From: Rob Biedenharn <Rob@...>
Date: 2008-06-11T01:25:12+09:00
Subject: Re: Random Number Stuff

On Jun 10, 2008, at 10:13 AM, Dave Bass wrote:
> David Stanislaus wrote:
>> How would you create a random number generator thats limited to a
>> specific rang? say
>>
>> 1930 - 1950
>
> If you want to do this a lot, you could add your own method to the
> builtin Range class:
>
>  class Range
>    def random
>      self.begin + rand(self.end - self.begin + 1)

>    end
>  end

It would be safer to do:
   class Range
     def random
       case self.begin <=> self.end
       when -1
         self.begin + rand(self.end - self.begin +  
(self.exclude_end? ? 0 : 1))
       when 0
         self.begin
       when +1
         self.begin - rand(self.begin - self.end +  
(self.exclude_end? ? 0 : 1))
       end
     end
   end

Since a Range does not require the endpoints to be ordered.  And  
1930..1950 is different from 1930...1950 according to whether 1950 is  
to be included.


> Then generate random numbers like this:
>
>  puts (1930...1950).random
>
> (For some reason this gives "warning: (...) interpreted as grouped
> expression"; I don't know why. Maybe someone more experienced could
> explain.)


Because your usage is to group the 1930...1950 so the random method is  
sent to the Range object and not to the 1950.  If you added the  
parentheses for the puts, the opening parenthesis would no longer be  
ambiguous:
   puts((1930...1950).random)

tries = Hash.new {|h,k| h[k] = 0 }
range = 1930..1950
500.times {|_| tries[range.random] += 1 }
tries.sort.each {|year,count| puts "%d %3d"%[year,count] }

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com