27 lines
672 B
Markdown
27 lines
672 B
Markdown
---
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title: count_by
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tags: list,intermediate
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---
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Groups the elements of a list based on the given function and returns the count of elements in each group.
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- Use `defaultdict()` to initialize a dictionary.
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- Use `map()` to map the values of the given list using the given function.
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- Iterate over the map and increase the element count each time it occurs.
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```py
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from collections import defaultdict
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def count_by(lst, fn=lambda x: x):
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count = defaultdict(int)
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for val in map(fn, lst):
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count[val] += 1
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return dict(count)
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```
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```py
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from math import floor
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count_by([6.1, 4.2, 6.3], floor) # {6: 2, 4: 1}
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count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
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```
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