2.5. Array Random¶

Since numpy v1.17: BitGenerator for the PCG-64 (Parallel Congruent Generator 64 bit) pseudo-random number generator
Before numpy v1.17: Mersenne Twister algorithm for pseudorandom number generation

2.5.1. Seed¶

Seed the generator

from datetime import datetime

def seed():
    timestamp = datetime.now().timestamp()
    return int(timestamp)

seed() % 10     # 3
seed() % 10     # 4
seed() % 10     # 5
seed() % 10     # 6

from datetime import datetime

def seed():
    timestamp = datetime.now().timestamp()
    cpu_temperature = 52.4
    return int(timestamp + cpu_temperature)

seed() % 10     # 7
seed() % 10     # 2
seed() % 10     # 5
seed() % 10     # 1

from datetime import datetime

def seed():
    timestamp = datetime.now().timestamp()
    cpu_temperature = 52.4
    ram_voltage = 68.8
    network_card_crc = 9876
    return int(timestamp + cpu_temperature + ram_voltage + network_card_crc)

seed() % 10     # 3
seed() % 10     # 0
seed() % 10     # 2
seed() % 10     # 8

import numpy as np

np.random.seed(0)

2.5.2. Generate¶

Random int from low (inclusive) to high (exclusive)
Random float in the half-open interval [0.0, 1.0)

Code 2.145. Generate pseudorandom int¶

import numpy as np


np.random.randint(0, 10)
# 5

np.random.randint(0, 10, size=5)
# array([4, 3, 0, 4, 3])

np.random.randint(0, 10, size=(2,3))
# array([[8, 8, 3],
#        [8, 2, 8]])

Code 2.146. Generate pseudorandom float¶

import numpy as np


np.random.random()
# 0.8472517387841254

np.random.random(size=5)
# array([0.88173536, 0.69253159, 0.72525428, 0.50132438, 0.95608363])

np.random.random(size=(2,3))
# array([[0.69947928, 0.29743695, 0.81379782],
#        [0.39650574, 0.8811032 , 0.58127287]])

2.5.3. Distributions¶

2.5.3.1. Uniform Distribution¶

Results are from the "continuous uniform" distribution over the stated interval
Random float in the half-open interval [0.0, 1.0)

../_images/random-distribution-uniform.png

Figure 2.9. Continuous Uniform Distribution [NP7]¶

import numpy as np


np.random.rand(5)
# array([0.5488135 , 0.71518937, 0.60276338, 0.54488318, 0.4236548 ])

np.random.rand(2,3)
# array([[0.5488135 , 0.71518937, 0.60276338],
#        [0.54488318, 0.4236548 , 0.64589411]])

np.random.rand(3,2)
# array([[0.5488135 , 0.71518937],
#        [0.60276338, 0.54488318],
#        [0.4236548 , 0.64589411]])

2.5.3.2. Normal (Gaussian) Distribution¶

Draw pseudorandom samples from a normal (Gaussian) distribution
Default:
- μ - loc=0.0
- σ - scale=1.0

import numpy as np


np.random.normal()
# 0.9500884175255894

np.random.normal(0.0, 1.0)
# 0.4001572083672233

np.random.normal(loc=0.0, scale=1.0)
# -0.977277879876411

import numpy as np


np.random.normal(size=5)
# array([-1.67215088, 0.65813053, -0.70150614, 0.91452499, 0.71440557])

np.random.normal(loc=0.0, scale=1.0, size=(2,3))
# array([[-0.99090328,  1.01788005,  0.3415874 ],
#        [-1.25088622,  0.92525075, -0.90478616]])

../_images/random-distribution-normal.png

Figure 2.10. Normal (Gaussian) distribution [NP3]¶

2.5.3.3. Poisson Distribution¶

Draw samples from a Poisson distribution

import numpy as np


np.random.poisson(6.0)
# 5

np.random.poisson(lam=6.0)
# 5

import numpy as np


np.random.poisson(lam=6.0, size=5)
# array([5, 7, 3, 5, 6])

np.random.poisson(lam=6.0, size=(2,3))
# array([[4, 9, 7],
#        [8, 5, 5]])

../_images/random-distribution-poisson.png

Figure 2.11. Poisson distribution [NP5]¶

2.5.4. Drawing and Sampling¶

Code 2.147. Choice¶

import numpy as np


np.random.choice([1, 2, 3])
# 2

np.random.choice([1, 2, 3], size=2)
# array([3, 1])

np.random.choice([1, 2, 3], size=2)
# array([3, 3])

np.random.choice([1, 2, 3], 2, replace=False)
# array([1, 3])

Code 2.148. Sample¶

import numpy as np


np.random.sample(size=5)
# array([0.44792617, 0.09956909, 0.35231166, 0.46924917, 0.84114013])

np.random.sample(size=(2,3))
# array([[0.90464774, 0.03755938, 0.50831545],
#        [0.16684751, 0.77905102, 0.8649333 ]])

np.random.sample(size=(3,2))
# array([[0.41139672, 0.13997259],
#        [0.03322239, 0.98257496],
#        [0.37329075, 0.42007537]])

2.5.5. Shuffle¶

Modify sequence in-place (!!)
Multi-dimensional arrays are only shuffled along the first axis

Code 2.149. 1-dimensional Array¶

import numpy as np

a = np.array([1, 2, 3])

np.random.shuffle(a)
# array([3, 1, 2])

Code 2.150. 2-dimensional Array¶

import numpy as np


a = np.array([[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]])

np.random.shuffle(a)
# array([[7, 8, 9],
#        [1, 2, 3],
#        [4, 5, 6]])

2.5.6. Assignments¶

2.5.6.1. Numpy Random Float¶

Assignment: Numpy Random Float
Last update: 2020-10-01
Complexity level: medium
Lines of code to write: 3 lines
Estimated time of completion: 3 min
Filename: solution/numpy_random_float.py

English:

Set random seed to zero
Print np.ndarray of 10 random floats

Polish:

Ustaw ziarno losowości na zero
Wypisz np.ndarray z 10 losowymi liczbami zmiennoprzecinkowymi

2.5.6.2. Numpy Random Int¶

Assignment: Numpy Random Int
Last update: 2020-10-01
Complexity level: easy
Lines of code to write: 4 lines
Estimated time of completion: 3 min
Filename: solution/numpy_random_int.py

English:

Set random seed to zero
Print np.ndarray of size 16x16 with random integers [0;9] (inclusive)

Polish:

Ustaw ziarno losowości na zero
Print np.ndarray o rozmiarze 16x16 z losowymi liczbami całkowitymi <0,9> (włącznie)

2.5.6.3. Numpy Random Sample¶

Assignment: Numpy Random Sample
Last update: 2020-10-01
Complexity level: medium
Lines of code to write: 5 lines
Estimated time of completion: 3 min
Filename: solution/numpy_random_sample.py

English:

Set random seed to zero
Print 6 random integers without repetition in range from 1 to 49

Polish:

Ustaw ziarno losowości na zero
Wyświetl 6 losowych i nie powtarzających się liczb całkowitych z zakresu od 1 do 49.