Array Flatten does not work (Instance variable nil)

Question

I was trying to write a simple array flatten method, but it does not work using instance variable. It works only using class variables. Can anyone tell me why? and how to make it work using instance variables.

 class Array
 @y = []
  def flatten_array
   self.each do |x|
    if x.class.to_s != 'Array'
    @y <<  x
     else
     x.flatten_array
    end
   end 
   return @y  
  end  
 end


 a =  [1,2,3,4,5]
 b =  [6,7,8]
 c =  [9,10]
 a1 = [12,13,a,b,c]
 puts a1.inspect
 b1 = a1.flatten_array
 puts b1.inspect

Answer 1

The reason why it doesn't work for instance variables is that x.flatten_array is using a new @y, not the one you were accumulating. Using an instance or class variable this way, however, is not good practice as it is effectively using a global variable for a local function.

The standard procedure for writing a recursive function that needs extra variables is to use a helper function:

def flatten_array
  int_flatten(self, [])
end

private 
def int_flatten(a, result)
  a.each do |elem|
    if (elem.is_a? Array)
      int_flatten(elem, result)
    else
      result << elem
    end
  end
  return result
end

or just be willing to use concatenation:

def flatten_array
  result = []
  each do |elem|
    if (elem.is_a? Array)
      result += elem.flatten_array
    else
      result << elem
    end
  end
  return result
end

Answer 2

Use native Array#flatten

a =  [1,2,3,4,5]
b =  [6,7,8]
c =  [9,10]
a1 = [12,13,a,b,c]

puts a1.inspect
#=> [12, 13, [1, 2, 3, 4, 5], [6, 7, 8], [9, 10]]

puts a1.flatten.inspect
#=> [12, 13, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]