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Algorithm to find unique prime factors #9935

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Algorithm to find unique prime factors #9935

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44 changes: 44 additions & 0 deletions maths/prime_factors.py
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,50 @@ def prime_factors(n: int) -> list[int]:
return factors


def unique_prime_factors(n: int) -> list[int]:
"""
Returns unique prime factors of n as a list.

>>> unique_prime_factors(0)
[]
>>> unique_prime_factors(100)
[2, 5]
>>> unique_prime_factors(2560)
[2, 5]
>>> unique_prime_factors(10**-2)
[]
>>> unique_prime_factors(0.02)
[]
>>> x = unique_prime_factors(10**241) # doctest: +NORMALIZE_WHITESPACE
>>> x == [2, 5]
True
>>> unique_prime_factors(10**-354)
[]
>>> unique_prime_factors('hello')
Traceback (most recent call last):
...
TypeError: '<=' not supported between instances of 'int' and 'str'
>>> unique_prime_factors([1,2,'hello'])
Traceback (most recent call last):
...
TypeError: '<=' not supported between instances of 'int' and 'list'

"""
i = 2
factors = []
while i * i <= n:
if n % i:
i += 1
else:
n //= i
if i not in factors:
factors.append(i)
if n > 1:
if n not in factors:
factors.append(n)
return factors


if __name__ == "__main__":
import doctest

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