An attempt at implementing Priority Queue through solely Ruby

heap_in_ruby

class Binary_heap
  def initialize(num = 10)
    @ary = Array.new
    num.times do @ary << rand(100) end
  end
  
  attr_reader :items, :ary
  
  def binary_heap()
    @ary.unshift(nil)
  end
  
  def build_heap()
    i = (@ary.size() - 1)/2
    begin 
      percolate_down(i)
      i -= 1
    end until i <= 1
  end

  
  def percolate_down(hole)
    tmp = @ary[hole]
    begin
      child = hole * 2
      if (child != @ary.size && @ary[child+1] < @ary[child])
        child += 1
      end
      if (@ary[child] < tmp)
        @ary[hole] = @ary[child]
      else break
      end
      hole = child
    end while (hole*2 <= @ary.size)
    @ary[hole] = tmp
  end
end

heap = Binary_heap.new
puts heap.ary
heap.binary_heap
puts heap.ary
heap.build_heap
puts heap.ary

I am not sure why it doesn't work. Why does @ary[child] return nil when it should return the element in the array?