Sum of marks based on ID of a generic in List in C#

Question

I am trying to do aggregate functions in a list of generics in C# based on an id

class Student{
  public int id { get; set; }
  public int marks { get; set; }
}

class Main {
  List<Student> StudentList = new List<Student>();
  List.Add(new Student(1,55);
  List.Add(new student(2,65);
  List.Add(new student(4,75);
  List.Add(new student(1,65);
  List.Add(new student(2,45);

  foreach(var st in StudentList) {
    Console.WriteLine(st.id + " " + st.marks);
  }
}

But this returns

I want to return the following result:

1 120
2 110
4 75

Take a look at https://stackoverflow.com/questions/16100900/select-multiple-fields-group-by-and-sum — Farshid Zaker, Mar 09 '19 at 23:17
Please take more care when posting code, this one is littered with errors. — DavidG, Mar 09 '19 at 23:18
Possible duplicate of [Select multiple fields group by and sum](https://stackoverflow.com/questions/16100900/select-multiple-fields-group-by-and-sum) — Richard II, Mar 10 '19 at 00:11

score 2 · Accepted Answer · answered Mar 09 '19 at 23:22

2

This can be done with the Linq GroupBy function:

var totals = StudentList
    .GroupBy(sl => sl.id) //Group by ID
    .Select(g => new Student(g.Key, g.Sum(s => s.marks)));

foreach (var st in totals)
{
    Console.WriteLine(st.id + " " + st.marks);
}

answered Mar 09 '19 at 23:22

DavidG

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Can you also add a solution without using Linq? – Learn AspNet Mar 09 '19 at 23:42
1

No, you should be able to do that yourself. – DavidG Mar 09 '19 at 23:42
I cannot. I am trying. Can you please help. I will really appreciate it – Learn AspNet Mar 09 '19 at 23:44
I've already answered your question, you can't keep asking for more. – DavidG Mar 09 '19 at 23:48
Can we convert var totals to List so I can perform further operations on it? – Learn AspNet Mar 09 '19 at 23:55
1

Yes, add `.ToList()` on the end. – DavidG Mar 09 '19 at 23:56

RoadRunner · Answer 2 · 2019-03-10T00:29:39.723

You can also group with a Dictionary<int, int>.

With LINQ:

var Totals = StudentList
    .GroupBy(x => x.Id)
    .ToDictionary(x => x.Key, x => x.Select(i => i.Mark).Sum());

Without LINQ:

var Totals = new Dictionary<int, int>();

foreach (var student in StudentList)
{
    var id = student.Id;
    if (!Totals.ContainsKey(id))
    {
        Totals[id] = 0;
    }

    Totals[id] += student.Mark;
}

Then you can simply print out the results like this:

foreach (var result in Totals)
{
    Console.WriteLine($"{result.Key} {result.Value}");
}

Which Outputs:

1 120
2 110
4 75

Sum of marks based on ID of a generic in List in C#

2 Answers2