Linq查询来做分组和最小

Question

I have a datastructure as shown below. For each (Person, Game) pairs, I need to find the lastest score in the past 24 hours. Is it possible to do that in LINQ? Something like (Person, Game, LatestScore)

+----------------+-----------------+---------------+-+------------+
| Person         |  Game           | Score         |EventTime     |
+-----------------------------------------------------------------+
|                |                 |               |              |
|                |                 |               |              |
+----------------+-----------------+---------------+--------------+

Any hints would be very helpful.

Answer 1

Assuming you have a class like this:

    class GameInfo
    {

        public DateTime EventTime { get; set; }
        public String Game { get; set; }
        public String Person { get; set; }
        public double Score { get; set; }
    }

You could do:

        List<GameInfo> data = new List<GameInfo> { 
        new GameInfo { Game = "G1", Person = "A", Score = 10, EventTime = new DateTime(2014, 10, 10, 10, 10, 10) },
        new GameInfo { Game = "G1", Person = "A", Score = 10, EventTime = new DateTime(2014, 10, 10, 10, 10, 10) },
        new GameInfo { Game = "G2", Person = "B", Score = 11, EventTime = new DateTime(2014, 10, 10, 20, 10, 10) },
        new GameInfo { Game = "G2", Person = "B", Score = 11, EventTime = new DateTime(2014, 10, 10, 20, 10, 10) }
    };

        var q = from game in data
                group game by new { G = game.Game, P = game.Person } into g

                select new {
                    Person = g.Key.P,
                    Game = g.Key.G,
                    Score = g.Aggregate((curmin, x) => (curmin == null || (x.EventTime) < curmin.EventTime ? x : curmin)).Score
                };

        foreach (var item in q)
        {
            Console.WriteLine("{0}, {1}, {2}", item.Game, item.Person, item.Score);
        }

As @Rawling pointed out, getting the scoce can be quite costly if you have lage data sets. So doing that efficiently might save alot of time in getting the desired output.

Answer 2

var dt = DateTime.Now.AddDays(-1);
var results = context.Where(x=>x.EventTime >= dt)
    .GroupBy(x=>new {x.Person,x.Game})
    .Select(x=>new 
    {
        x.Key.Person,
        x.Key.Game,
        LatestScore = x.Where(d=>d.EventTime == x.Max(l=>l.EventTime))
                       .Select(d=>d.Score)
                       .FirstOrDefault()
    });