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Design #1608

Domain of definition of NextPrime (and PrevPrime)

Added by John Abbott over 2 years ago. Updated over 2 years ago.

Status:
Closed
Priority:
Normal
Assignee:
John Abbott
Category:
Tidying
Target version:
CoCoALib-0.99800
Start date:
16 Sep 2021
Due date:
% Done:

100%

Estimated time:
1.01 h
Spent time:
1.00 h

Description

Not so important: what is the domain of defn of NextPrime and that of PrevPrime?

I tried to compute NextPrime(0) but obtained an error saying the arg must be (strictly) positive.
Also PrevPrime(2) in CoCoA-5 gave an unhelpful/misleading error message (ArgTooBig).

Decide defn, and improve error mesgs.

History

#1 Updated by John Abbott over 2 years ago

  • Description updated (diff)
  • Status changed from New to In Progress
  • Assignee set to John Abbott
  • % Done changed from 0 to 10

I am now inclined to allow 0 as arg to NextPrime; perhaps this is related to my preference to consider 0 as a "natural number".

I have doubts about PrevPrime returning 0 when given 2 (or 1 or 0) as input. Surely an error would be more appropriate?

I wonder why I wrote the code the way I did...? (Documentation? Pah!)

#2 Updated by John Abbott over 2 years ago

  • Status changed from In Progress to Feedback
  • % Done changed from 10 to 90
I have spoken to Anna who agrees to change the behaviour:
  • both fns throw BadArg (or similar) if given (strictly) negative args
  • PrevPrime throws OutOfRange if arg is 0 <= n <= 2.
  • NextPrime returns 0 if no next small prime exists
  • Similar mods to NextProbPrime and PrevProbPrime.

Modified doc. Modified test-NumTheory1.C.

#3 Updated by John Abbott over 2 years ago

  • Status changed from Feedback to Closed
  • % Done changed from 90 to 100
  • Estimated time set to 1.01 h

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