input: def bubble_sort(arr):
      """ Sorts a given array in ascending order using bubble sort.
      >>> bubble_sort([3, 7, 4, 1])
      [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]
      return arr
  
  arr = [3, 7, 4, 1]
  bubble_sort(arr)
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]
      return arr