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snippets/python/s/group-by.md

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+---
+title: Group list elements
+type: snippet
+language: python
+tags: [list,dictionary]
+cover: body-of-water
+dateModified: 2020-11-02T19:28:05+02:00
+---
+
+Groups the elements of a list based on the given function.
+
+- Use `collections.defaultdict` to initialize a dictionary.
+- Use `fn` in combination with a `for` loop and `dict.append()` to populate the dictionary.
+- Use `dict()` to convert it to a regular dictionary.
+
+```py
+from collections import defaultdict
+
+def group_by(lst, fn):
+  d = defaultdict(list)
+  for el in lst:
+    d[fn(el)].append(el)
+  return dict(d)
+```
+
+```py
+from math import floor
+
+group_by([6.1, 4.2, 6.3], floor) # {4: [4.2], 6: [6.1, 6.3]}
+group_by(['one', 'two', 'three'], len) # {3: ['one', 'two'], 5: ['three']}
+```