---
title: sum_by
tags: math,list,beginner
---

Calculates the sum of a list, after mapping each element to a value using the provided function.

- Use `map()` with `fn` to map each element to a value using the provided function.
- Use `sum()` to return the sum of the values.

```py
def sum_by(lst, fn):
  return sum(map(fn, lst))
```

```py
sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20
```