# Pseudorandom Number Generator

Author

Published

April 24, 2023

Pseudorandom number generator is a proxy for truly random number generator. It is not truly random because the numbers generated are determined by an initial value call seed. Therefore, the numbers generated are deterministed if we know the seed. The randomness comes from value of the seed. In other words, if we set the seed to a fixed number such as seed = 123, then we can gurantee to generate the exact same sequence any time we set the seed to 123. For example:

random.seed(123)
r1 = random.random()
random.seed(123)
r2 = random.random()
r1 == r2
True

There is no raltionship between the pseudorandom numbers generated, as the plot below shows:

plt.plot([random.random() for _ in range(50)]);

Also, the pseudorandom numbers generated follow standard uniform distribution.

plt.hist([random.random() for _ in range(10000)]);

Simple implementation to set the random_state using the seed as well as generating random numbers that follow the standard uniform distribution:

rnd_state = None # starting value

def seed(a):
"Set the random state"
global rnd_state
a, x = divmod(a, 30268)
a, y = divmod(a, 30306)
a, z = divmod(a, 30322)
rnd_state = int(x) + 1, int(y) + 1, int(z) + 1

def rand():
global rnd_state
x, y, z = rnd_state
x = (171 * x) % 30269
y = (172 * y) % 30307
z = (170 * z) % 30323
rnd_state = x, y, z
return (x / 30269 + y / 30307 + z / 30323) % 1.0

Both torch and numpy fail at creating different random numbers in both the parent and the child processes because both processes have the same random_state, which is used to generate the random number.. But Python works fine because it takes care of changing the random_state in the child process(es) as the code segments below illustrates:

if os.fork():
print(f"In parent: {torch.rand(1)}")
else:
print(f"In child: {torch.rand(1)}")
os._exit(os.EX_OK)
In parent: tensor([0.3910])
In child: tensor([0.3910])
if os.fork():
print(f"In parent: {np.random.randn(1)}")
else:
print(f"In child: {np.random.randn(1)}")
os._exit(os.EX_OK)
In parent: [-0.51009584]
In child: [-0.51009584]
if os.fork():
print(f"In parent: {random.random()}")
else:
print(f"In child: {random.random()}")
os._exit(os.EX_OK)
In parent: 0.6560074137612377
In child: 0.3243934113820527

As a result, we need to be careful when using multiprocessing/threading with pytorch and numpy that requires generating different random numbers in different threads/processes.