总结列表清单？ [英] Sum a list of lists?

问题描述

我知道我可以将两个列表合计为：

  zipWith（+）[1,2,3] [2,3,4] 
  

但是列表的列表呢？例如：

  let k = [[1,1,1]，[2,2,2]，[3， 3,3]] 
 sumFoo k 
> [6,6,6] 
  

我试过类似的东西：

  foldr（\xs ys  - > zipWith（+）xs ys）[] k 
  

但是这给了我一个空的列表！

编辑：抱歉，我选择了一个不好的例子！它应该总结不同列表的相同标记。查看更新后的示例。 现在更清楚你实际需要什么:)你有一个矩阵表示为列表的列表并且想要提取列的总和。要做到这一点，只需对矩阵进行转置并计算行和：

  Prelude> import Data.List 
 Prelude Data.List>地图总和。转置$ [[1,1,1]，[2,2,2]，[3,3,3]] 
 [6,6,6] 
  

How can I sum a list of lists with?

I know that I can sum two lists as:

zipWith (+) [1,2,3] [2,3,4]

But what about a list of list? For example:

let k = [ [1,1,1], [2,2,2], [3,3,3] ]
sumFoo k
> [ 6, 6, 6]

I tried something like:

foldr (\xs ys -> zipWith (+) xs ys) [] k

but that gives me an empty list!

EDIT: Sorry I picked a bad example! It should sum over the same indicies of DIFFERENT lists. See updated example.

解决方案

EDIT: Now it's more clear what you actually need :) You have a matrix represented as a list of lists and want to extract the column sums. To do this, simply transpose the matrix and compute the row sums:

Prelude> import Data.List
Prelude Data.List> map sum . transpose $ [ [1,1,1], [2,2,2], [3,3,3] ]
[6,6,6]

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