使用LINQ得到一个集合的最后N个元素？ [英] Using Linq to get the last N elements of a collection?

问题描述

给定一个集合，有没有办法让该集合的最后N个元素？如果没有在框架的方法，这将是编写扩展方法来做到这一点的最好方法是什么？

解决方案

  collection.Skip（Math.Max​​（0，collection.Count（） -  N））;
 

此方法preserves项目为了不会对任何排序的依赖，并拥有跨多个LINQ提供广泛的兼容性。

要注意不要叫跳过与负数是很重要的。当$ P $带负参数psented一些供应商，如实体框架，将产生一个ArgumentException。到 Math.Max​​ 的调用避免了这种整齐。

下面的类有所有必需品扩展方法，它们是：静态类，静态方法，并使用这个关键字

。

 公共静态类MiscExtensions
{
    //例如：collection.TakeLast（5）;
    公共静态的IEnumerable＆LT; T＆GT; TakeLast＆LT; T＆GT;（这个IEnumerable的＆LT; T＆GT;源，INT N）
    {
        返回source.Skip（Math.Max​​（0，source.Count（） -  N））;
    }
}
 

对性能的简要说明：

由于调用计数（）可能会导致某些数据结构的枚举，这种方法有导致在数据两遍的风险。这是不是真的大多数枚举接口问题;事实上，已经存在优化的列表，数组，甚至EF查询评估计数（）在O（1）时间的操作。

如果，但是，你必须使用只进枚举，并想避免两遍，考虑喜欢的拉塞五卡尔森或马克拜尔斯形容。这两种方法都使用一个临时缓冲区，以存放在枚举项，一旦集合的末尾被发现它们产生

Given a collection, is there a way to get the last N elements of that collection? If there isn't a method in the framework, what would be the best way to write an extension method to do this?

解决方案

collection.Skip(Math.Max(0, collection.Count() - N));

This approach preserves item order without a dependency on any sorting, and has broad compatibility across several LINQ providers.

It is important to take care not to call Skip with a negative number. Some providers, such as the Entity Framework, will produce an ArgumentException when presented with a negative argument. The call to Math.Max avoids this neatly.

The class below has all of the essentials for extension methods, which are: a static class, a static method, and use of the this keyword.

public static class MiscExtensions
{
    // Ex: collection.TakeLast(5);
    public static IEnumerable<T> TakeLast<T>(this IEnumerable<T> source, int N)
    {
        return source.Skip(Math.Max(0, source.Count() - N));
    }
}

A brief note on performance:

Because the call to Count() can cause enumeration of certain data structures, this approach has the risk of causing two passes over the data. This isn't really a problem with most enumerables; in fact, optimizations exist already for Lists, Arrays, and even EF queries to evaluate the Count() operation in O(1) time.

If, however, you must use a forward-only enumerable and would like to avoid making two passes, consider a one-pass algorithm like Lasse V. Karlsen or Mark Byers describe. Both of these approaches use a temporary buffer to hold items while enumerating, which are yielded once the end of the collection is found.

这篇关于使用LINQ得到一个集合的最后N个元素？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！