Java-使用流转换列表列表 [英] Java - transposing a list of list using streams

问题描述

出于改进目的，我正尝试仅使用流来转置列表列表.

For improvement purposes, I am trying to use exclusively streams to transpose a list of list.

我的意思是，我有一个包含例如双打列表的列表

By that I mean, that I have a List of List of Doubles that contains for example

1 2 3 4
5 6 7 8

我想获取包含以下内容的双打列表

And I would like to obtain a List of List of Doubles that contains

1 5
2 6
3 7
4 8

以下堆栈溢出问题中提供了一种迭代方法:如何转置List< List> ?

An iterative way is provided in the following Stack Overflow question : How to transpose List<List>?

到目前为止，我只想到了丑陋的解决方案，例如将Double替换为包含列表中的值和索引的自定义对象，使用flatMap进行展平，再使用groupBy使用保存的索引再次构建它，然后转到回到双打.

I have only thought of ugly solutions so far, such as replacing the Double with a custom object that holds the value and the index in the list, flatten using flatMap, build it again using groupBy with the index I saved and then go back to Doubles.

如果您知道任何干净的方法，请告诉我.谢谢.

Please let me know if you are aware of any clean way. Thanks.

推荐答案

我喜欢你的问题！只要List是正方形(每个List包含相同数量的元素)，这是一种简单的实现方法:

I like your question! Here's a simple way to achieve it, as long as the Lists are square (every List contains the same number of elements):

List<List<Integer>> list = List.of(List.of(1, 2, 3, 4), List.of(5, 6, 7, 8));

IntStream.range(0, list.get(0).size())
         .mapToObj(i -> list.stream().map(l -> l.get(i)).collect(Collectors.toList()))
         .collect(Collectors.toList());

上面的代码返回以下内容:

The above code returns the following:

[[1, 5], [2, 6], [3, 7], [4, 8]]

注意:这对于大型列表而言效果不佳.

Note: This will not perform well for large lists.

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