count_by

snippets/count_by.md

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+### count_by
+
+Groups the elements of a list based on the given function and returns the count of elements in each group.
+
+Use `map()` to map the values of the list using the given function. Iterate over the map and increase the the elements count each time it occurs.
+
+```python
+def count_by(arr, fn=lambda x: x):
+    key = {}
+    for el in map(fn, arr):
+        key[el] = 0 if not el in key else key[el]
+        key[el] += 1
+    return key
+```
+
+``` python
+from math import floor
+count_by([6.1, 4.2, 6.3], floor) # {4: 1, 6: 2}
+count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
+```