25 lines
670 B
Markdown
25 lines
670 B
Markdown
---
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title: List intersection based on function
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tags: list,function
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expertise: intermediate
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firstSeen: 2019-08-20T15:16:27+03:00
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lastUpdated: 2020-11-02T19:28:05+02:00
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---
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Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.
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- Create a `set`, using `map()` to apply `fn` to each element in `b`.
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- Use a list comprehension in combination with `fn` on `a` to only keep values contained in both lists.
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```py
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def intersection_by(a, b, fn):
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_b = set(map(fn, b))
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return [item for item in a if fn(item) in _b]
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```
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```py
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from math import floor
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intersection_by([2.1, 1.2], [2.3, 3.4], floor) # [2.1]
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```
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