input: Write a function to read an array of integers and calculate their sum. If the array is empty, return 0.
  def calculate_sum(arr):
      sum = 0
      for i in arr:
          sum += i
      return sum
  
  assert calculate_sum([1, 3, 5, 7]) == 16
  assert calculate_sum([]) == 0
  assert calculate_sum([10, -5, 3, 2]) == 10
  assert calculate_sum([-1, -2, -3, -4]) == -10
  assert calculate_sum([0, 0, 0, 0]) == 0
output: def calculate_sum(arr):
      sum = 0
      for i in arr:
          sum += i
      return sum