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]
  
  # Example Usage:
  # arr = [3,7,4,1]
  # bubble_sort(arr)
  # print ("Sorted array :")
  # for i in range(len(arr)):
  #     print ("%d" %arr[i]),
  
  # Output:
  # Sorted array :
  # 1 3 4 7
output:     return arr