LINQ查询返回每个类别中价格最高的商品

Question

这是对我不久前发布的一个问题的跟进。我得到了一个答案，但我意识到我已经简化了我的示例类，以至于我失去了最初的意图。在接受了原来问题的答案后，我认为最好再开始另一个问题。

这是我的新课：

public class Art
{
    public string Type { get; set; }
    public string Name { get; set; }
    public int Price { get; set; }
}

...and，下面是列表创建：

public static void Example0000()
{
    List<Art> art = new List<Art>();
    art.Add(new Art() { Price = 45, Type = "painting", Name = "Still Life in Maryland" });
    art.Add(new Art() { Price = 123, Type = "sculpture", Name = "Dying Sheep" });
    art.Add(new Art() { Price = 12, Type = "icon", Name = "Perplexed Smiley" });
    art.Add(new Art() { Price = 460, Type = "sculpture", Name = "Waves on Sand" });
    art.Add(new Art() { Price = 2030, Type = "painting", Name = "Robert in the Morning" });
    art.Add(new Art() { Price = 10, Type = "icon", Name = "Smiley Picking Nose" });
    art.Add(new Art() { Price = 700, Type = "painting", Name = "Birds in Autumn" });
    art.Add(new Art() { Price = 1400, Type = "sculpture", Name = "Holding Hands" });
    art.Add(new Art() { Price = 46, Type = "painting", Name = "Reeling Him In" });
    art.Add(new Art() { Price = 12000, Type = "sculpture", Name = "Old Dog" });
    art.Add(new Art() { Price = 6, Type = "icon", Name = "Hiding Smiley" });
    art.Add(new Art() { Price = 810, Type = "sculpture", Name = "Rhinestone Cowgirl" });
    art.Add(new Art() { Price = 250, Type = "painting", Name = "Upstairs, Downstairs" });
    art.Add(new Art() { Price = 3, Type = "icon", Name = "Dopey Smiley" });
    art.Add(new Art() { Price = 1000, Type = "painting", Name = "Young Love" });
    art.Add(new Art() { Price = 260, Type = "sculpture", Name = "Taking a Spill" });
}

我想要的是一个对象集合，每个类型都有一个对象，它们有三个属性: ArtType、ArtName和MostExpensivePrice。对于每种类型，我想要的名称和价格的最高价格的项目的类型。

所以我的清单应该是：

painting______Robert_in_the_Morning______2030

sculpture_____Old Dog__________________12000

icon_________Perplexed Smiley______________12

那么LINQ会是什么样的呢？我开始的示例如下所示：

var categories4 =
    from a in art
    group a by a.Type into g
    let maxPrice = g.Max(p => p.Price)
    select new { ArtType = g.Key, MostExpensive = g.Where(a => a.Price == maxPrice) };

Answer 1

请参见Enumerable.Aggregate()方法。

到目前为止，给出的另一个答案只是返回最高价格，这不是你在这里要求的。如果您像这样使用Enumerable.Aggregate()：

MostExpensive = g.Aggregate((art1, art2) => (art1.Price > art2.Price) ? art1 : art2)

然后，LINQ结果将包含Art实例，而不是int实例，因此您可以以最大值而不是仅显示价格来显示所有信息。

编辑：

如果上面的内容并不明显，那么完整的表达式可以是：

var artprices =
    from a in art
    group a by a.Type into g
    let mostExpensive = g.Aggregate((art1, art2) => (art1.Price > art2.Price) ? art1 : art2)
    select new { ArtType = g.Key, ArtName = mostExpensive.Name, MostExpensivePrice = mostExpensive.Price };

你会得到一个结果，元素有你想要的三个值。

编辑2：

最后，作为新的网站，我不能添加评论到其他答案，但我会尽可能客观地指出，他们都有缺陷，以不同的方式。

一个答案是，对原始集合中的每个元素进行一次评估，然后对每个Type值(即每个组)进行一次评估。这是一个典型的O(N^2)场景，它将在非常小的数据集上执行得很好，但是对于任何非平凡的数据集合来说都是非常糟糕的。

另外两个答案建议对每一组中的元素进行排序。这样做更好，但仍然需要该排序的内存和性能开销。典型的排序是O(N log N)，这比O(N^2)好得多，但仍然不如使用聚合()获得的线性O(N)好。同样，小数据集完全没有问题，但是与更有效的方法相比，一个非平凡的集合将导致显着的性能下降。

希望这能帮上忙！

Answer 2

这个对你有用吗？

var query =
    art
        .OrderByDescending(x => x.Price)
        .GroupBy(x => x.Type)
        .Select(x => x.First());

我得到了这个结果：

​

​

Answer 3

好吧，你把第一部分说对了，你只是有点偏离了第二部分。您需要做的第一件事是了解GroupBy方法返回的内容。GroupBy本质上返回列表列表(数组数组或可枚举的可枚举列表)。

使用声明为您的类型是：

public class Art
{
    public string Type { get; set; }
    public string Name { get; set; }
    public int Price { get; set; }
}

根据这些数据：

List<Art> art = new List<Art>()
{
    new Art() { Price = 45, Type = "painting", Name = "Still Life in Maryland" }),
    new Art() { Price = 123, Type = "sculpture", Name = "Dying Sheep" }),
    new Art() { Price = 12, Type = "icon", Name = "Perplexed Smiley" }),
    new Art() { Price = 460, Type = "sculpture", Name = "Waves on Sand" });,
    new Art() { Price = 2030, Type = "painting", Name = "Robert in the Morning" }),
    new Art() { Price = 10, Type = "icon", Name = "Smiley Picking Nose" }),
    new Art() { Price = 700, Type = "painting", Name = "Birds in Autumn" }),
    new Art() { Price = 1400, Type = "sculpture", Name = "Holding Hands" }),
    new Art() { Price = 46, Type = "painting", Name = "Reeling Him In" }),
    new Art() { Price = 12000, Type = "sculpture", Name = "Old Dog" }),
    new Art() { Price = 6, Type = "icon", Name = "Hiding Smiley" }),
    new Art() { Price = 810, Type = "sculpture", Name = "Rhinestone Cowgirl" }),
    new Art() { Price = 250, Type = "painting", Name = "Upstairs, Downstairs" }),
    new Art() { Price = 3, Type = "icon", Name = "Dopey Smiley" }),
    new Art() { Price = 1000, Type = "painting", Name = "Young Love" }),
    new Art() { Price = 260, Type = "sculpture", Name = "Taking a Spill" })
}

将艺术品列表分组，结果如下所示：

IEnumerable<IGrouping<string, Art>> groupedByType = art.GroupBy(a => a.Type);

其中，每个IGrouping<string, Art>包含一个Art列表，其中列表中的每个部分都具有相同的Type。在第二步中，我们只需要从每一组中选择最高价格：

IEnumerable<Art> maxFromEachGroup = groupedByType
    // Take a single piece of art from each group
    .Select(group =>
        // Get the maximum piece of art by ordering from largest to smallest 
        // and taking the first
        group.OrderByDescending(a => a.Price).First() 
    );

现在您有了一个Art列表，其中包含了每个组中最昂贵的部分。使用最大值需要注意的是，它返回最高价格的值，而不是以最大价格返回Art的部分。因此，LINQ表达式表单中的整个表达式是：

var maxFromEachGroup = from a in art
                       group a by a.Type into g
                       select (from a in g orderby a.Price descending select a).First();