True, False])
b[rows_on, :] # Rows 0 and 2, all columns. Equivalent to b[(0, 2), :]
array([[ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11],
[24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]])