如何在 C#.Net 泛型集合中按索引处理嵌套列表的元素？

Question

这似乎是一件简单的事情，但是如何按索引寻址嵌套列表的元素呢？

我最近与一位同事共享了一个返回 List<List<int>> 的 C# 类，该同事不知道如何处理生成的 List<List<int>> 集合，并且无法在 StackOverflow 上找到它。 find index of an int in a list 描述了如何索引List<int>，但没有描述如何寻址嵌套列表。

Answer 1

使用索引器从父列表中获取子列表，然后使用另一个索引器获取列表中的实际项目。也可以使用ElementAt 或其他list functions，以及foreach 或其他iterators。

    // create a jagged array for demo
    List<List<int>> NestedListOfLists = (new List<int>[] {
        (new int[] { 1, 2, 3 }).ToList(),
        (new int[] { 4, 5 }).ToList(),
        (new int[] { 6 }).ToList()
    }).ToList();

    // one way to address a List of Lists, returns 3:5:6
    Console.WriteLine("{0}:{1}:{2}",
        NestedListOfLists[0][2], // NestedListOfLists[0] returns the first list, which then can be indexed with [2] for the third element
        NestedListOfLists[1][1], // NestedListOfLists[1] returns the first list, which is then indexed with [1]
        NestedListOfLists[2][0]  // NestedListOfLists[2] returns the first list, which is then indexed with [0]
        );

    /*Console.WriteLine("{0}",
        NestedListOfLists[1,0]    // this doesn't compile
        );*/

    // another way to address a List of Lists
    Console.WriteLine("{0}",
        (NestedListOfLists.ElementAt(0)).ElementAt(2) // NestedListOfLists.ElementAt(0) returns the first list, which then can be indexed with ElementAt(2) for the third element
        );

    // sometimes its practical to iterate through the lists
    foreach( List<int> IntList in NestedListOfLists)
    {
        Console.Write("List of {0}: \t", IntList.Count() );
        foreach (int i in IntList)
        {
            Console.Write("{0}\t", i );
        }
        Console.Write("\n");
    }