How to select a single object using Linq in vb.net

Question

I have done a lot of searching to what appears to be a simple LINQ problem but I can't figure out how to do grab an object out of a collection that has a specified minimum (or max value) without resorting to a sort like this:

dim customers= GetCustomers()

dim youngest = (From c in customers
                 Order By c.age Ascending).ToList.First

This (untested code) structure works fine with the exception that the entire customer array must be sorted and placed into a list for the only purpose of extracting the first value. That can't be the best way to get the minimum!

Note that I want the whole c record in this case, not the minumum age of a customer that can be done like this (a typical example):

dim customers= GetCustomers()

dim youngest = (From c in customers
                 Select c.age).Min

Or even

dim customers= GetCustomers()

dim youngest = (From c in customers
                 Select c).Min(Function(x) x.age)

I can't for the life of me figure out how to get the whole object (or even the index) without resorting to the sort...

Sanjeevakumar Hiremath · Accepted Answer · 2011-03-19T00:06:52.653

6

Again, C# code, I'm not sure of I got it right in VB.NET

C#

  Customer youngest = customers.Aggregate((c1, c2) => (c1.age < c2.age) ? c1 : c2);

VB.NET

  dim youngest = customers.Aggregate( Function(ByVal c1, ByVal c2) IF( (c1.age < c2.age) , c1 , c2  ) );

edited Mar 19 '11 at 00:06

answered Mar 18 '11 at 23:26

Sanjeevakumar Hiremath

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**Edited** I got the question wrong first time around, now it is O(n) selection. – Sanjeevakumar Hiremath Mar 18 '11 at 23:48
I ended up using aggregate and it seems to be pretty clean. Thanks. – Hucker Mar 19 '11 at 05:37
I assume its your first question, you need to accept an answer that solved your problem. – Sanjeevakumar Hiremath Mar 19 '11 at 05:46

score 1 · Answer 2 · edited May 23 '17 at 11:50

1

There is no such operator in regular LINQ which will avoid sorting of the entire IEnumerable. But you're not the first one who needs a solution. For example check out Jason's answer here (though it's MaxBy and C# but you'll get an idea): Simple LINQ question in C#

Or MinBy from MoreLinq

edited May 23 '17 at 11:50

Community

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answered Mar 18 '11 at 23:32

Snowbear

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Aggregate is one iteration and with aggregate you could select without sorting, check my answer. – Sanjeevakumar Hiremath Mar 18 '11 at 23:49
@Sanje, yes, agree. Looks a little bit sophisticated as most queries with `Aggregate`, but still ok. Thanks for the idea – Snowbear Mar 19 '11 at 00:18

score 0 · Answer 3 · answered Mar 18 '11 at 22:59

0

You are almost there. You should find that

dim youngest = (From c in customers
                Order By c.age Ascending
                Select c).First

does what you are looking for. (I am a C# guy, not a VB.NET guy, so my syntax may be off a bit.)

answered Mar 18 '11 at 22:59

DocMax

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That is basically what my first query does. Because you are doing the 'Order by c.age' there is an implied sort which adds an O(NlogN) performance hit vs the O(N) behavior I'm hoping to see with the Min function. – Hucker Mar 18 '11 at 23:27
I get that this has an implied sort. I had read your question to mean that you were trying to avoid the ToList, not that you were looking to avoid the sort altogether. Other answers give O(N) behavior, so there is no benefit to me updating mine. – DocMax Mar 19 '11 at 02:20
There's actually no reason to assume order by sorts the list first. The first and only call to MoveNext() could possibly just return the smallest item rather than sort the whole list and this would be O(N). This was not possible with the original version that had a ToList. In lazy eval languages like Haskell, it doesn't sort the list first and this would be O(N). Sadly, even in .NET 4.0, it seems it will QSORT first making people play games with LINQ rather than being declarative – Tony Lee Mar 20 '11 at 00:01

How to select a single object using Linq in vb.net

3 Answers3