How do I get the Nth largest element from a list with duplicates, using LINQ?

问题

I've seen in various StackOverflow answers that I can get the Nth largest element in a list by doing something like

var nthFromTop = items.OrderByDescending().Skip(N-1).First();

But wouldn't this only work if there are no duplicates in the list? If the list contains duplicates, is there a way to get the Nth largest element (or set of elements) using LINQ? If not, what would be the most efficient way to do this in C#?

回答1:

To get the set of all items equal to the Nth largest item you'll need to group the items, order the groups, and then decrement N by the group size while N is positive. When N reaches zero, you've hit the group containing the Nth largest item.

public static IEnumerable<T> Foo<T>(this IEnumerable<T> source, int n)
{
    return source.GroupBy(x => x)
        .OrderByDescending(group => group.Key)
        .SkipWhile(group =>
        {
            n -= group.Count();
            return n > 0;
        })
        .First();
}

回答2:

If you want to get a set of all the elements, use GroupBy

 var items = new[] {1, 1, 2, 2, 3, 4, 4};
 var thirdLargest = items
   .GroupBy(x => x)
   .OrderByDescending(group => group.Key)
   .ElementAt(2);

回答3:

If you would like to avoid duplicates, how about using group by?

回答4:

If you want to get all values that are the Nth largest if there are duplicates do this:

EDIT

List<int> ints = new List<int>()
{
     1,2,5,8,12,34,12,52,34
};

int NthLargest = 1;
var queryresult = ints
                   .GroupBy(e => e)
                   .OrderByDescending(f => f.Count())
                   .ThenByDescending(k => k.Key)
                   .ElementAt(NthLargest - 1);

来源：https://stackoverflow.com/questions/24455950/how-do-i-get-the-nth-largest-element-from-a-list-with-duplicates-using-linq

标签

linq