From: mental@...
Date: 2005-12-13T05:05:03+09:00
Subject: Re: A question about recursive programming

Quoting Hank Gong <hankgong@gmail.com>:

> You program works.

It was just intended to illustrate a trivial transformation from
iterative to recursive.

> Mine one listed here:
>
>  def max(arr)
>     case arr.size
>         when 1 then return arr.first   ＃<---Wrong
>         when arr.first>max(arr[1..-1]) then return arr.first
>         else return max(arr[1..-1])
>     end
> end

That's a Ruby problem, not a recursion problem; you can't mix forms
of case/when like that.

There's a choice of either:

 case
   when [boolean condition]
   ...
 end

or:

 case value
   when [other value to compare with ===]
   ...
 end

If you rewrite this using the first form:

 def max( arr )
   case
     when arr.size == 1 then return arr.first
     when arr.first > max( arr[1..-1] ) then return arr.first
     else return max( arr[1..-1] )
   end
 end

It will work, although it's not optimal.  For one thing, return is
unnecessary here, and more importantly max( arr[1..-1] ) will be
called twice needlessly...

-mental