# Python: Symmetric difference between two lists, after applying the provided function to each list element of both

## Python List: Exercise - 227 with Solution

Write a Python program to get the symmetric difference between two lists, after applying the provided function to each list element of both.

- Create a set by applying fn to each element in every list.
- Use a list comprehension in combination with fn on each of them to only keep values not contained in the previously created set of the other.

**Sample Solution:**

**Python Code:**

```
# Define a function 'symmetric_difference_by' that takes two lists 'a' and 'b' and a function 'fn' as input.
def symmetric_difference_by(a, b, fn):
# Create sets '_a' and '_b' containing the results of applying the 'fn' function to each element in 'a' and 'b'.
(_a, _b) = (set(map(fn, a)), set(map(fn, b)))
# Find the elements in 'a' that are not in '_b' and the elements in 'b' that are not in '_a'.
# Combine these two sets and return the result as a list.
return [item for item in a if fn(item) not in _b] + [item for item in b if fn(item) not in _a]
# Import the 'floor' function from the 'math' module.
from math import floor
# Display the result of applying 'symmetric_difference_by' to two example lists using the 'floor' function.
print(symmetric_difference_by([2.1, 1.2], [2.3, 3.4], floor))
```

Sample Output:

[1.2, 3.4]

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