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comp / comp.lang.python / Re: Relatively prime integers in NumPy

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o Re: Relatively prime integers in NumPyOscar Benjamin

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Subject: Re: Relatively prime integers in NumPy
From: Oscar Benjamin
Newsgroups: comp.lang.python
Date: Thu, 11 Jul 2024 21:22 UTC
References: 1 2
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From: oscar.j.benjamin@gmail.com (Oscar Benjamin)
Newsgroups: comp.lang.python
Subject: Re: Relatively prime integers in NumPy
Date: Thu, 11 Jul 2024 22:22:30 +0100
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(posting on-list this time)

On Thu, 11 Jul 2024 at 15:18, Popov, Dmitry Yu via Python-list
<python-list@python.org> wrote:
>
> Dear Sirs.
>
> Does NumPy provide a simple mechanism to identify relatively prime integers, i.e. integers which don't have a common factor other than +1 or -1? For example, in case of this array:
> [[1,5,8],
> [2,4,8],
> [3,3,9]]
> I can imagine a function which would return array of common factors along axis 0: [1,2,3]. Those triples of numbers along axis 1 with the factor of1 or -1 would be relatively prime integers.

It sounds like you want the gcd (greatest common divisor) of each row.
The math module can do this:

In [1]: a = [[1,5,8],
...: [2,4,8],
...: [3,3,9]]

In [2]: import math

In [3]: [math.gcd(*row) for row in a]
Out[3]: [1, 2, 3]

NumPy can also do it apparently:

In [10]: np.gcd.reduce(np.transpose(a))
Out[10]: array([1, 2, 3])

https://en.wikipedia.org/wiki/Greatest_common_divisor

--
Oscar

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