C# – When to use a SortedList over a SortedDictionary

cnetsorteddictionarysortedlist

This may appear to be a duplicate of this question, which asks "What’s the difference between SortedList and SortedDictionary?" Unfortunately, the answers do nothing more than quote the MSDN documentation (which clearly states that there are performance and memory use differences between the two) but don't actually answer the question.

In fact (and so this question doesn't get the same answers), according to MSDN:

The SortedList<TKey, TValue> generic
class is a binary search tree with
O(log n) retrieval, where n is the
number of elements in the dictionary.
In this, it is similar to the
SortedDictionary<TKey, TValue> generic
class. The two classes have similar
object models, and both have O(log n)
retrieval. Where the two classes
differ is in memory use and speed of
insertion and removal:

SortedList<TKey, TValue> uses less
memory than SortedDictionary<TKey, TValue>.

SortedDictionary<TKey, TValue> has
faster insertion and removal
operations for unsorted data, O(log n)
as opposed to O(n) for
SortedList<TKey, TValue>.

If the list is populated all at once
from sorted data, SortedList<TKey, TValue> is faster than
SortedDictionary<TKey, TValue>.

So, clearly this would indicated that SortedList<TKey, TValue> is the better choice unless you need faster insert and remove operations for unsorted data.

The question still remains, given the information above what are the practical (real-world, business case, etc.) reasons for using a SortedDictionary<TKey, TValue>? Based on the performance information, it would imply that there really is no need to have SortedDictionary<TKey, TValue> at all.

Best Answer

I'm not sure how accurate the MSDN documentation is on SortedList and SortedDictionary. It seems to be saying both are implemented using a binary search tree. But if the SortedList uses a binary search tree, why would it be much slower on additions than SortedDictionary?

Anyway, here are some performance test results.

Each test operates on a SortedList / SortedDictionary containing 10,000 int32 keys. Each test is repeated 1,000 times (Release build, Start without Debugging).

The first group of tests add keys in sequence from 0 to 9,999. The second group of tests add random shuffled keys between 0 to 9,999 (every number is added exactly once).

***** Tests.PerformanceTests.SortedTest

SortedDictionary Add sorted: 4411 ms
SortedDictionary Get sorted: 2374 ms


SortedList Add sorted: 1422 ms
SortedList Get sorted: 1843 ms

***** Tests.PerformanceTests.UnsortedTest

SortedDictionary Add unsorted: 4640 ms
SortedDictionary Get unsorted: 2903 ms


SortedList Add unsorted: 36559 ms
SortedList Get unsorted: 2243 ms

As with any profiling, the important thing is the relative performance, not the actual numbers.

As you can see, on sorted data the sorted list is faster than the SortedDictionary. On unsorted data the SortedList is slightly quicker on retrieval, but about 9 times slower on adding.

If both are using binary trees internally, it is quite surprising that the Add operation on unsorted data is so much slower for SortedList. It is possible that sorted list may also be adding items to a sorted linear data structure at the same time, which would slow it down.

However, you would expect the memory usage of a SortedList to be equal or greater than or at least equal to a SortedDictionary. But this contradicts what the MSDN documentation says.

Related Solutions

C# – In .NET, which loop runs faster, ‘for’ or ‘foreach’

Patrick Smacchia blogged about this last month, with the following conclusions:

for loops on List are a bit more than 2 times cheaper than foreach loops on List.

Looping on array is around 2 times cheaper than looping on List.

As a consequence, looping on array using for is 5 times cheaper than looping on List using foreach (which I believe, is what we all do).

C# – SortedList class in .NET

Maybe I'm slow, but isn't this the easiest implementation ever?

class SortedList<T> : List<T>
{
    public new void Add(T item)
    {
        Insert(~BinarySearch(item), item);
    }
}

http://msdn.microsoft.com/en-us/library/w4e7fxsh.aspx

Unfortunately, Add wasn't overrideable so I had to new it which isn't so nice when you have List<T> list = new SortedList<T>; which I actually needed to do.... so I went ahead and rebuilt the whole thing...

class SortedList<T> : IList<T>
{
    private List<T> list = new List<T>();

    public int IndexOf(T item)
    {
        var index = list.BinarySearch(item);
        return index < 0 ? -1 : index;
    }

    public void Insert(int index, T item)
    {
        throw new NotImplementedException("Cannot insert at index; must preserve order.");
    }

    public void RemoveAt(int index)
    {
        list.RemoveAt(index);
    }

    public T this[int index]
    {
        get
        {
            return list[index];
        }
        set
        {
            list.RemoveAt(index);
            this.Add(value);
        }
    }

    public void Add(T item)
    {
        list.Insert(~list.BinarySearch(item), item);
    }

    public void Clear()
    {
        list.Clear();
    }

    public bool Contains(T item)
    {
        return list.BinarySearch(item) >= 0;
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        list.CopyTo(array, arrayIndex);
    }

    public int Count
    {
        get { return list.Count; }
    }

    public bool IsReadOnly
    {
        get { return false; }
    }

    public bool Remove(T item)
    {
        var index = list.BinarySearch(item);
        if (index < 0) return false;
        list.RemoveAt(index);
        return true;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return list.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return list.GetEnumerator();
    }
}

Or perhaps something like this is a more appropriate Remove function...

    public bool Remove(T item)
    {
        var index = list.BinarySearch(item);
        if (index < 0) return false;
        while (((IComparable)item).CompareTo((IComparable)list[index]) == 0)
        {
            if (item == list[index])
            {
                list.RemoveAt(index);
                return true;
            }
            index++;
        }
        return false;
    }

Assuming items can compare equal but not be equal...

Best Answer

Related Solutions

C# – In .NET, which loop runs faster, ‘for’ or ‘foreach’

C# – SortedList class in .NET

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