From: Paul Brannan <pbrannan@...>
Date: 2002-04-25T06:25:53+09:00
Subject: Re: Numberic#prev

On Thu, Apr 25, 2002 at 05:01:32AM +0900, Hal E. Fulton wrote:
> Let me ask these questions:
> 1. What do we do when we can't figure out a predecessor?
> Raise an exception? That would be my first guess.
> 2. Are there cases where String#succ is undefined? What 
> does it do then?
> 3. What are the major domains where String#succ is defined?
> "A".."Z", "a".."z", "0".."9", and are there more?
> 4. Do the rules for concatenation of such strings make things
> more difficult? For example: Can you guess the value of 
> "Z9".succ ? It is "AA0". Therefore "AA0".pred would have to be
> "Z9"...

These are very good questions.  I don't have an answer to any of them.

In question #1, by "when we can't figure out" do you mean "when there is
more than one predecessor"?

Perhaps posting this is not wise, but here is an implementation, with
some examples.  Maybe this will make the problem more concrete, or maybe
it is a Pandora's box.  Read it at your own risk.

Paul

---

class Fixnum
  def pred
    return self - 1
  end
end

class String
  def pred
    case self[-1]
    when nil
      return '' # is this the right thing to return?
    when ?0
      return pred_borrow('9')
    when ?A
      return pred_borrow('Z')
    when ?a
      return pred_borrow('z')
    else
      result = self.dup
      result[-1] = self[-1].pred
      return result
    end
  end

  def pred_borrow(end_str)
    result = self[0..-2]
    if result.length == 0 then
      return ""
    else
      return result.pred + end_str
    end
  end
end

p ''.pred       #=> ""
p '9'.pred      #=> "8"
p '42'.pred     #=> "41"
p '40'.pred     #=> "39"
p '10'.pred     #=> "09"  (should be both '9' and '09')
p 'A0'.pred     #=> "9"   (should be ????)
p 'AA0'.pred    #=> "Z9"  (should be ????)
p 'AA'.pred     #=> "Z"   (should be ????)