使用（单个）LINQ 查询计算与组中前一项的差异

当数据需要分组时，我试图弄清楚如何计算与前一项的差异。

我有这样的数据

City      Area  Date        Citizens
New York    1   2010.11.20  5
New York    1   2010.11.21  8
New York    1   2010.11.22  12
New York    1   2010.11.23  17
New York    1   2010.11.24  23
New York    1   2010.11.25  29

Chicago 1   2010.11.20  5
Chicago 1   2010.11.21  10
Chicago 1   2010.11.22  15
Chicago 1   2010.11.23  20
Chicago 1   2010.11.24  25
Chicago 1   2010.11.25  30

New York    2   2010.11.20  6
New York    2   2010.11.21  7
New York    2   2010.11.22  9
New York    2   2010.11.23  7
New York    2   2010.11.24  10
New York    2   2010.11.25  15

Chicago 2   2010.11.20  5
Chicago 2   2010.11.21  15
Chicago 2   2010.11.22  25
Chicago 2   2010.11.23  20
Chicago 2   2010.11.24  25
Chicago 2   2010.11.25  30

我需要为每个城市添加一列“增加”，其计算方法是从当前公民数量中减去以前的公民数量。

The expected result is like this

City    Area    Date    Citizens    Increase
New York    1   2010.11.20  5   5
New York    1   2010.11.21  8   3
New York    1   2010.11.22  12  4
New York    1   2010.11.23  17  5
New York    1   2010.11.24  23  6
New York    1   2010.11.25  29  7

Chicago 1   2010.11.20  5   5
Chicago 1   2010.11.21  10  5
Chicago 1   2010.11.22  15  5
Chicago 1   2010.11.23  20  5
Chicago 1   2010.11.24  25  5
Chicago 1   2010.11.25  30  5

New York    2   2010.11.20  6   6
New York    2   2010.11.21  7   1
New York    2   2010.11.22  9   2
New York    2   2010.11.23  7   -2
New York    2   2010.11.24  10  3
New York    2   2010.11.25  15  5

Chicago 2   2010.11.20  5   5
Chicago 2   2010.11.21  15  10
Chicago 2   2010.11.22  25  10
Chicago 2   2010.11.23  20  -5
Chicago 2   2010.11.24  25  5
Chicago 2   2010.11.25  30  5

我想知道这是否可以通过单个 linq 查询来完成，从而避免 “foreach（城市中的 c） foreach（区域内的a） ……”

问题是如何计算“7”行，其中与先前记录的简单差异将为 -24，而此时应该为 5。

这是一个示例代码：

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    public class MyObject
    {
        public int ID { get; set; }
        public string City { get;set; }
        public DateTime Date { get; set; }
        public int Value { get; set; }
        public int DiffToPrev { get; set; }
    }

    class Program
    {

        static void Main()
        {
            var list = new List<MyObject>
              {
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now,          Value = 5},
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now.AddDays(1),Value = 8},
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now.AddDays(2),Value = 12},
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now.AddDays(3),Value = 17},
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now.AddDays(4),Value = 23},
                new MyObject {ID= 1, City = "New York",Date = DateTime.Now.AddDays(5),Value = 29},

                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now,           Value = 5},
                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now.AddDays(1),Value = 10},
                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now.AddDays(2),Value = 15},
                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now.AddDays(3),Value = 20},
                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now.AddDays(4),Value = 25},
                new MyObject {ID= 1, City = "Chicago",Date = DateTime.Now.AddDays(5),Value = 30},

                new MyObject {ID= 2, City = "New York",Date = DateTime.Now,          Value = 6},
                new MyObject {ID= 2, City = "New York",Date = DateTime.Now.AddDays(1),Value = 7},
                new MyObject {ID= 2, City = "New York",Date = DateTime.Now.AddDays(2),Value = 9},
                new MyObject {ID= 2, City = "New York",Date = DateTime.Now.AddDays(3),Value = 7},
                new MyObject {ID= 2, City = "New York",Date = DateTime.Now.AddDays(4),Value = 10},
                new MyObject {ID= 2, City = "New York",Date = DateTime.Now.AddDays(5),Value = 15},

                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now,           Value = 5},
                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now.AddDays(1),Value = 15},
                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now.AddDays(2),Value = 25},
                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now.AddDays(3),Value = 20},
                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now.AddDays(4),Value = 25},
                new MyObject {ID= 2, City = "Chicago",Date = DateTime.Now.AddDays(5),Value = 30},               
            };

        }
    }
}

你可以这样做：

var newList = list.GroupBy(x => new { x.City, x.ID })
.Select
(
    x =>
    {
        var subList = x.OrderBy(y => y.Date).ToList();
        return subList.Select((y, idx) => new MyObject
        {
            ID = y.ID,
            City = y.City,
            Date = y.Date,
            Value = y.Value,
            DiffToPrev = (idx == 0) ? y.Value : y.Value - subList.ElementAt(idx-1).Value 
        });
    }
)
.SelectMany(x => x)
.ToList();

无论如何，我认为在这种情况下 foreach 语句更清晰（而且不是更长），例如：

List<MyObject> newList = new List<MyObject>();
foreach (var grp in list.GroupBy(x => new { x.City, x.ID }))
{
    MyObject prev = null;
    foreach (var obj in grp.OrderBy(y => y.Date))
    {
        newList.Add(new MyObject
        {
            ID = obj.ID,
            City = obj.City,
            Date = obj.Date,
            Value = obj.Value,
            DiffToPrev = (prev == null) ? obj.Value : obj.Value - prev.Value
        });
        prev = obj;
    }
}

P.S.
显然（根据您的需要）在 foreach 代码中您可以设置DiffToPrev直接在现有对象上（obj）而不是创建一个新的，而是使创建成为可选newList.

Results:

ID: 1,City: New York,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 1,City: New York,Date: 22/11/2010 12:52:40,Value: 8,Diff: 3
ID: 1,City: New York,Date: 23/11/2010 12:52:40,Value: 12,Diff: 4
ID: 1,City: New York,Date: 24/11/2010 12:52:40,Value: 17,Diff: 5
ID: 1,City: New York,Date: 25/11/2010 12:52:40,Value: 23,Diff: 6
ID: 1,City: New York,Date: 26/11/2010 12:52:40,Value: 29,Diff: 6
ID: 1,City: Chicago,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 1,City: Chicago,Date: 22/11/2010 12:52:40,Value: 10,Diff: 5
ID: 1,City: Chicago,Date: 23/11/2010 12:52:40,Value: 15,Diff: 5
ID: 1,City: Chicago,Date: 24/11/2010 12:52:40,Value: 20,Diff: 5
ID: 1,City: Chicago,Date: 25/11/2010 12:52:40,Value: 25,Diff: 5
ID: 1,City: Chicago,Date: 26/11/2010 12:52:40,Value: 30,Diff: 5
ID: 2,City: New York,Date: 21/11/2010 12:52:40,Value: 6,Diff: 6
ID: 2,City: New York,Date: 22/11/2010 12:52:40,Value: 7,Diff: 1
ID: 2,City: New York,Date: 23/11/2010 12:52:40,Value: 9,Diff: 2
ID: 2,City: New York,Date: 24/11/2010 12:52:40,Value: 7,Diff: -2
ID: 2,City: New York,Date: 25/11/2010 12:52:40,Value: 10,Diff: 3
ID: 2,City: New York,Date: 26/11/2010 12:52:40,Value: 15,Diff: 5
ID: 2,City: Chicago,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 2,City: Chicago,Date: 22/11/2010 12:52:40,Value: 15,Diff: 10
ID: 2,City: Chicago,Date: 23/11/2010 12:52:40,Value: 25,Diff: 10
ID: 2,City: Chicago,Date: 24/11/2010 12:52:40,Value: 20,Diff: -5
ID: 2,City: Chicago,Date: 25/11/2010 12:52:40,Value: 25,Diff: 5
ID: 2,City: Chicago,Date: 26/11/2010 12:52:40,Value: 30,Diff: 5
ID: 1,City: New York,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 1,City: New York,Date: 22/11/2010 12:52:40,Value: 8,Diff: 3
ID: 1,City: New York,Date: 23/11/2010 12:52:40,Value: 12,Diff: 4
ID: 1,City: New York,Date: 24/11/2010 12:52:40,Value: 17,Diff: 5
ID: 1,City: New York,Date: 25/11/2010 12:52:40,Value: 23,Diff: 6
ID: 1,City: New York,Date: 26/11/2010 12:52:40,Value: 29,Diff: 6
ID: 1,City: Chicago,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 1,City: Chicago,Date: 22/11/2010 12:52:40,Value: 10,Diff: 5
ID: 1,City: Chicago,Date: 23/11/2010 12:52:40,Value: 15,Diff: 5
ID: 1,City: Chicago,Date: 24/11/2010 12:52:40,Value: 20,Diff: 5
ID: 1,City: Chicago,Date: 25/11/2010 12:52:40,Value: 25,Diff: 5
ID: 1,City: Chicago,Date: 26/11/2010 12:52:40,Value: 30,Diff: 5
ID: 2,City: New York,Date: 21/11/2010 12:52:40,Value: 6,Diff: 6
ID: 2,City: New York,Date: 22/11/2010 12:52:40,Value: 7,Diff: 1
ID: 2,City: New York,Date: 23/11/2010 12:52:40,Value: 9,Diff: 2
ID: 2,City: New York,Date: 24/11/2010 12:52:40,Value: 7,Diff: -2
ID: 2,City: New York,Date: 25/11/2010 12:52:40,Value: 10,Diff: 3
ID: 2,City: New York,Date: 26/11/2010 12:52:40,Value: 15,Diff: 5
ID: 2,City: Chicago,Date: 21/11/2010 12:52:40,Value: 5,Diff: 5
ID: 2,City: Chicago,Date: 22/11/2010 12:52:40,Value: 15,Diff: 10
ID: 2,City: Chicago,Date: 23/11/2010 12:52:40,Value: 25,Diff: 10
ID: 2,City: Chicago,Date: 24/11/2010 12:52:40,Value: 20,Diff: -5
ID: 2,City: Chicago,Date: 25/11/2010 12:52:40,Value: 25,Diff: 5
ID: 2,City: Chicago,Date: 26/11/2010 12:52:40,Value: 30,Diff: 5