input: def bubble_sort(arr): 
      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] 
  
  """Create a program in Python to sort a given array in ascending order using bubble sort.
  >>> bubble_sort([3, 7, 4, 1])
  [1, 3, 4, 7]
  """
output:     return arr