"Select followed by First" versus "First followed by Select" when summing the data for a student

Question

I am wondering which one I should choose between the following two options when I need to calculate the sum of Data of the student with Id of 1.

var out1 = students
            .Select(x => new { Id = x.Id, Sum = x.Data.SelectMany(n => n).Sum() })
            .FirstOrDefault(s => s.Id == 1)
            .Sum;

var out2 = students
            .FirstOrDefault(s => s.Id == 1)
            .Data.SelectMany(n => n)
            .Sum();

Any comments are welcome.

class Student
{
    public int Id { get; set; }
    public List<List<int>> Data { get; set; }
}

class Program
{
    static void Main()
    {
        var a = new Student
        {
            Id = 1,
            Data = new List<List<int>>
            {
                new List<int>{ 1,2,3},
                new List<int>{ 4,5,6}
            }
        };

        var b = new Student
        {
            Id = 2,
            Data = new List<List<int>>
            {
                new List<int>{ 7,8,9},
                new List<int>{ 10,11,12}
            }
        };

        var c = new Student
        {
            Id = 3,
            Data = new List<List<int>>
            {
                new List<int>{ 13,14,15},
                new List<int>{ 16,17,18}
            }
        };

        var students = new List<Student> { a, b, c };

        var out1 = students
                    .Select(x => new { Id = x.Id, Sum = x.Data.SelectMany(n => n).Sum() })
                    .FirstOrDefault(s => s.Id == 1)
                    .Sum;

        var out2 = students
                    .FirstOrDefault(s => s.Id == 1)
                    .Data.SelectMany(n => n)
                    .Sum();

        Console.WriteLine(out1);
        Console.WriteLine(out2);

        Console.ReadLine();
    }
}

Answer 1

Option 1

students
       .Select(x => new { Id = x.Id, Sum = x.Data.SelectMany(n => n).Sum() })
       .FirstOrDefault(s => s.Id == 1)
       .Sum;

• .Select: Transforms all Student object in the collection into anonymous objects
• .FirstOrDefault: Finds the first anonymous object where the Id is matching with the predefined value
• .Sum: Accesses the Sum property of the anonymous object
  • This is error-prone since FirstOrDefault might return with null if there is no matching element in the collection
  • The fix could be ?.Sum

So, you might need to iterate through twice of the collection (once to transform all elements then if the Id == 1 matches with the last element). So, the order of this function is O(2n).

Option 2

students
       .FirstOrDefault(s => s.Id == 1)
       .Data.SelectMany(n => n)
       .Sum();

• FirstOrDefault: Finds the first Student object where the Id is matching with the predefined value
• .Data.SelectMany: Flattens the nested collection of collections to a collection
  • It is error-prone since FirstOrDefault might return with null
• .Sum(): Aggregates the items in the flatten collection

The order of this function is O(n), since Id == 1 might find the last element.

Alternatives

• If you would store your Students in a Dictionary then the lookup would be O(1)

Dictionary<int, List<List<int>>> students = ...
var result = students[1].SelectMany(n => n)?.Sum();

• You could use two Sum calls instead of SelectMany + Sum

var result = students[1].Sum(n => n.Sum());

Please be aware that the indexer operator could throw KeyNotFoundException rather than return with null