List Union Based on Function

Introduction

In Python, we can use the set() function to get the union of two lists. However, sometimes we need to apply a function to each element of both lists before getting the union. In this challenge, you will create a function that returns the union of two lists based on a provided function.

List Union Based on Function

Write a function union_by(a, b, fn) that takes in two lists a and b, and a function fn. The function should return a list that contains every element that exists in any of the two lists once, after applying the provided function to each element of both.

To solve this problem, you can follow these steps:

1. Create a set by applying fn to each element in a.
2. Use a list comprehension in combination with fn on b to only keep values not contained in the previously created set, _a.
3. Finally, create a set from the previous result and a and transform it into a list.

The function should have the following input parameters:

• a: a list of elements
• b: a list of elements
• fn: a function that takes an element and returns a value

The function should return a list of elements.

def union_by(a, b, fn):
  _a = set(map(fn, a))
  return list(set(a + [item for item in b if fn(item) not in _a]))

from math import floor

union_by([2.1], [1.2, 2.3], floor) ## [2.1, 1.2]

Summary

In this challenge, you learned how to create a function that returns the union of two lists based on a provided function. You also learned how to apply a function to each element of a list, create a set from a list, and transform a set into a list.