From: Matthew Desmarais <desmarm@...>
Date: 2005-12-13T04:14:05+09:00
Subject: Re: A question about recursive programming

mental@rydia.net wrote:

>Quoting Hank Gong <hankgong@gmail.com>:
>
>  
>
>>And also I think it's not easy to think every loop as recursive
>>way.
>>    
>>
>
>  
>
>>For example it's very difficult to write max function by
>>recursive function.
>>    
>>
>
>iterative:
>
> def max( *values )
>   case values.size
>   when 0
>     nil
>   when 1
>     values[0]
>   else
>     i = 1
>     max = values[0]
>     while i < values.size
>       max = values[i] if values[i] > max
>       i = i + 1
>     end
>     max
>   end
> end
>
>recursive:
>
> def max( *values )
>   case values.size
>   when 0
>     nil
>   when 1
>     values[0]
>   else
>     max_helper( values, 1, values[0] )
>   end
> end
>
> def max_helper( values, i, max )
>   if i < values.size
>     max = values[i] if values[i] > max
>     max_helper( values, i + 1, max )
>   else
>     max
>   end
> end
>
>-mental
>  
>
So:
An empty list has no max.
The max of a list containing one element is that element.
The max of a list containing more than one element is the greater of the 
first element and the max of the rest of the list.

def recursive_max(an_array)
  if (an_array.size < 2)
    an_array.first
  else
    rest_max = recursive_max(an_array[1..-1])
    an_array.first > rest_max ? an_array.first : rest_max
  end
end

Regards,
Matthew