From: Robert Dober <robert.dober@...>
Date: 2009-03-08T02:36:59+09:00
Subject: [ruby-core:22720] Re: [Bug #1243] 1 is prime

On Fri, Mar 6, 2009 at 8:42 PM, Jacob Fugal <lukfugl@gmail.com> wrote:
> I have a hard time imagining a situation where asking if a negative
> number is prime would be necessary. Code the cares about prime numbers
> almost by design implies that it expects positive integers. In that
> mindset, I call YAGNI.
>
> There is no compelling argument (to me) to have the Ruby standard
> library definition of prime be inconsistent with the mathematical
> definition. A prime number -- not a prime factor or a prime element of
> the ring of integers or a prime element of any other arbitrary ring --
> is a natural number (that is, a positive -- or non-negative, it
> doesn't matter here -- integer) which has exactly two distinct natural
> number divisors: 1 and itself. 1 is not prime and negative integers
> are not prime. Rationals or Floats not coinciding with an integer are
> not prime. That should be the definition of "prime?".
>
> In the rare case where someone does need to check if the absolute
> value of a negative integer x is prime, x.abs.prime? is not a horrible
> burden, and it reveals the intent that negative numbers are acceptable
> input to boot.
>
> Jacob Fugal
>
>

I tend to agree, I was taken away by the "challenge" to find a
meaningful definition for primes < 0. And when I failed to do so I
asked for help, stupid somehow, is it not, LOL.

Cheers
Robert