Use LINQ to find MAX or MIN of a field from all child records

Question

Please review my brief example code below and tell me if there is better, tighter way to write the LINQ query. The parent class is simply a student's name and a list of test score data. The child class is a single test score datum. I want to find the worst (or best) score out of all of the TestScore values and then identify the student who had the best score.

Thanks!

Dim query = From s In studentList _
            Where s.ScoreList.Select(Function(d) d.TestScore).Min _
                = studentList.SelectMany(Function(g) g.ScoreList).Select(Function(h) h.TestScore).Min _
            Select StudentName = s.Student, _
                   WorstScore = s.ScoreList.Select(Function(g) g.TestScore).Min

You really didn't need all that boilerplate code in there. The query itself makes it pretty easy to infer what we need to know about your classes. — Justin Morgan - On strike, Apr 20 '11 at 20:12
OT: Does anyone but me think that the phrase "LINQ Query" is redundant? But I can't think of a better way to say it! — Chris Dunaway, Apr 21 '11 at 21:38

score 2 · Accepted Answer · answered Apr 20 '11 at 20:12

I'm not a VB person so there may be some syntax errors, but I can suggest two improvements:

Rather than using Select then Min, use the overload of Min which accepts a projection

Don't find the overall minimum on every iteration; just do it once:

Dim worstScore = studentList.SelectMany(Function(g) g.ScoreList) _
                            .Min(Function(h) h.TestScore)

Dim query = From s In studentList _
            Where s.ScoreList.Min(Function(d) d.TestScore) = worstScore
            Select s.Student

Note that I've removed the anonymous type from the query - we know the worstScore already, so we don't really need it in with the query result. Note that there could still be multiple students with that worst score though.

I prefer your answer because it removes the redundant data (worstScore) from the query. Thanks for pointing out the Min projection. — blueshift, Apr 21 '11 at 12:56

score 1 · Answer 2 · answered Apr 20 '11 at 21:20

1

How about

Dim query = studentList.OrderBy(Function(d) d.ScoreList.Select(Function(d) d.TestScore).Min).First()

Using OrderByDescending and Max to find the best score.

answered Apr 20 '11 at 21:20

BlueRaja - Danny Pflughoeft

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Thanks for your response. It is pretty minimalistic, which is what I originally was seeking. – blueshift Apr 21 '11 at 13:00

score 0 · Answer 3 · edited May 23 '17 at 11:55

The first thing I would do if I were you is add a MinimumScore and MaximumScore property to StudentScores make AddScore keep track of the minimum and maximum score that you've seen so far. Now for a given student you can find these in O(1) time instead of in O(n) time

Then what you want to do is write an implementation of MaxBy and MinBy. I don't speak VB fluently, but you can find an implementation of MaxBy in C# in a previous answer and convert to VB accordingly.

Finally, having done all this, you can say

studentWithBestScore = studentList.MaxBy(s => s.MaximumScore);
var bestScore = studentWithBestScore.MaximumScore;

(Again, you'll have to convert this to VB.)

Thanks for taking time to respond. The MaxBy and MinBy might be useful to me at another time. For now, I'll pursue an approach that uses native LINQ syntax. — blueshift, Apr 21 '11 at 12:59

Use LINQ to find MAX or MIN of a field from all child records

3 Answers3