18 lines
459 B
Markdown
18 lines
459 B
Markdown
---
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title: sum_by
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tags: math,list,function,beginner
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---
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Returns the sum of a list, after mapping each element to a value using the provided function.
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Use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `sum()` to return the sum of the values.
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```py
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def sum_by(lst, fn):
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return sum(list(map(fn,lst)))
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```
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```py
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sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20
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```
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