From: Robert Klemme <shortcutter@...>
Date: 2010-10-21T01:45:17+09:00
Subject: Re: about handling args in block

On 20.10.2010 16:35, Jeremy Bopp wrote:
> On 10/20/2010 9:16 AM, salamond wrote:
>> Hi, guys.
>>
>> I'm writing a Node class for a Binary Tree.
>> Coding comes below.
>>
>> There's a method remove_leaf.
>> Now I want to do it this way:
>> =begin
>>                         [@left_node, @right_node].each do |node|
>>                                 if node.is_leaf?
>>                                         node = nil
>>                                 end
>>                         end
>> =end
>>
>> But it doesn't work.
>> Is there a way to do it ?
>
> In your example, node is just a reference to the same objects referenced
> by @left_node and @right_node.  Assigning nil to node only discards the
> reference held by node.  It looks like you're trying to do this too much
> the C/C++ way, where you would have a pointer to a pointer that you
> could then nullify.
>
> The alternative code you're already using (where you unrolled this loop)
> is probably the easiest and most straightforward way to deal with this
> issue.

OP, you can somewhat simplify the code

Node = Struct.new :value, :left, :right do

   # We usually do not use prefix "is" which
   # you might be used to in Java.
   def leaf?
     @left.nil? && @right.nil?
   end

   # Actually this will remove all leaves hence the
   # changed name.
   def remove_leaves
     self.left = nil if left && left.leaf?
     self.right = nil if right && right.leaf?
     self
   end

   def to_s; value.to_s end

   # Useful for recursive algorithms.
   def visit(val = nil, &b)
     val = @left.visit(val, &b) if @left
     val = b[self, val]
     val = @right.visit(val, &b) if @right
     val
   end
end

:-)

Have fun!

Btw, you original implementation had a flaw because it would break if a 
Node had only one child.  For the other the test ".is_leaf?" would fail 
because NilClass does not have that method.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/