25 lines
738 B
Markdown
25 lines
738 B
Markdown
---
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title: union_by
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tags: list,intermediate
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firstSeen: 2019-08-21T09:10:39+03:00
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lastUpdated: 2020-11-02T19:28:35+02:00
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---
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Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.
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- Create a `set` by applying `fn` to each element in `a`.
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- Use a list comprehension in combination with `fn` on `b` to only keep values not contained in the previously created set, `_a`.
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- Finally, create a `set` from the previous result and `a` and transform it into a `list`
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```py
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def union_by(a, b, fn):
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_a = set(map(fn, a))
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return list(set(a + [item for item in b if fn(item) not in _a]))
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```
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```py
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from math import floor
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union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]
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```
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