# Write a Python program to calculate the average of a given list, after mapping each element to a value using the provided function.  
def average_by(lst, fn = lambda x: x):
  return sum(map(fn, lst), 0.0) / len(lst)

print(average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n']))
print(average_by([{ 'n': 10 }, { 'n': 20 }, { 'n': -30 }, { 'n': 60 }], lambda x: x['n']))
