user3113376 - 1 year ago 83
C# Question

# Getting average value of groups with LINQ

I am trying to split my List into different groups based on a certain value each item in the List has, and then find the average of another value in those groups.

To better put it, I have a list of Students:

``````List<Student> students = new List<Student> {
new Student { Name="Bob", Level=Level.Sophomore, GPA=3.2f },
new Student { Name="Cathy", Level=Level.Freshman, GPA=3.6f },
new Student { Name="James", Level=Level.Senior, GPA=3.8f },
new Student { Name="Jessica", Level=Level.Senior, GPA=3.7f },
new Student { Name="Derek", Level=Level.Junior, GPA=2.8f },
new Student { Name="Sam", Level=Level.Junior, GPA=3.1f }
};
``````

And I want to group them by their Class Level, so they'll be grouped into Freshman, Sophomore, Junior, and Senior. And then I want to be able to get the Average GPA for those groups.

So a possible result set for these students would be:

``````Senior: 3.7
Junior: 2.9
Sophomore: 3.2
Freshman : 3.6
``````

I'm not quite too sure how to go about getting this result though. I have tried things like
`students.GroupBy(x => x.Level).Average();`
but it does not work.

Any thoughts on this would be greatly appreciated. Thank you!

You have to use an additional `Select`, so you're looking for something like this:

``````var result = students.GroupBy(s => s.Level)
.Select(g => new {Level=g.Key, Avg=g.Average(s => s.GPA)});
``````

`Select(g => new {...})` creates an instance of a new anonymous type for each group.

That type has two properties:

• `Level`, which is the `Key` property of the group
• `Avg`, which is the average GPA of the group

Just "imagine" there's this type in use (more or less):

``````class GroupAverage
{
public Level Level { get; set; }
public float Avg { get; set; }
}

var result = students.GroupBy(s => s.Level)
.Select(g => new GroupAverage { Level=g.Key, Avg=g.Average(s => s.GPA) } );
``````

but it simply has no name.

`result` is now:

Feel free to round the values if you need to.

To get the `Level` with the highest average, simply use

``````var highest = result.OrderByDescending(a => a.Avg).First().Level;
``````

(Note that this will crash if there are no items in `students`)

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