# Write a Python program to get a list of elements that exist in both lists, after applying the provided function to each list element of both.  
 def intersection_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) in _b]
from math import floor
print(intersection_by([2.1, 1.2], [2.3, 3.4], floor))
