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LINQ к объектам индексируют в группе + для различных группировок (иначе ROW_NUMBER с РАЗДЕЛОМ эквивалентом)

Несколько человек упомянули , используя блоки, но я думаю, что они гораздо полезнее, чем думали люди. Думайте о них как об инструменте АОП бедного человека. У меня есть множество простых объектов, которые фиксируют состояние в конструкторе, а затем восстанавливают его в методе Dispose () . Это позволяет мне обернуть часть функциональности в блок , используя блок , и быть уверенным, что состояние восстанавливается в конце. Например: 

using(new CursorState(this, BusyCursor));
{
    // Do stuff
}

CursorState фиксирует текущий курсор, используемый формой, затем устанавливает форму для использования поставляемого курсора. В конце он восстанавливает исходный курсор. Я делаю множество таких вещей, как, например, захват выборок и текущей строки в сетке перед обновлением и т. Д.

7
задан Justin Grant 25 July 2009 в 22:21
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3 ответа

I think jpbochi missed that you want your groupings to be by pairs of values (Title+SourceId then Title+Index). Here's a LINQ query (mostly) solution:

var selectedFew = 
    from doc in docs
    group doc by new { doc.Title, doc.SourceId } into g
    from docIndex in g.Select((d, i) => new { Doc = d, Index = i })
    group docIndex by new { docIndex.Doc.Title, docIndex.Index } into g
    select g.Aggregate((a,b) => (a.Doc.SourceId <= b.Doc.SourceId) ? a : b);

First we group by Title+SourceId (I use an anonymous type because the compiler builds a good hashcode for the grouping lookup). Then we use Select to attach the grouped index to the document, which we use in our second grouping. Finally, for each group we pick the lowest SourceId.

Given this input:

var docs = new[] {
    new { Title = "ABC", SourceId = 0 },
    new { Title = "ABC", SourceId = 4 },
    new { Title = "ABC", SourceId = 2 },
    new { Title = "123", SourceId = 7 },
    new { Title = "123", SourceId = 7 },
    new { Title = "123", SourceId = 7 },
    new { Title = "123", SourceId = 5 },
    new { Title = "123", SourceId = 5 },
};

I get this output:

{ Doc = { Title = ABC, SourceId = 0 }, Index = 0 }
{ Doc = { Title = 123, SourceId = 5 }, Index = 0 }
{ Doc = { Title = 123, SourceId = 5 }, Index = 1 }
{ Doc = { Title = 123, SourceId = 7 }, Index = 2 }

Update: I just saw your question about grouping by Title first. You can do this using a subquery on your Title groups:

var selectedFew =
    from doc in docs
    group doc by doc.Title into titleGroup
    from docWithIndex in
        (
            from doc in titleGroup
            group doc by doc.SourceId into idGroup
            from docIndex in idGroup.Select((d, i) => new { Doc = d, Index = i })
            group docIndex by docIndex.Index into indexGroup
            select indexGroup.Aggregate((a,b) => (a.Doc.SourceId <= b.Doc.SourceId) ? a : b)
        )
    select docWithIndex;
6
ответ дан 6 December 2019 в 23:11
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To be honest, I'm quite confused with your question. Maybe if you should explain what you're trying to solve. Anyway, I'll try to answer what I understood.

1) First, I'll assume that you already have a list of documents grouped by Title+SourceId. For testing purposes, I hardcoded a list as follow:

var docs = new [] {
    new { Title = "ABC", SourceId = 0 },
    new { Title = "ABC", SourceId = 4 },
    new { Title = "ABC", SourceId = 2 },
    new { Title = "123", SourceId = 7 },
    new { Title = "123", SourceId = 5 },
};

2) To get put a index in every item, you can use the Select extension method, passing a Func selector function. Like this:

var docsWithIndex
    = docs
    .Select( (d, i) => new { Doc = d, Index = i } );

3) From what I understood, the next step would be to group the last result by Title. Here's how to do it:

var docsGroupedByTitle
    = docsWithIndex
    .GroupBy( a => a.Doc.Title );

The GroupBy function (used above) returns an IEnumerable>. Since a group is enumerable too, we now have an enumerable of enumerables.

4) Now, for each of the groups above, we'll get only the item with the minimum SourceId. To make this operation we'll need 2 levels of recursion. In LINQ, the outer level is a selection (for each group, get one of its items), and the inner level is an aggregation (get the item with the lowest SourceId):

var selectedFew
    = docsGroupedByTitle
    .Select(
        g => g.Aggregate(
            (a, b) => (a.Doc.SourceId  <= b.Doc.SourceId) ? a : b
        )
    );

Just to ensure that it works, I tested it with a simple foreach:

foreach (var a in selectedFew) Console.WriteLine(a);
//The result will be:
//{ Doc = { Title = ABC, SourceId = 0 }, Index = 0 }
//{ Doc = { Title = 123, SourceId = 5 }, Index = 4 }

I'm not sure that's what you wanted. If not, please comment the answer and I can fix the answer. I hope this helps.

Obs.: All the classes used in my tests were anonymous. So, you don't really need to define a DocumentWithIndex type. Actually, I haven't even declared a Document class.

3
ответ дан 6 December 2019 в 23:11
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Синтаксис на основе метода: 

var selectedFew = docs.GroupBy(doc => new {doc.Title, doc.SourceId}, doc => doc)
                      .SelectMany((grouping) => grouping.Select((doc, index) => new {doc, index}))
                              .GroupBy(anon => new {anon.doc.Title, anon.index})
                              .Select(grouping => grouping.Aggregate((a, b) =>    a.doc.SourceId <= b.doc.SourceId ? a : b));

Можно ли сказать, что приведенный выше синтаксис эквивалентен синтаксису на основе метода?

1
ответ дан 6 December 2019 в 23:11
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