input: Write a function to read an array from the user and then determine the mean and median of the array. The function should return both values. If the array is empty, the function should return None for both mean and median.
  
  def mean_median(arr):
      if not arr:
          return None, None
      
      arr_sum = 0
      for i in range(len(arr)):
          arr_sum += arr[i]
      
      mean = arr_sum / len(arr)
      
      arr = sorted(arr)
      if len(arr) % 2 != 0:
          median = arr[len(arr) // 2]
      else:
          median = (arr[len(arr) // 2] + arr[len(arr) // 2 - 1]) / 2
      
      return mean, median
  
  assert mean_median([10, 15, 20, 25, 30, 35]) == (22.5, 22.5)
  assert mean_median([3, 5, 7, 12, 13, 14, 21, 23, 23, 23, 23, 29, 40, 56]) == (22.0, 22.0)
  assert mean_median([100]) == (100.0, 100.0)
  assert mean_median([]) == (None, None)
output: def mean_median(arr):
      if not arr:
          return None, None
      
      arr_sum = 0
      for i in range(len(arr)):
          arr_sum += arr[i]
      
      mean = arr_sum / len(arr)
      
      arr = sorted(arr)
      if len(arr) % 2 != 0:
          median = arr[len(arr) // 2]
      else:
          median = (arr[len(arr) // 2] + arr[len(arr) // 2 - 1]) / 2
      
      return mean, median