Issue
There does not seem to be a PyTorch function for computing a factorial. Is there a method to do this in PyTorch? I am looking to manually compute a Poisson distribution in Torch (I am aware this exists: https://pytorch.org/docs/stable/generated/torch.poisson.html) and the formula requires a factorial in the denominator.
Poisson Distribution: https://en.wikipedia.org/wiki/Poisson_distribution
Solution
I think you can find it as torch.jit._builtins.math.factorial
BUT pytorch
as well as numpy
and scipy
(Factorial in numpy and scipy) uses python
's builtin math.factorial
:
import math
import numpy as np
import scipy as sp
import torch
print(torch.jit._builtins.math.factorial is math.factorial)
print(np.math.factorial is math.factorial)
print(sp.math.factorial is math.factorial)
True
True
True
But, in contrast, scipy
in addition to "mainstream" math.factorial
contains the very "special" factorial function scipy.special.factorial
. Unlike function from math
module it operates on arrays:
from scipy import special
print(special.factorial is math.factorial)
False
# the all known factorial functions
factorials = (
math.factorial,
torch.jit._builtins.math.factorial,
np.math.factorial,
sp.math.factorial,
special.factorial,
)
# Let's run some tests
tnsr = torch.tensor(3)
for fn in factorials:
try:
out = fn(tnsr)
except Exception as err:
print(fn.__name__, fn.__module__, ':', err)
else:
print(fn.__name__, fn.__module__, ':', out)
factorial math : 6
factorial math : 6
factorial math : 6
factorial math : 6
factorial scipy.special._basic : tensor(6., dtype=torch.float64)
tnsr = torch.tensor([1, 2, 3])
for fn in factorials:
try:
out = fn(tnsr)
except Exception as err:
print(fn.__name__, fn.__module__, ':', err)
else:
print(fn.__name__, fn.__module__, ':', out)
factorial math : only integer tensors of a single element can be converted to an index
factorial math : only integer tensors of a single element can be converted to an index
factorial math : only integer tensors of a single element can be converted to an index
factorial math : only integer tensors of a single element can be converted to an index
factorial scipy.special._basic : tensor([1., 2., 6.], dtype=torch.float64)
Answered By - trsvchn
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