How to perform a binary search on IList<T>?

Asked 2009-06-08T21:09:09.183

Simple question - given an IList<T> how do you perform a binary search without writing the method yourself and without copying the data to a type with build-in binary search support. My current status is the following.

List<T>.BinarySearch() is not a member of IList<T>
There is no equivalent of the ArrayList.Adapter() method for List<T>
IList<T> does not inherit from IList, hence using ArrayList.Adapter() is not possible

I tend to believe that is not possible with build-in methods, but I cannot believe that such a basic method is missing from the BCL/FCL.

If it is not possible, who can give the shortest, fastest, smartest, or most beatiful binary search implementation for IList<T>?

UPDATE

We all know that a list must be sorted before using binary search, hence you can assume that it is. But I assume (but did not verify) it is the same problem with sort - how do you sort IList<T>?

CONCLUSION

There seems to be no build-in binary search for IList<T>. One can use First() and OrderBy() LINQ methods to search and sort, but it will likly have a performance hit. Implementing it yourself (as an extension method) seems the best you can do.

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3 Answers

I doubt there is a general purpose binary search method in .NET like that, except for the one being present in some base classes (but apparently not in the interfaces), so here's my general purpose one.

public static Int32 BinarySearchIndexOf<T>(this IList<T> list, T value, IComparer<T> comparer = null)
{
    if (list == null)
        throw new ArgumentNullException(nameof(list));

    comparer = comparer ?? Comparer<T>.Default;

    Int32 lower = 0;
    Int32 upper = list.Count - 1;

    while (lower <= upper)
    {
        Int32 middle = lower + (upper - lower) / 2;
        Int32 comparisonResult = comparer.Compare(value, list[middle]);
        if (comparisonResult == 0)
            return middle;
        else if (comparisonResult < 0)
            upper = middle - 1;
        else
            lower = middle + 1;
    }

    return ~lower;
}

This of course assumes that the list in question is already sorted, according to the same rules that the comparer will use.

answered 2009-06-08T21:21:29.097

You are going to have a couple of problems binary-searching an IList<T>, First ,like you mentioned, the BinarySearch method on the List<T> is not a member of the IList<T> interface. Second, you have no way of sorting the list prior to searching (which you must do for a binary search to work).

I think your best bet is to create a new List<T>, sort it, and then search. It's not perfect but you don't have to many options if you have an IList<T>.

answered 2009-06-08T21:16:02.320

If you need a ready-made implementation for binary search on IList<T>s, Wintellect's Power Collections has one (in Algorithms.cs):

/// <summary>
/// Searches a sorted list for an item via binary search. The list must be sorted
/// by the natural ordering of the type (it's implementation of IComparable&lt;T&gt;).
/// </summary>
/// <param name="list">The sorted list to search.</param>
/// <param name="item">The item to search for.</param>
/// <param name="index">Returns the first index at which the item can be found. If the return
/// value is zero, indicating that <paramref name="item"/> was not present in the list, then this
/// returns the index at which <paramref name="item"/> could be inserted to maintain the sorted
/// order of the list.</param>
/// <returns>The number of items equal to <paramref name="item"/> that appear in the list.</returns>
public static int BinarySearch<T>(IList<T> list, T item, out int index)
        where T: IComparable<T>
{
    // ...
}

answered 2009-09-28T12:29:32.657

How to perform a binary search on IList<T>?

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