Here's mine:

MAXNUM = (( ARGV[0].to_i == 0 ) ? 1000000 : ARGV[0].to_i)

numbers = (0...MAXNUM).to_a
divs = Array.new

t = Time.now

MAXNUM.downto(0) do |n|
    nts = n.to_s
    next if numbers[n].nil? or
            ((nts[-1]-48) == 0) or
            (nts[0]-48) < (nts[-1]-48)*2 or
            nts == nts.reverse

    if n % (v = nts.reverse.to_i) == 0
      divs << [n,v]
      numbers.delete(v)
    end
end

puts "Benchmark = #{Time.now - t} secondi"
puts divs.inspect


On Sun, May 4, 2008 at 7:40 PM, Ken Bloom <kbloom@[...].com>  wrote:
>  On Fri, 02 May 2008 10:28:00 -0500, Matthew Moss wrote:
> 
> 
> 
>  > This is a fairly simple quiz this week, as I'm in the middle of putting
>   > together a new website to replace the existing RQ2 website, which was
>   > supposed to be temporary. Hopefully the new one should be in place next
>   > week.
>   >
>   > -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>   >
>   > The three rules of Ruby Quiz 2:
>   >
>   > 1.  Please do not post any solutions or spoiler discussion for this quiz
>   > until 48 hours have passed from the time on this message.
>   >
>   > 2.  Support Ruby Quiz 2 by submitting ideas as often as you can! (A
>   > permanent, new website is in the works for Ruby Quiz 2. Until then,
>   > please visit the temporary website at
>   >
>   >      <http://matthew.moss.googlepages.com/home>.
>   >
>   > 3.  Enjoy!
>   >
>   > Suggestion:  A [QUIZ] in the subject of emails about the problem helps
>   > everyone on Ruby Talk follow the discussion.  Please reply to the
>   > original quiz message, if you can.
>   >
>   > -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>   >
>   > Quiz #161
>   > Reverse Divisible Numbers
>   >
>   > This week's quiz is borrowed from Jon Galloway (http://tinyurl.com/
>   > 5jvf37).
>   > Don't read the comments or solution there without trying this first!
>   >
>   > Your task is to write a bit of Ruby code that will find and report all
>   > integers that are divisible by their reverse. For example, 9801 is
>   > divisible by 1089.
>   >
>   > Your script should accept a single, optional argument to specify the
>   > upper
>   > limit of your search. If not provided, the default should be one
>   > million.
>   >
>   > There are two extra rules for finding these "reverse divisible" numbers:
>   >
>   > 1. Don't count palindromes; they are obviously reverse-divisible. 2.
>   > Don't count numbers ending with zero. Reversing such numbers forces you
>   >    to drop leading zeros on the divisor, and that's not as much fun.
> 
>   Everyone's posted benchmarks, but nobody's posted code yet. It's been
>   more than 48 hours, so I guess I'll start.
> 
>   MAX=1_000_000
> 
>   #we want pairs of reverse,num where num.to_s==reverse.to_s.reverse and
>   #num*multiplier == reverse for some integer multiplier where
>   #multiplier >= 2
>   #
>   #if multiplier is a fraction, then we would swap num and reverse to get an
>   #integer, so we'll do it in one direction only
>   #
>   #if multiplier is 1, then num is a palindrome
>   #
>   #if the most significant digit of num > 4 then all num*multiplier
>   #have more digits than reverse has, so we need not check those
> 
>   DIGITS=(Math.log10(MAX)).to_i
>   ranges=(0..DIGITS).collect{ |digits| 10**digits ...  5*10**digits}
>   ranges[-1]=ranges[-1].begin..MAX if MAX < ranges[-1].end
> 
>   ranges.each do |range|
>    range.each do |num|
>      ns=num.to_s
>      reverse=ns.reverse.to_i
>      next if num==reverse
>      next if ns[-1,1]=='0'
>      next unless (reverse/num)*num==reverse
>      puts "#{num} * #{reverse/num} = #{reverse}"
>    end
>   end
> 
> 
> 
> 
>   --
>   Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
>   Department of Computer Science. Illinois Institute of Technology.
>   http://www.iit.edu/~kbloom1/
> 
> 



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