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snippets/python/s/count-by.md

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+---
+title: Count grouped elements
+type: snippet
+language: python
+tags: [list]
+cover: rabbit-call
+dateModified: 2020-11-02T19:27:07+02:00
+---
+
+Groups the elements of a list based on the given function and returns the count of elements in each group.
+
+- Use `collections.defaultdict` to initialize a dictionary.
+- Use `map()` to map the values of the given list using the given function.
+- Iterate over the map and increase the element count each time it occurs.
+
+```py
+from collections import defaultdict
+
+def count_by(lst, fn = lambda x: x):
+  count = defaultdict(int)
+  for val in map(fn, lst):
+    count[val] += 1
+  return dict(count)
+```
+
+```py
+from math import floor
+
+count_by([6.1, 4.2, 6.3], floor) # {6: 2, 4: 1}
+count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
+```