Recipe5.10.Sorting a Dictionary s Keys andor Values

Recipe 5.10. Sorting a Dictionary's Keys and/or Values

Problem

You want to sort the keys and/or values contained in a Hashtable in order to display the entire Hashtable to the user, sorted in either ascending or descending order.

Solution

Use the Keys and Values properties of a Dictionary<T,U> object to obtain an ICollection of its key and value objects. The methods shown here return a List<T> of objects containing the keys or values of a Dictionary<T,U>:

 using System; using System.Collections; using System.Collections.Generic; public static List<T> GetKeys<T,U>(Dictionary<T,U> table) {     return (new List<T>(table.Keys)); }

The method shown here returns a List<U> of objects containing the values in a Dictionary<T,U>:

 using System; using System.Collections; using System.Collections.Generic; public static List<U> GetValues<T,U>(Dictionary<T,U> table) {     return (new List<U>(table.Values)); }

The following code creates a Dictionary<T,U> object and displays it sorted in ascending and descending order:

 public static void TestSortKeyValues() {     // Define a Dictionary<T,U> object.     Dictionary<string, string> hash = new Dictionary<string, string>();     hash.Add(2, "two");     hash.Add(1, "one");     hash.Add(5, "five");     hash.Add(4, "four");     hash.Add(3, "three");     // Get all the keys in the Dictionary<T,U> and sort them.     List<string> keys = GetKeys(hash);     keys.Sort( );     // Display sorted list.     foreach (object obj in keys)         Console.WriteLine("Key: " + obj + " Value: " + hash[obj]);     // Reverse the sorted list.     Console.WriteLine( );     keys.Reverse( );     // Display reversed list.     foreach (object obj in keys)         Console.WriteLine("Key: " + obj + " Value: " + hash[obj]);     Console.WriteLine( );     Console.WriteLine( );     // Get all the values in the Dictionary<T,U> and sort them.     List<string> values = GetValues(hash);     values.Sort( );     // Display sorted list.     foreach (string obj in values)         Console.WriteLine("Value: " + obj);     // Reverse the sorted value list.     Console.WriteLine( );     values.Reverse( );     // Display sorted list.     foreach (string obj in values)         Console.WriteLine("Value: " + obj);}

The key/value pairs are displayed as shown:

 Key: 1 Value: one Key: 2 Value: two Key: 3 Value: three Key: 4 Value: four Key: 5 Value: five Key: 5 Value: five Key: 4 Value: four Key: 3 Value: three Key: 2 Value: two Key: 1 Value: one Value: five      Notice that the values are sorted alphabetically Value: four Value: one Value: three Value: two Value: two Value: three Value: one Value: four Value: five

Discussion

The Dictionary<T,U> object exposes two useful properties for obtaining a collection of its keys or values. The Keys property returns an ICollection containing all the keys currently in the Dictionary<T,U>. The Values property returns the same for all values currently contained in the Dictionary<T,U>.

The GetKeys method uses the Keys property. Once the ICollection of keys is returned through this property, a new List<T> is created to hold the keys. This List<T> is then returned to the caller. The GetValues method works in a similar manner except that it uses the Values property.

The GetValues method uses the Values property. Once the ICollection of values is returned through this property, a new List<U> is created of the same size to hold the values. This List<U> is then returned to the caller.

The ICollection object returned from either the Keys or Values property of a Dictionary<T,U> object contains direct references to the key and value collections within the Dictionary<T,U>. This means that if the keys and/or values change in a Dictionary<T,U>, the key and value collections will be altered accordingly.