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]
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]