在c＃字典中只进行一次查找的查找或插入操作 [英] Find-or-insert with only one lookup in c# dictionary

问题描述

我是一个以前的C ++ / STL程序员，试图使用c＃/。NET技术编写一个快速前进算法...

I'm a former C++/STL programmer trying to code a fast marching algorithm using c#/.NET technology...

如果不存在，在给定键处插入一个值的STL方法map :: insert，否则返回现有键值对的迭代器。

I'm searching for an equivalent of STL method "map::insert" that insert a value at given key if not exists, else returns an iterator to the existing key-value pair.

我发现这有两个查找：一个在TryGetValue和另一个在添加方法：

The only way I found does this with two lookups : one inside TryGetValue and another one in Add method :

List<Point> list;
if (!_dictionary.TryGetValue (pcost, out list))
{
    list = new List<Point> ();
    dictionary.Add (pcost, list);
}
list.Add (new Point { X = n.x, Y = n.y });

有什么解释为什么这是不可能使用.NET容器？

Is there something that explains why this is not possible using .NET containers ? Or did I missed some point ?

感谢。

推荐答案

可以按照以下方式分配值：

You can just assign your value in the following way:

var dict = new Dictionary<int, int>();
dict[2] = 11;

如果键2的值不存在 - 将被添加，否则它将被覆盖。

if value with key 2 does not exist - it will be added and otherwise it will be just overriden.

字典没有GetOrAdd方法，但来自C＃4.0的ConcurrentDictionary会执行：

Dictionary does not have method GetOrAdd, but ConcurrentDictionary from C# 4.0 does:

var dict = new ConcurrentDictionary<int, int>();
dict[2] = 10;
int a = dict.GetOrAdd(2, 11);// a == 10

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