Sorting Algorithms In C#

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Preface

By Marc Clifton

After my last article on a QuickSort algorithm with customizable swapping, Jonathan de Halleux made some improvements to my original design and added some sorting algorithms. As I was writing this article and waiting for Jonathan's blessing, Robert Rohde provided some additional sorting algorithms which I have included here as well. Rather than updating the original article, I felt it would be better to represent Jonathan's and Robert's work separately from my original article, as they have contributed so much that it is quite a different beast. My contribution at this point is the article itself, some additions to the ISwap interface, and integrating both Jonathan's and Robert's work into a unified namespace. All kudos should go to Jonathan's and Robert's excellent work--I learned a few things writing this article!

Introduction

This library provides a very nice and flexible package of sorting algorithms from which the developer can choose. The algorithms presented here have been ported to C# and are based on selected algorithms in Java found here. Click on the pictures to run an applet that shows the algorithm running!

The sorting algorithms are:

• Bidirectional Bubble Sort
• Bubble Sort
• ComboSort11
• Double Storage Merge Sort (utilizes setter)
• Fast Quick Sort (utilizes setter)
• Heap Sort
• In Place Merge Sort (utilizes setter)
• Insertion Sort (utilizes setter)
• Odd-Even Transport Sort
• Quick Sort
• Quick Sort With Bubble Sort (utilizes setter)
• Selection Sort
• Shaker Sort
• Shear Sort
• Shell Sort

Sorting algorithms such as InPlaceMergeSort, InsertionSort, and ShellSort perform set operations rather than swap operations. For this reason, the ISwap interface includes two "Set" methods. If you aren't using one of the algorithms that uses a setter, then you can ignore them.

All of these sort algorithms are thread safe.

The Object Model

The object model easily allows for additional sorting algorithms to be added. The following UML diagram illustrates this with the FastQuickSorter implementation:

Additional sorters are derived from SwapSorter.

Implementation

The following illustrates some of the key implementation features of the NSort library.

Interfaces

Two interfaces provide the necessary abstraction of concrete instances.

ISorter

using System;
using System.Collections;

namespace NSort
{
    /// <SUMMARY>
    /// Summary description for ISorter.
    /// </SUMMARY>
    public interface ISorter
    {
        void Sort(IList list);
    }
}

This interface abstracts the concrete sorting algorithms.

ISwap

using System;
using System.Collections;

namespace NSort
{
    /// <SUMMARY>
    /// Object swapper interface
    /// </SUMMARY>
    public interface ISwap
    {
        void Swap(IList array, int left, int right);
        void Set(IList array, int left, int right);
        void Set(IList array, int left, object obj);
    }
}

This interface abstracts the concrete swapper. In some cases, additional work needs to be done when swapping elements of the array. Defining your own swapping algorithm derived from the ISwap interface allows you to accomplish this work.

SwapSorter

This is the abstract base class for all the sorting algorithms. It implements the management of the concrete ISwap and ISorter instances. Each concrete sorting algorithm implements two constructors--a default constructor and a constructor in which you can specify your own comparer and swapper functions. In the SwapSorter class, these are implemented as follows:

public SwapSorter()
{
    this.comparer = new ComparableComparer();
    this.swapper = new DefaultSwap();
}

As you can see from the default constructor above, the default comparer and swapper are implemented.

public SwapSorter(IComparer comparer, ISwap swapper)
{
    if (comparer == null)
        throw new ArgumentNullException("comparer");
    if (swapper==null)
        throw new ArgumentNullException("swapper");

    this.comparer = comparer;
    this.swapper = swapper;
}

The above code illustrates specifying your own comparer and swapper.

An instance of the default comparer or swapper can also be passed in, should you only need to specify your own instance of one or the other, but not both.

DefaultSwap

The default swapper implementation is straight-forward:

using System;
using System.Collections;

namespace NSort
{
    /// <SUMMARY>
    /// Default swap class
    /// </SUMMARY>
    public class DefaultSwap : ISwap
    {
        public void Swap(IList array, int left, int right)
        {
            object swap=array[left];
            array[left]=array[right];
            array[right]=swap;
        }

        public void Set(IList array, int left, int right)
        {
            array[left]=array[right];
        }

        public void Set(IList array, int left, object obj)
        {
            array[left]=obj;
        }
    }
}

ComparableComparer

The default comparer is very powerful: 

using System;
using System.Collections;

namespace NSort
{
    /// <SUMMARY>
    /// Default <SEE cref="IComparable" /> object comparer.
    /// </SUMMARY>
    public class ComparableComparer : IComparer
    {
        public int Compare(IComparable x, Object y)
        {
            return x.CompareTo(y);
        }

        #region IComparer Members
        int IComparer.Compare(Object x, Object y)
        {
            return this.Compare((IComparable)x,y);
        }

        #endregion
    }
}

Notice in this implementation, the IComparer.Compare method invokes the object's ICompareTo function. This has the advantage of putting the comparison "smarts" into the object being compared against. There are several advantages to this. If the object is a class, only certain fields in the class might be involved in the comparison. Also, the object itself can determine what other types of objects it can be compared with and provide necessary translation/conversion services. Given this flexibility, overriding the comparer is probably only necessary when comparing third party objects that do not implement CompareTo or in cases where you wish to extend the native CompareTo capability.

Usage

The usage is best illustrated by looking at the NUnit-compatible unit tests that are provided with the download.

Instantiating the Sorter

Each sorting algorithm has its own test fixture:

[TestFixture]
public class QuickSorterTest : SorterTest
{
    [SetUp]
    public void SetUp()
    {
        this.Sorter = new QuickSorter();
    }
}

Creating, Sorting, And Verifying Some Sample Data

The test, verifying that the sorter is behaving correctly:

[Test]
public void SortInt()
{
    Random rnd = new Random();
    int[] list = new int[1000];
    int i;
    for(i = 0; i<list.Length; ++i)
        list[i] = rnd.Next();

    // create sorted list
    SortedList sl = new SortedList();
    foreach(int key in list)
        sl.Add(key,null);

    // sort table
    Sorter.Sort(list);

    i = 0;
    foreach(int val in sl.Keys)
    {
        Assert.AreEqual(val,list[i]);
        ++i;
    }
}

Unit Testing

The code includes unit tests written for Marc's unit test framework. In the code included in this article, the unit tests are part of the NSort assembly.

Speaking of which, beware of the fast quicksort algorithm--the unit test fail on this once, but since the test data is randomly generated, it hasn't been reproduced!

What's Next

Jonathan has written an excellent performance benchmark framework that will readily integrate into Marc's unit test framework. The next step is to do so, and illustrate that effort by benchmarking the various sorting algorithms.

History

• 5^th February, 2004: Initial version

License

This article has no explicit license attached to it, but may contain usage terms in the article text or the download files themselves. If in doubt, please contact the author via the discussion board below.

A list of licenses authors might use can be found here.