Add average_by snippet

snippets/average_by.md

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+---
+title: average_by
+tags: math,list,function,intermediate
+---
+
+Returns the average of a list, after mapping each element to a value using the provided function.
+
+Use `map()` to map each element to the value returned by `fn`.
+Use `sum()` to sum all of the mapped values, divide by `len(lst)`.
+
+```py
+def average_by(lst, fn=lambda x: x):
+  return sum(map(fn, lst), 0.0) / len(lst)
+```
+
+```py
+average_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda x: x['n']) # 5.0
+```