input: Write a function to double each element in an array.
  assert double_array_elements([2, 3, 4]) == [4, 6, 8]
  assert double_array_elements([1, 2, 3, 4, 5]) == [2, 4, 6, 8, 10]
  assert double_array_elements([10, 20, 30]) == [20, 40, 60]
output: def double_array_elements(arr):
      return [x * 2 for x in arr]