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List Difference Based on Function

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Introduction

In Python, we can easily find the difference between two lists using the built-in set() function. However, what if we want to find the difference based on a specific function applied to each element in both lists? In this challenge, you will create a function that takes in two lists and a function, and returns the difference between the two lists after applying the provided function to each list element of both.

Skills Graph

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List Difference Based on Function

Create a function called difference_by(a, b, fn) that takes in three parameters:

  • a: a list of elements
  • b: a list of elements
  • fn: a function that will be applied to each element in both lists

The function should return a list of elements that are in list a but not in list b, after applying the provided function fn to each element in both lists.

To solve this problem, you can follow these steps:

  1. Create a set, using map() to apply fn to each element in b.
  2. Use a list comprehension in combination with fn on a to only keep values not contained in the previously created set, _b.
def difference_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) not in _b]
from math import floor

difference_by([2.1, 1.2], [2.3, 3.4], floor) ## [1.2]
difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
## [ { x: 2 } ]

Summary

In this challenge, you have learned how to find the difference between two lists based on a specific function applied to each element in both lists. You have also practiced using map(), set(), and list comprehension to solve the problem.

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