如何选择集合中多个子类别的最后两个实例？

Question

Stackoverflow

使用C# LINQ，如何选择集合中多个子类别的最后两个实例？

在下面的代码中，phaseRepository是一个阶段列表。

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每个阶段都有一些属性，包括方向和动量(其中Momentum.Undefined应该被认为与Momentum.Time相同)。您将如何为四个子类别选择最后两个阶段实例：

• Direction.Up & Momentum.Price
• Direction.Up & Momentum.Time (平等对待.Time和.Undefined )
• Direction.Down & Momentum.Price
• Direction.Down & Momentum.Time (平等对待.Time和.Undefined )

答案类似于(不需要PhaseNumber命令)：

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为了提供帮助，下面的代码是一个LINQPad C#程序，它编译但缺少正确的答案：

void Main()
{
    var phaseRepository = new PhaseRepository();
    phaseRepository.Collection = GetCollection();

    // answer in its current state is incorrect...
    var answer = phaseRepository.Collection.Select(p => p);

    answer.Dump();  
}

private List<Phase> GetCollection() 
{
    return new List<Phase>
    {
        new Phase ()
        {
            PhaseNumber = 1,
            Direction = Dir.Up,
            Momentum = Mom.Price,
        },
        new Phase ()
        {
            PhaseNumber = 2,
            Direction = Dir.Down,
            Momentum = Mom.Time,
        },
        new Phase ()
        {
            PhaseNumber = 3,
            Direction = Dir.Up,
            Momentum = Mom.Price,
        },
        new Phase ()
        {
            PhaseNumber = 4,
            Direction = Dir.Down,
            Momentum = Mom.Time,
        },
        new Phase ()
        {
            PhaseNumber = 5,
            Direction = Dir.Up,
            Momentum = Mom.Time,
        },
        new Phase ()
        {
            PhaseNumber = 6,
            Direction = Dir.Down,
            Momentum = Mom.Undefined,
        },
        new Phase ()
        {
            PhaseNumber = 7,
            Direction = Dir.Up,
            Momentum = Mom.Price,
        },
        new Phase ()
        {
            PhaseNumber = 8,
            Direction = Dir.Down,
            Momentum = Mom.Price,
        },
        new Phase ()
        {
            PhaseNumber = 9,
            Direction = Dir.Up,
            Momentum = Mom.Price,
        },
        new Phase ()
        {
            PhaseNumber = 10,
            Direction = Dir.Down,
            Momentum = Mom.Time,
        },
        new Phase ()
        {
            PhaseNumber = 11,
            Direction = Dir.Up,
            Momentum = Mom.Time,
        },
        new Phase ()
        {
            PhaseNumber = 12,
            Direction = Dir.Down,
            Momentum = Mom.Undefined,
        }
    };
}

public class PhaseRepository
{
    public List<Phase> Collection = new List<Phase>();
}   

public class Phase
{
    public int PhaseNumber { get; set; }
    public Dir Direction { get; set; }
    public Mom Momentum { get; set; }
}

public enum Dir
{
    Up,
    Down,
    Undefined
}

public enum Mom
{
    Price,
    Time,
    Undefined
}

Answer 1

将GroupBy和SelectMany结合起来很容易做到：

var answer = phaseRepository.Collection
    .OrderBy(p => p.PhaseNumber); // prepare to enforce "last two" requirement
    .GroupBy(p =>
         new { 
            p.Direction, 
            Momentum = p.Momentum == Mom.Undefined ? Mom.Time : p.Momentum
         })
    .SelectMany(g => g.Skip(g.Count() - 2)) // skip all but 2 from each group
    .OrderBy(p => p.PhaseNumber); // optional, to make verification easier

这里最重要的一点是，GroupBy是有保证，可以在每个组中保留源序列中元素的相对顺序：

IGrouping对象是根据源元素的顺序生成的，这些元素产生每个IGrouping的第一个键。分组中的元素是按照生成它们的元素出现在源中的顺序产生的。

因此，初始的逐阶段排序最初就足以满足“每个组中的最后两个”的要求。

Answer 2

对于Direction.Up & Momentum.Price：

phaseRepository.Where(phase =>
{
    phase.Direction == Direction.Up &&
    phase.Momentum == Momentum.Price
}).Reverse().Take(2);

您可以将此应用于所有其他需求。如果您希望未定义的时间与时间相同，只需检查phase.Momentum == Momentum.Time || phase.Momentum == Momentum.Undefined

如果您想要最后一个2 PhaseNumbers，那么在这个数字上使用OrderByDescending(x => x.PhaseNumber)，然后从它中使用.Take(2)。

如果Where()返回的元素少于2个，则Take(2)仍将返回一个空列表(如果Where()返回1项，则返回1个元素)。

Answer 3

像这样的东西应该能起作用

phaseRepository.Collection
    .GroupBy(x => Tuple.Create(x.Direction, x.Momentum))
    .Where(x => x.Key == //filter on your desired sub catagories
    .Select(x => x.OrderBy(y=>y.PhaseNumber).Take(2).ToList()) //get just the last 2 items
    .SelectMany(x => x)
    .OrderBy(y=>PhaseNumber) //return in complete order