How do you sort a C# dictionary by value?

Question

I often have a Dictionary of keys & values and need to sort it by value. For example, I have a hash of words and their frequencies, and want to order them by frequency.

There's SortedList which is good for a single value (frequency), but I want to map it back to the word.

SortedDictionary orders by key, not value. Some resort to a custom class, but what's the cleanest way?

Answer 1

Looking around, and using some C# 3.0 features we can do this:

foreach (KeyValuePair<string, int> item in keywordCounts.OrderBy(key => key.Value))
        { 
               // do something with item.Key and item.Value
        }

This is the cleanest way I've seen and is similar to the Ruby way of handling hashes.

Answer 2

On a high level, you have no other choice then to walk through the whole Dictionary and look at each value.

Maybe this helps: http://bytes.com/forum/thread563638.html Copy/Pasting from John Timney:

Dictionary<string, string> s = new Dictionary<string, string>();
s.Add("1", "a Item");
s.Add("2", "c Item");
s.Add("3", "b Item");

List<KeyValuePair<string, string>> myList = new List<KeyValuePair<string, string>>(s);
myList.Sort(
    delegate(KeyValuePair<string, stringfirstPair>,
    KeyValuePair<string, stringnextPair>)
    {
        return firstPair.Value.CompareTo(nextPair.Value);
    }
);

Answer 3

@Michael: Thanks for the reply! I did see that post, but was hoping to find a solution without creating an intermediate list.

I'm more familiar with Ruby than C#, which is like this

hash.sort{|a,b| a[1]<=>b[1]}

so I prefer the OrderBy method because it functions similarly to this. Thanks for the tip though.

Answer 4

myList.Sort(
        delegate(KeyValuePair<string, string> firstPair,
        KeyValuePair<string, string> nextPair)
        {
            return firstPair.Value.CompareTo(nextPair.Value);
        }
    );

Since you're targeting .net 2.0 or above, you can simplify this into lambda syntax -- it's equivalent but shorter. If you're targeting .net 2.0 you can only use this syntax if you're using the compiler from vs2008 (or above).

myList.Sort((firstPair,nextPair) =>
    {
        return firstPair.Value.CompareTo(nextPair.Value);
    }
);

Answer 5

@Leon: Thanks for the tip. This is for an internal app so we're using .NET 3.5. I like the lambda syntax as I'm not getting lost in the delegate/tag soup :).

Answer 6

Why not use LINQ:

System.Collections.Generic.Dictionary<string, int> myDict = new Dictionary<string, int>();
    myDict.Add("one", 1);
    myDict.Add("four", 4);
    myDict.Add("two", 2);
    myDict.Add("three", 3);

    var sortedDict = (from entry in myDict orderby entry.Value ascending select entry);

This would also allow for great flexibility in that you can select the top 10, 20 10% etc. Or if you are using your word frequency index for type-ahead, you could also include StartsWith clause as well.

Answer 7

You'd never be able to sort a dictionary anyway. They are not actually ordered. The guarantees for a dictionary are that the key and value collections are iterable, and values can be retrieved by index or key, but here is no guarantee of any particular order. Hence you would need to get the name value pair into a list.

Answer 8

var sortedDict = (from entry in myDict orderby entry.Value ascending select entry);

in this case what can i use insted of var, and how can i reach first element with his value?

Answer 9

The easiest way to get a sorted Dictionary is to use the built in SortedDictionary class:

        //Sorts sections according to the key value stored on "sections" unsorted dictionary, which is passed as a constructor argument
        System.Collections.Generic.SortedDictionary<int, string> sortedSections = null;
        if (sections != null)
        {
            System.Collections.Generic.sortedSections = new SortedDictionary<int, string>(sections);
        }

sortedSections will contains the sorted version of "sections"

Answer 10

Sorting a SortedDictionary list to bind into a ListView control using VB.Net:

    Dim MyDictionary As SortedDictionary(Of String, MyDictionaryEntry)

        MyDictionaryListView.ItemsSource = MyDictionary.Values.OrderByDescending(Function(entry) entry.MyValue)

Public Class MyDictionaryEntry ' Need Property for GridViewColumn DisplayMemberBinding
    Public Property MyString As String
    Public Property MyValue As Integer
End Class

Xaml:

<ListView Name="MyDictionaryListView">
    <ListView.View>
        <GridView>
            <GridViewColumn DisplayMemberBinding="{Binding Path=MyString}" Header="MyStringColumnName"></GridViewColumn>
            <GridViewColumn DisplayMemberBinding="{Binding Path=MyValue}" Header="MyValueColumnName"></GridViewColumn>
         </GridView>
    </ListView.View>
</ListView>

Answer 11

Or for fun you could use some LINQ extension goodness:

var dictionary = new Dictionary<string, int> { { "c", 3 }, { "a", 1 }, { "b", 2 } };
dictionary.OrderBy(x => x.Value)
  .ForEach(x => Console.WriteLine("{0}={1}", x.Key,x.Value));

Answer 12

You can use the LINQ to sort the Dictionary by value.

Below link provide a good details about the same.

http://www.a2zmenu.com/CSharp/How-to-sort-CSharp-dictionary-by-value.aspx