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+---
+title: intersection_by
+tags: list,function,intermediate
+---
+
+Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.
+
+Create a `set` by applying `fn` to each element in `b`, then use list comprehension in combination with `fn` on `a` to only keep values contained in both lists.
+
+```py
+def intersection_by(a, b, fn):
+  _b = set(map(fn, b))
+  return [item for item in a if fn(item) in _b]
+```
+
+```py
+from math import floor
+intersection_by([2.1, 1.2], [2.3, 3.4],floor) # [2.1]
+```
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