C#获取集合中2个项目的最常用组合

Question

我有一个项目集合，如下所示：

    ID    ELECT1        ELECT2
    1      FDI           AED     
    2      BPG           AED
    3      AED           FDI
    4      FDI           AED 
    5      GPH           AED
    6      AED           BPG
    7      GPH           FDI

我想将elect1和elect2分组，然后输出两个最受欢迎的项目，就像这些项目被选择了3次一样

FDI AED 
AED FDI
FDI AED

第二个最受欢迎的组合是

AED BPG
BPG AED

所以输出应该是

2 Most popular combinations are
    FDI AED 
    AED BPG

我写了一些代码，但我不知道我怎么可能做到这一点

var groups = cStudent.GroupBy(x => new { x.elective1, x.elective2 });    
foreach (var group in groups)
{
    Console.WriteLine("{0} / {1}:", group.Key.elective1, group.Key.elective2);
    foreach (var item in group)
    {
        Console.WriteLine("  {0} ({1})", item.elective1, item.elective2);
    }
}

这就是我用我的代码得到的结果，它已经完成了一半，只是不知道如何结束。IMAGE

Answer 1

IMHO，解决这个问题的最干净的方法是实现一个自定义的相等比较器，它提供了解决当前问题所需的相等语义：

class IgnoreElectiveOrderStudentEqualityComparer 
    : IEqualityComparer<Student>
{
    public bool Equals(Student x, Student y)
        => (x.ElectiveOne == y.ElectiveOne &&
            x.ElectiveTwo == y.ElectiveTwo) ||
           (x.ElectiveOne == y.ElectiveTwo &&
            x.ElectiveTwo == y.ElectiveOne);

    public int GetHashCode(Student obj)
        => obj.ElectiveOne.GetHashCode() ^
           obj.ElectiveTwo.GetHashCode();
}

现在，您只需使用GroupBy和自定义比较器：

var mostPopular = 
    students.GroupBy(s => s,
                     new IgnoreElectiveOrderStudentEqualityComparer())
            .OrderByDescending(g => g.Count())
            .Select(g => new
            {
                g.Key.ElectiveOne, 
                g.Key.ElectiveTwo
            })
            .Take(2);

Answer 2

诀窍是让两个选修课的顺序不重要，所以alpha第一个总是第一个(因此，如果你有(数学，艺术)，它与(艺术，数学)是一样的：

var most_pop = cStudent.GroupBy(x => { if (string.Compare(x.elective1,x.elecitive2) > 0) 
                                         return(new Tuple(x.elective1.x.elective2);
                                       else
                                         return(new Tuple(x.elective2,x.elective1);
                                     },
                                (b, a) => new {  Key = b, Count = a.Count() })
                .OrderyByDesending(x => x.Count).Take(2);

如果你不想要一个带有计数的对象，你可以添加一个键的选择。

Answer 3

这对你来说应该是可行的：

cStudent
    .Select(x => new[] { x.elective1, x.elective2 }.OrderBy(y => y).ToArray())
    .GroupBy(x => Tuple.Create(x[0], x[1]), (k, g) => new { Elective = k, Count = g.Count() })
    .OrderByDescending(x => x.Count)
    .Select(x => new { elective1 = x.Elective.Item1, elective2 = x.Elective.Item2 })
    .Take(2)