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python/snippets/average-by.md

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+---
+title: Mapped list average
+type: snippet
+tags: [math,list]
+cover: flower-vase
+dateModified: 2020-11-02T19:27:07+02:00
+---
+
+Calculates 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)`.
+- Omit the last argument, `fn`, to use the default identity function.
+
+```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
+```