Add group_by snippet

snippets/group_by.md

@@ -0,0 +1,23 @@
+---
+title: group_by
+tags: list,object,beginner
+---
+
+Groups the elements of a list based on the given function.
+
+Use `list()` in combination with `map()` and `fn` to map the values of the list to the keys of an object.
+Use list comprehension to map each element to the appropriate `key`.
+
+```py
+def group_by(lst, fn):
+  groups = {}
+  for key in list(map(fn,lst)):
+    groups[key] = [item for item in lst if fn(item) == key]
+  return groups
+```
+
+```py
+import math
+group_by([6.1, 4.2, 6.3], math.floor); # {4: [4.2], 6: [6.1, 6.3]}
+group_by(['one', 'two', 'three'], 'length'); # {3: ['one', 'two'], 5: ['three']}
+```