Question

I’m porting some C++ code to C#.

Does C# have an equivalent to std::nth_element() or do I need to roll my own?

Answer 1

I presume you are looking for an accessor that returns the Nth element of an unordered collection by performing a partial-sort on the collection. This tends to be useful when you have a very large collection and are interested in one of the first elements based on some ordering predicate.

To my knowledge, neither the .NET BCL or LINQ extensions offer an equivalent. All of the sorting methods (including Enumerable.OrderBy) perform a complete ordering of the collection.

If you need an efficient version of Nth, you will need to roll your own extension method on IEnumerable to do so. If you are going to roll you own you may want to look into the Quick Select algorithm, which has O(n) performance.

If the brute-force version is sufficient, you could use LINQ:

var someCollection = new []{ 5, 2, 8, 9, 0, 1, 3, 12, 4 };

var fifthItem = someCollection.NthItem(5);

public static class NthExtensions 
{
    public static T NthItem(this IEnumerable<T> coll, int n) 
    {
        return coll.OrderBy(x => x).Skip(n - 1).First();
    }
}