NumPy

import numpy as np

np.random.seed(seed=1234)

Array

x = np.array(6)
x.ndim
x.shape
x.size
x.dtype

np.zeros((2, 2))
np.ones((2, 2))
np.eye((2))
np.random.random((2, 2))

Index

x = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]])
x[:, 1]    # [2, 6, 10]
x[0, :]    # [1, 2, 3, 4]
x[:3, 1:3] # [[2, 3], [6, 7], [10, 11]]
x[[0, 1, 2], [0, 2, 1]] # [1, 7, 10]

# Boolean array indexing
x = np.array([[1,2], [3, 4], [5, 6]])
print ("x:\n", x)
print ("x > 2:\n", x > 2)
print ("x[x > 2]:\n", x[x > 2])
# x:
#  [[1 2]
#  [3 4]
#  [5 6]]
# x > 2:
#  [[False False]
#  [ True  True]
#  [ True  True]]
# x[x > 2]:
#  [3 4 5 6]

Matrix

• math: x+y/x-y/x*y np.add/subtract/multiply
• dot product: a.dot(b)
• sum: np.sum(x)
• column sum: np.sum(x, axis=0)
• row sum: np.sum(x, axis=1)
• transposing: x.T
• reshape: np.reshape(x, (2, 3))