input: Write a function to calculate the sum of an array of integers. If the array is empty, return 0. The function should take in an array and return an integer representing the sum of the array elements.
  
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([-1, -2, -3, -4])==-10
  assert calculate_sum([])==0
output: def calculate_sum(arr):
      return sum(arr)