Group List by properties and get number of times grouped

Question

Lets say I have a class called Person:

public class Person
{
    public int Age { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

And a list of persons:

Person { Age = 20, FirstName = "John", LastName = "Joe" }
Person { Age = 20, FirstName = "John", LastName = "Joe" }
Person { Age = 10, FirstName = "James", LastName = "Dokes" }

What I want to have is a (new or old with a new property) list that groups the person by age, first name and last name AND I also want to know how many times that object has been grouped.

So the result of the above would be:

Person { Age = 20, FirstName = "John", LastName = "Joe", Count = 2 }
Person { Age = 10, FirstName = "James", LastName = "Dokes", Count = 1 }

I got as far as this answer http://stackoverflow.com/a/491832/668521 but I am unsure about counting the number of times grouped. — Dumpen, Feb 20 '13 at 08:22

score 6 · Accepted Answer · answered Feb 20 '13 at 08:23

6

Simple:

people.GroupBy(x => new { x.Age, x.FirstName, x.LastName })
      .Select(x => new { x.Key.Age, x.Key.FirstName, x.Key.LastName, Count = x.Count() });

answered Feb 20 '13 at 08:23

JustAnotherUserYouMayKnow

2,554
16
14

1

Should be `x.Key.Age`, `x.Key.LastName` and `x.Key.FirstName`? – MarcinJuraszek Feb 20 '13 at 08:24
I am sorry, you are correct. The code is corrected too. Thank you kind sir! – JustAnotherUserYouMayKnow Feb 20 '13 at 08:25
Well that was easier than I thought! Thank you. – Dumpen Feb 20 '13 at 08:31

score 1 · Answer 2 · answered Feb 20 '13 at 08:31

Just before I posted I saw answer already from JustAnotherUserYouMayKnow, so was going to cancel, but am posting anyway just to show same result using only the GroupBy, with the resultSelector parameter (instead of separate projection)

var personList = new List<Person> {
    new Person { Age = 20, FirstName = "John", LastName = "Joe" },
    new Person { Age = 20, FirstName = "John", LastName = "Joe" },
    new Person { Age = 10, FirstName = "James", LastName = "Dokes"}};

var results = personList.GroupBy(per => new { per.Age, per.FirstName, per.LastName },
    (key, items) => new { key.Age, key.FirstName, key.LastName, Count = items.Count() } );

score 1 · Answer 3 · answered Feb 20 '13 at 08:33

Non-proc alternative:

List<Person> ls = new List<Person>();
ls.Add (new Person() { Age = 20, FirstName = "John", LastName = "Joe" });
ls.Add(new Person() { Age = 20, FirstName = "John", LastName = "Joe" });
ls.Add(new Person() { Age = 10, FirstName = "James", LastName = "Dokes" });

var result = (from cur in ls
                group cur by new { Age = cur.Age, Name = cur.FirstName, LastName = cur.LastName }
                into tmpResult
                select new { tmpResult.Key.Age,tmpResult.Key.LastName,tmpResult.Key.Name,Count = tmpResult.Count()});

Group List by properties and get number of times grouped

3 Answers3