Group Elements by Function

Introduction

In Python, you can group elements of a list based on a given function. This can be useful when you want to count the number of occurrences of each group. In this challenge, you will be tasked with creating a function that groups the elements of a list based on a given function and returns the count of elements in each group.

Count Grouped Elements

Write a function count_by(lst, fn = lambda x: x) that takes a list lst and a function fn as arguments. The function should group the elements of the list based on the given function and return a dictionary with the count of elements in each group.

To solve this problem, you can follow these steps:

1. Initialize a dictionary using collections.defaultdict.
2. Use map() to apply the given function to each element of the list.
3. Iterate over the mapped values and increase the count of each element in the dictionary.

The function should return the resulting dictionary.

from collections import defaultdict

def count_by(lst, fn = lambda x: x):
  count = defaultdict(int)
  for val in map(fn, lst):
    count[val] += 1
  return dict(count)

from math import floor

count_by([6.1, 4.2, 6.3], floor) ## {6: 2, 4: 1}
count_by(['one', 'two', 'three'], len) ## {3: 2, 5: 1}

Summary

In this challenge, you learned how to group elements of a list based on a given function and return the count of elements in each group. You also learned how to use collections.defaultdict and map() to solve this problem.