java 使用 lambdas 的 List<> 的 Java8 子列表
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Java8 sublists of a List<> using lambdas
提问by Peter Andersson
I have a problem which I feel would be perfect for streams and/or lambdas. On the other hand I don't want to overcomplicate this, but since will use this specific technique in many variations (run function on a sublist), I would like some ideas about how to get it right from the beginning.
我有一个问题，我觉得它非常适合流和/或 lambda。另一方面，我不想将其复杂化，但由于将在许多变体中使用此特定技术（在子列表上运行函数），因此我想要一些有关如何从一开始就正确使用它的想法。
I have a List<Product> productList.
我有一个List<Product> productList.
I want to be able to iterate over all sublists in productList. For example all sublists with size=30. This sublist should then be used as an argument to a function.
我希望能够遍历productList. 例如所有大小为 30 的子列表。然后应将此子列表用作函数的参数。
This is my current, naive, solution:
这是我目前的天真解决方案：
List<Product> products=...
// This example uses sublists of size 30
for (int i = 0; i < products.size() - 29; i++) {
// sublist start index is inclusive, but end index is exclusive
List<Product> sublist = products.subList(i, i + 30);
Double res = calc(sublist);
}
// an example of a function would be moving average
How would this be implemented using lambdas?
这将如何使用 lambda 实现？
EDIT I tried to come up with the simplest possible example to illustrate the problem. After some comments, I realized that a perfect example is calculating a moving average. First MAVG is calculated on sublist [0..29], second on [1..30], third on [2..31] and so on.
编辑我试图想出一个最简单的例子来说明这个问题。经过一些评论后，我意识到一个完美的例子是计算移动平均线。第一个 MAVG 在子列表 [0..29] 上计算，第二个在 [1..30] 上，第三个在 [2..31] 上，依此类推。
回答by Eugene
Unless I'm missing something obvious...
除非我遗漏了一些明显的东西......
IntStream.range(0, products.size() - 29)
.mapToObj(i -> products.subList(i, i + 30))
.map(list -> calc(list))
.forEach... // or any other terminal op
Well if you want to run more then a single function to these sublists, like:
好吧，如果您想对这些子列表运行多个函数，例如：
double result = calc(sublist)
log(sublist) // void
double res = diffCalc(sublist)
you are probably off staying with the usual for loop.
您可能不会继续使用通常的 for 循环。
a mapoperation does a single action on a sublist, it could be made to do more in a stream, but that will look really ugly IMO.
一个map操作确实在子列表一个动作，它可以进行做多流，但看起来真难看IMO。
回答by ZhekaKozlov
回答by oleg.cherednik
I think that code could be less clear with using lambdas. Maybe plain old ifis better that modern lambda?
我认为使用 lambdas 代码可能不太清楚。如果比现代lambda更好，也许是旧的？
int sizeSubList = 30;
for (int i = 0; i < products.size(); i++)
calc(products.subList(i, Math.min(i + sizeSubList, products.size())));
