Bug #7395: Negative numbers can't be primes by definition

• Status: Closed
• Assignee: yugui (Yuki Sonoda)
• Target version: 2.2.0
• ruby -v: 2.0.0

[zzak (zzak _)]

from github:
https://github.com/ruby/ruby/pull/187

By definition, a prime number can be a positive integer greater than 1.

http://mathworld.wolfram.com/PrimeNumber.html

[usa (Usaku NAKAMURA)]

• Status changed from Open to Assigned

[ko1 (Koichi Sasada)]

• Subject changed from Negative numbers can't be primes by definition to Negative numbers can't be primes by definition
• Target version changed from 2.0.0 to 2.1.0

[hsbt (Hiroshi SHIBATA)]

• Target version changed from 2.1.0 to 2.2.0

[ayumin (Ayumu AIZAWA)]

• Status changed from Assigned to Closed
• % Done changed from 0 to 100

Applied in changeset r45878.

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• lib/prime.rb (Prime#prime?): negative numbers can't be primes
  by definition. reported by Ivan Kataitsev. [Bug #7395]
• test/test_prime.rb: add test.

[hsbt (Hiroshi SHIBATA)]

ayumin

this issue seems incompatible changes. it should be add NEWS.

[mame (Yusuke Endoh)]

• Status changed from Closed to Assigned

Read the rdoc carefully:

Returns true if +self+ is a prime number, false for a composite.

By definition, a composite number can be also a positive integer greater than 1.

http://mathworld.wolfram.com/CompositeNumber.html

Thus,

1.prime? #=> false
0.prime? #=> false
(-1).prime? #=> false

looks all buggy (or undefined behavior) to me.
If we can change the spec, it would be good to raise an exception, return nil, or at least update the rdoc.
Anyway, the maintainer (Yugui) should treat this, I think.

--
Yusuke Endoh [e-mail]

[ayumin (Ayumu AIZAWA)]

Endo-san

Your opinion seems right.

IMHO, I think Rdoc should be updated.
This method should evaluate the number is prime number or not.
It should not refer the number is composite number or not.

We may have to confirm Yugui's intention anyway.
Luckily I'm going to meet her on Wed, so I will ask her about this issue.

Shibata-san
I will add information to NEWS, once change was accepted by Yugui.

[ayumin (Ayumu AIZAWA)]

• Status changed from Assigned to Closed

Applied in changeset r46061.

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add information of incompatibility about Prime.prime? [Bug #7395]