import numpy as np
# create array with 8 integers
a = 2 * np.arange(8)**2 + 1
# [ 1 3 9 19 33 51 73 99]
# create array with 16 integers
b = np.arange(16)
b.shape = (4, 4)
# [[ 0 1 2 3]
# [ 4 5 6 7]
# [ 8 9 10 11]
# [12 13 14 15]]
# fancy indexing
a[[3, -1, 1]]
# [19 99 3]
b[:, [2, -1, 0]]
# [[ 2 3 0]
# [ 6 7 4]
# [10 11 8]
# [14 15 12]]
# get fibonacci sequence
fib = np.array([0, 1, 1, 2, 3])
a[fib]
# [ 1 3 3 9 19]
b[fib]
# [[ 0 1 2 3]
# [ 4 5 6 7]
# [ 4 5 6 7]
# [ 8 9 10 11]
# [12 13 14 15]]
# get diagonal
i = np.arange(4)
b[i, i]
# [ 0 5 10 15]
# get diagonal of upper part
b[i[:3], i[:3] + 1]
# [ 1 6 11]
# get diagonal of lower part
b[i[1:], i[1:] - 1]
# [ 4 9 14]