# Count grouped elements

Python, List · Nov 2, 2020

Groups the elements of a list based on the given function and returns the count of elements in each group.

• Use `collections.defaultdict` to initialize a dictionary.
• Use `map()` to map the values of the given list using the given function.
• Iterate over the map and increase the element count each time it occurs.
```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}```

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