How to "flatten" a Ruby array

[Garrett Rowe]

Ok so I'm trying to learn Ruby. My plan was to get a feel for the syntax by trying to work through Ninety-Nine Lisp Problems in Ruby. However I got stuck on Problem 7.

P07 (**) Flatten a nested list structure.
    Transform a list, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively).
 
    Example:
    * (my-flatten '(a (b (c d) e)))
    (A B C D E)
 
    Hint: Use the predefined functions list and append.

My test code looks like:

   def test_my_flaten
      mt_arr = [[]]
      assert_equal( [], my_flatten( mt_arr ) )
      
      non_nested = [1, 2, 3]
      assert_equal( non_nested, my_flatten( non_nested ) )
      
      flat = [1, 2, 3, 4, 5]
      
      nested = [1 , [ 2, 3] , 4, 5]
      assert_equal( flat, my_flatten( nested ) )
      
      multi_nested = [1, [2, 3], [4, 5]]
      assert_equal( flat, my_flatten( multi_nested ) )
      
      deeply_nested = [1, [2, [3, 4]], 5]
      assert_equal( flat, my_flatten( deeply_nested ) )
    end
 

And my "my_flatten" method looks like:

def my_flatten( arr )
  flat = Array.new
  arr.each {|x| (x.kind_of? Array) ? flat + ( my_flatten(x) ) : flat << x}
end

But my code fails at my first assertion (Expected [] but was [[]]),

and if I comment that out it fails on my third assertion too:

Expected [1, 2, 3, 4, 5] but was [1, [2, 3], 4, 5].

Can any of you guys give me a nudge int the right direction?
[ February 18, 2008: Message edited by: Garrett Rowe ]

[Jim Yingst]

It looks like your my_flatten will create a new array every time it's invoked - that's the first line, after all. But is that really what you want? A flattened array should have just one array, no matter how many times the method recurses, yes? In many cases you want to append new elements to an existing array, not create a new one. Can you think of a way to do that?