How can I retrieve first n elements from Dictionary<string, int>?

Question

有没有办法从C＃中的Dictionary中检索前n个元素？

Answer 1

Dictionaries are not ordered per se , you can't rely on the "first" actually meaning that. From MSDN: "For enumeration... The order in which the items are returned is undefined."

You may be able to use an OrderedDictionary depending on your platform version, and it's not a particularly complex thing to create as a custom descendant class of Dictionary.

Answer 2

Note that there's no explicit ordering for a Dictionary , so although the following code will return n items, there's no guarantee as to how the framework will determine which n items to return.

using System.Linq;

yourDictionary.Take(n);

The above code returns an IEnumerable<KeyValuePair<TKey,TValue>> containing n items. You can easily convert this to a Dictionary<TKey,TValue> like so:

yourDictionary.Take(n).ToDictionary();

Answer 3

You can't really take the first N elements from a Dictionary<TKey,TValue> because it is not an ordered collection. So it really has no concept of First, Last, etc ... But as others have pointed out, if you just want to take N elements regardless of order the LINQ take function works fine

var map = GetTheDictionary();
var firstFive = map.Take(5);

Answer 4

Oftentimes omitting the cast to dictionary won't work:

dictionary = dictionary.Take(n);

And neither will a simple case like this:

dictionary = dictionary.Take(n).ToDictionary();

The surest method is an explicit cast:

dictionary = dictionary.Take(n).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

Answer 5

Could use Linq for example?

 var dictionary = new Dictionary<string, int>();

 /// Add items to dictionary

 foreach(var item in dictionary.Take(5))
 {
      // Do something with the first 5 pairs in the dictionary
 }