24 lines
665 B
Markdown
24 lines
665 B
Markdown
---
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title: count_by
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tags: list
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---
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:information_source: Already implemented via `collections.Counter`
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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 `map()` to map the values of the list using the given function. Iterate over the map and increase the the elements count each time it occurs.
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```py
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def count_by(arr, fn=lambda x: x):
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key = {}
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for el in map(fn, arr):
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key[el] = 0 if el not in key else key[el]
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key[el] += 1
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return key
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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) # {4: 1, 6: 2}
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count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
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``` |