Reverse Sorted Dictionary in .NET

Question

Is there any way I can iterate backwards (in reverse) through a SortedDictionary in c#?

Or is there a way to define the SortedDictionary in descending order to begin with?

Answer 1

The SortedDictionary itself doesn't support backward iteration, but you have several possibilities to achieve the same effect.

1. Use .Reverse-Method (Linq). (This will have to pre-compute the whole dictionary output but is the simplest solution)

   var Rand = new Random();
   
   var Dict = new SortedDictionary<int, string>();
   
   for (int i = 1; i <= 10; ++i) {
       var newItem = Rand.Next(1, 100);
       Dict.Add(newItem, (newItem * newItem).ToString());
   }
   
   foreach (var x in Dict.Reverse()) {
       Console.WriteLine("{0} -> {1}", x.Key, x.Value);
   }
   

2. Make the dictionary sort in descending order.

   class DescendingComparer<T> : IComparer<T> where T : IComparable<T> {
       public int Compare(T x, T y) {
           return y.CompareTo(x);
       }
   }
   
   // ...
   
   var Dict = new SortedDictionary<int, string>(new DescendingComparer<int>());
   

3. Use SortedList<TKey, TValue> instead. The performance is not as good as the dictionary's (O(n) instead of O(logn)), but you have random-access at the elements like in arrays. When you use the generic IDictionary-Interface, you won't have to change the rest of your code.

Edit :: Iterating on SortedLists

You just access the elements by index!

var Rand = new Random();


var Dict = new SortedList<int, string>();

for (int i = 1; i <= 10; ++i) {
    var newItem = Rand.Next(1, 100);
    Dict.Add(newItem, (newItem * newItem).ToString());
}

// Reverse for loop (forr + tab)
for (int i = Dict.Count - 1; i >= 0; --i) {
    Console.WriteLine("{0} -> {1}", Dict.Keys[i], Dict.Values[i]);
}

Answer 2

The easiest way to define the SortedDictionary in the reverse order to start with is to provide it with an IComparer<TKey> which sorts in the reverse order to normal.

Here's some code from MiscUtil which might make that easier for you:

using System.Collections.Generic;

namespace MiscUtil.Collections
{
    /// <summary>
    /// Implementation of IComparer{T} based on another one;
    /// this simply reverses the original comparison.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public sealed class ReverseComparer<T> : IComparer<T>
    {
        readonly IComparer<T> originalComparer;

        /// <summary>
        /// Returns the original comparer; this can be useful
        /// to avoid multiple reversals.
        /// </summary>
        public IComparer<T> OriginalComparer
        {
            get { return originalComparer; }
        }

        /// <summary>
        /// Creates a new reversing comparer.
        /// </summary>
        /// <param name="original">The original comparer to 
        /// use for comparisons.</param>
        public ReverseComparer(IComparer<T> original)
        {
            if (original == null)
            { 
                throw new ArgumentNullException("original");
            }
            this.originalComparer = original;
        }

        /// <summary>
        /// Returns the result of comparing the specified
        /// values using the original
        /// comparer, but reversing the order of comparison.
        /// </summary>
        public int Compare(T x, T y)
        {
            return originalComparer.Compare(y, x);
        }
    }
}

You'd then use:

var dict = new SortedDictionary<string, int>
     (new ReverseComparer<string>(StringComparer.InvariantCulture));

(or whatever type you were using).

If you only ever want to iterate in one direction, this will be more efficient than reversing the ordering afterwards.

Answer 3

There is also a very simple approach if you are dealing with numeric values as the key which is to simply negate them when you create the dictionary.

Answer 4

Briefly create a reversed sorted dictionary in one line.

var dict = new SortedDictionary<int, int>(Comparer<int>.Create((x, y) => y.CompareTo(x)));

There's a way to create a IComparer<T> using System.Collections.Generic.Comparer<T>. Just pass a IComparision<T> delegate to its Create method to build a IComparer<T>.

var dict = new SortedDictionary<int, TValue>(
    Comparer<int>.Create(
        delegate(int x, int y)
        {
            return y.CompareTo(x);
        }
    )
);

You can use a lambda expression/local function/method to replace the delegate if their significance are (TKey, TKey) => int.

Answer 5

If you're using .NET 3.5, you can use the OrderByDescending extension method:

        var dictionary = new SortedDictionary<int, string>();
        dictionary.Add(1, "One");
        dictionary.Add(3, "Three");
        dictionary.Add(2, "Two");
        dictionary.Add(4, "Four");



        var q = dictionary.OrderByDescending(kvp => kvp.Key);
        foreach (var item in q)
        {
            Console.WriteLine(item.Key + " , " + item.Value);
        }