如何使用LinQ获取列表的前N个元素？

Question

I have an ordered list by exam points and I want to have the top N element of this list.
If the N(th) and N+1(th) students have the same exam points, the list must have them both.

For example I have a list like this:

john.   80  
mike.   75  
james.  70  
ashley. 70
kate.   60

Top 3 should return john, mike, james, ashley
I tried Take() but it returns only john, mike, james

English is not my main language, sorry if I couldn't tell correctly
Thanks

Answer 1

Here's a one-pass-only implementation:

public static IEnumerable<TSource> TopWithTies<TSource, TValue>(
    this IEnumerable<TSource> source,
    int count,
    Func<TSource, TValue> selector)
{
    if (source == null) throw new ArgumentNullException("source");
    if (selector == null) throw new ArgumentNullException("selector");
    if (count < 0) throw new ArgumentOutOfRangeException("count");
    if (count == 0) yield break;
    using(var iter = source.OrderByDescending(selector).GetEnumerator())
    {
        if(iter.MoveNext())
        {
            yield return iter.Current;
            while (--count >= 0)
            {
                if(!iter.MoveNext()) yield break;
                yield return iter.Current;    
            }
            var lastVal = selector(iter.Current);
            var eq = EqualityComparer<TValue>.Default;
            while(iter.MoveNext() && eq.Equals(lastVal, selector(iter.Current)))
            {
                yield return iter.Current;
            }
        }
    }
}

Example usage:

var data = new[]
{
    new { name = "john", value = 80 },
    new { name = "mike", value = 75 },
    new { name = "james", value = 70 },
    new { name = "ashley", value = 70 },
    new { name = "kate", value = 60 }
};
var top = data.TopWithTies(3, x => x.value).ToList();
foreach(var row in top)
{
    Console.WriteLine("{0}: {1}", row.name, row.value);
}

Answer 2

What you probably want to do is

1. Get the nth
2. get all when >= nth

ie

var nth = users.Skip(n-1).FirstOrDefault()
var top = users.TakeWhile(user => user.Score >= nth.Score)

(This assumes that the list is ordered descending, as in the example given in the question. Also will throw an error if there are < n elements in the input list)

Answer 3

What if more than two students have the same marks? Will you take them all? OP: Yes

You can group by the points, then use OrderByDescending + Take + SelectMany :

var topThreePoints = users.GroupBy(u => u.Points)
                          .OrderByDescending(g => g.Key)
                          .Take(3)
                          .SelectMany(g => g);

Answer 4

I have created a sample case in LINQPad.

var a = new List<Tuple<string,int>>();
a.Add(new Tuple<string,int>("john",80));
a.Add(new Tuple<string,int>("mike",75));
a.Add(new Tuple<string,int>("james",70));
a.Add(new Tuple<string,int>("ashley",70 ));
a.Add(new Tuple<string,int>("kate",60  ));

a.Where(x=>x.Item2>=a.OrderBy(i=>i.Item2).Skip(2).Take(1).SingleOrDefault ().Item2).Dump();

Don't know if it is efficient enough though.

Answer 5

也许是这样的？

list.TakeWhile((item, index) => index < N || list[index] == list[index + 1]);