input: 
  def bubble_sort(arr):
      """ Sorts a given array in ascending order using the bubble sort algorithm.
      >>> arr = [3,7,4,1]
      >>> bubble_sort(arr)
      >>> print(arr)
      [1, 3, 4, 7]
      """
      n = len(arr)
      for i in range(n-1):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
  
  arr = [3,7,4,1]
  bubble_sort(arr)
  print("Sorted array :")
  for i in range(len(arr)):
      print("%d" %arr[i]),
  
output:     n = len(arr)
      for i in range(n-1):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]