reduce number of LINQ queries

Question

void Main()
    {
        List<SomeContainer> someList = new List<SomeContainer>();
        someList.Add(new SomeContainer { a = true, b = true, c = true });
        someList.Add(new SomeContainer { a = false, b = true, c = false });
        someList.Add(new SomeContainer { a = true, b = true, c = false });
        someList.Add(new SomeContainer { a = true, b = false, c = false });
        someList.Add(new SomeContainer { a = true, b = false, c = false });
        someList.Add(new SomeContainer { a = true, b = true, c = false });
        someList.Add(new SomeContainer { a = true, b = true, c = false });

        var q1 = from container in someList where container.a == true select container.a;
        var q2 = from container in someList where container.b == true select container.b;
        var q3 = from container in someList where container.c == true select container.c;
        q1.Count().Dump();
        q2.Count().Dump();
        q3.Count().Dump();
    }

    class SomeContainer
    {
        public Boolean a { get; set; }
        public Boolean b { get; set; }
        public Boolean c { get; set; }
    }

is it possible to generate something like this with one querry:
a | b | c
6 | 5 | 1

Snowbear · Accepted Answer · 2011-03-23 22:52:29Z

4

Not sure whether you will treat this as optimization, but this will iterate over your list only once:

var result = someList
        .Select(i => new [] {i.a? 1:0, i.b? 1:0, i.c? 1:0,})
        .Aggregate((i, acc) => new [] {i[0]+acc[0], i[1]+acc[1], i[2]+acc[2]});

int countA = result[0];
int countB = result[1];
int countC = result[2];

answered Mar 23, 2011 at 22:52

Snowbear

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3 Comments

BrokenGlass Over a year ago

+1 this answers the question, I'd personally settle for int aCount = someList.Count(x => x.a); (b,c) though for readability.

Snowbear Over a year ago

@BrokenGlass, agree, but it wasn't about readability. It is anyway good to know this idea since it works with plain SQL also so it can optimize regular SQL queries.

Przemysław Banaszek Over a year ago

Now I know everything about Aggregate. BrokenGlass's solution rocks for it's simplicity. Thanks for quick response.

Sanjeevakumar Hiremath · Accepted Answer · 2011-03-24 08:15:45Z

1

int a = 0, b = 0, c = 0;
var qAll = (from ct in someList
            select new{ ac  = ct.a ? a++ : 0,  
                        bc =  ct.b ? b++ : 0,
                        cc = ct.c ? c++ : 0}).ToList();

//Now a, b, c variables should have the count of a==true, b==true & c==true in somelist.
//Console.WriteLine("A= {0}, B={1}, C={2}", a, b, c);

answered Mar 24, 2011 at 8:15

Sanjeevakumar Hiremath

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1 Comment

Przemysław Banaszek Over a year ago

So it is possible to do it in one query! Perfect solution.

user180326 · Accepted Answer · 2011-03-23 22:55:58Z

0

The Aggregate extension methods allow you combine values from enumerables in arbitrary ways. You can aggregate a tuple of three integers representing a count, and provide a function that will increment the three counters on the tuple depending on the input.

It suspect it may could look contrived for this example, and it may be slower than a regular for loop.

Sometimes Aggregate shines though. I used it to compute the bounding rectangle of a list of rectangles: var union = rects.Aggregate(Rectangle.Union);

answered Mar 23, 2011 at 22:55

user180326

Comments

Przemysław Banaszek · Accepted Answer · 2011-03-24 01:23:26Z

0

someList.Count(container => container.a).Dump();
someList.Count(container => container.b).Dump();
someList.Count(container => container.c).Dump();
//even when looking for false values it looks nice...
someList.Where(container => container.a == false).Count(container => container.a).Dump();

answered Mar 24, 2011 at 1:23

Przemysław Banaszek

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