LINQ可以在收集被排序时使用二进制搜索吗？ [英] Can LINQ use binary search when the collection is ordered?

问题描述

我可以以某种方式指示LINQ使用二进制搜索时，我试图搜索的集合是有序的。我使用 ObservableCollection< T> ，填充有序数据，我试图使用 Enumerable.First（< Predicate>）。在我的谓词，我过滤的字段我的集合的值排序。

Can I somehow "instruct" LINQ to use binary search when the collection that I'm trying to search is ordered. I'm using an ObservableCollection<T>, populated with ordered data, and I'm trying to use Enumerable.First(<Predicate>). In my predicate, I'm filtering by the value of the field my collection's sorted by.

推荐答案

据我所知，它不可能与内置的方法。但是，编写一个扩展方法比较容易，这样可以写出如下代码：

As far as I know, it's not possible with the built-in methods. However it would be relatively easy to write an extension method that would allow you to write something like that :

var item = myCollection.BinarySearch(i => i.Id, 42);

（假设你的集合实现了IList;没有办法执行二叉搜索if您不能随机访问这些项目）

(assuming, of course, that you collection implements IList ; there's no way to perform a binary search if you can't access the items randomly)

这里有一个示例实现：

public static T BinarySearch<T, TKey>(this IList<T> list, Func<T, TKey> keySelector, TKey key)
        where TKey : IComparable<TKey>
{
    if (list.Count == 0)
        throw new InvalidOperationException("Item not found");

    int min = 0;
    int max = list.Count;
    while (min < max)
    {
        int mid = min + ((max - min) / 2);
        T midItem = list[mid];
        TKey midKey = keySelector(midItem);
        int comp = midKey.CompareTo(key);
        if (comp < 0)
        {
            min = mid + 1;
        }
        else if (comp > 0)
        {
            max = mid - 1;
        }
        else
        {
            return midItem;
        }
    }
    if (min == max &&
        min < list.Count &&
        keySelector(list[min]).CompareTo(key) == 0)
    {
        return list[min];
    }
    throw new InvalidOperationException("Item not found");
}

（未经测试...可能需要进行一些调整） 现在测试并修复;）

它引发 InvalidOperationException 的事实可能看起来很奇怪，这是 Enumerable.First 在没有匹配项时执行的操作。

The fact that it throws an InvalidOperationException may seem strange, but that's what Enumerable.First does when there's no matching item.

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