支持查找最接近键的C＃排序字典对象？

Question

Is there any ordered dictionary collection available in C# that provides a ready means of finding the first key greater than a value if the sought value isn't present?

Ie, if (!Dictionary.ContainsKey(some_key)) then return the next key > some_key based on the dictionary's ordering predicate?

If there's a clever way of doing this with delegates an example of that would be equally appreciated!

Answer 1

As Vadim suggested, your best bet would be the SortedDictionary implementation which stores the keys sorted. From there you could do the following:

var next = dictionary.ContainsKey(key)
                ? dictionary[key]
                : dictionary.FirstOrDefault(kvp => kvp.Key > key).Value;

The dictionary.FirstOrDefault will return the first key value pair where the key is greater than the desired key. If there are none, then a blank key-value pair is returned {,} and the value returned should be the default value of the type stored. Since I was playing with a SortedDictionary, it returned null.

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var dictionary = new SortedDictionary<int, string> {{1, "First"}, {2, "Second"}, {10, "10th"}};
            Console.WriteLine(GetNext(1, dictionary));
            Console.WriteLine(GetNext(3, dictionary));
            Console.WriteLine(GetNext(11, dictionary));

            Console.ReadLine();
        }

        private static string GetNext(int key, SortedDictionary<int, string> dictionary)
        {
            return dictionary.ContainsKey(key)
                ? dictionary[key]
                : dictionary.FirstOrDefault(kvp => kvp.Key > key).Value;
        }
    }
}

Answer 2

Here is a great binary-search implementation for any sorted IList : If the exact key doesn't exist, it returns the ~index of the next greatest key.

With that class in scope one can just do something like the following:

SortedList myList;
int nextBiggestKey; // Index of key >= soughtValue
if((nextBiggestKey = myList.Keys.BinarySearch(soughtValue)) < 0)
{
   if(~nextBiggestKey > myList.Count) continue; // soughtValue is larger than largest key in myList
   nextBiggestKey = ~nextBiggestKey
}