nested arrays and strings in Ruby

Question

I'm trying to figure out nested arrays, particularly of the multi dimensional type. I've gone through two articles on them and are getting tripped up when the iteration is checking for strings.

# Array - 
more_nested_array = [["hello", ["world", "new york"]], ["love", "ruby"]]

# Iteration-
more_nested_array.each do |element|
 element.each do |inner_element|
    if inner_element.is_a?(Array)
      inner_element.each do |third_layer_element|
     end
   end
  end
end

So it is using the if statement because supposedly in some iteration there are strings. This reference to strings is confusing me since it looks like it's just a bunch of arrays. Can someone explain please?

Answer 1

NoMethodError otherwise

The check is needed because the loops are hard-coded for this given tree (or nested arrays, if you prefer).

Removing the check :

more_nested_array = [["hello", ["world", "new york"]], ["love", "ruby"]]

# Iteration-
more_nested_array.each do |element|
  element.each do |inner_element|
    inner_element.each do |third_layer_element|
      puts third_layer_element
    end
  end
end

outputs:

undefined method `each' for "hello":String (NoMethodError)

because not every inner_element is an array or responds to each.

Recursive version

With this kind of structure, it would be preferrable to write a recursive method in order to parse the tree, instead of hard-coding the tree depth and node classes.

more_nested_array = [["hello", ["world", "new york"]], ["love", "ruby"]]

def parse_tree(node, current_depth = 0)
  if node.respond_to?(:each)
    node.each do |child|
      parse_tree(child, current_depth + 1)
    end
  else
    puts "Found #{node.inspect} at depth #{current_depth}"
  end
end

parse_tree(more_nested_array)

It outputs:

Found "hello" at depth 2
Found "world" at depth 3
Found "new york" at depth 3
Found "love" at depth 2
Found "ruby" at depth 2