C# - Splitting a list into n amount of sub-lists

Question

I would like to split a list into 'n' amount of sub-lists.

I have a list of Form teachers and a list of Students. Every Student is assigned to one Form teacher and each Form teacher can have more than one Student. The list of Form teachers is dynamic - it is being populated based on checkbox selection on the form (i.e: there may be one, three, six, etc. in the list).

//A method to assign the Selected Form teachers to the Students
private void AssignFormTeachers(List<FormTeacher> formTeacherList, List<Student> studentList)
{
    int numFormTeachers = formTeacherList.Count;

    //Sort the Students by Course - this ensures cohort identity.
    studentList = studentList.OrderBy(Student => Student.CourseID).ToList();

    //Split the list according to the number of Form teachers
    List<List<Student>> splitStudentList = splitList(numFormTeachers , studentList);

The splitList() method is where I'm attempting to split the list into a list of Student lists, but I'm having a problem. Let's say there are 3 Form teachers - I can't seem to split the list into 3 sub-lists, but rather end up with lists of 3 Students.

I would really appreciate some help with this. I have searched for a possible solution, but every time I end up with lists of size 'n', rather than 'n' amount of lists. If this question has been answered before, please point me in the direction of that answer.

Answer 1

You're trying to partition your list into n parts with equal number of elements?

Try GroupBy:

var splitStudentList = studentList.Select((s, i) => new { s, i })
                                  .GroupBy(x => x.i % numFormTeachers)
                                  .Select(g => g.Select(x => x.s).ToList())
                                  .ToList();

Or you can create your own extension method to do that. I've described how to do it right on my blog: Partitioning the collection using LINQ: different approaches, different performance, the same result.

public IEnumerable<IEnumerable<T>> Partition<T>(IEnumerable<T> source, int size)
{
    var partition = new List<T>(size);
    var counter = 0;

    using (var enumerator = source.GetEnumerator())
    {
        while (enumerator.MoveNext())
        {
            partition.Add(enumerator.Current);
            counter++;
            if (counter % size == 0)
            {
                yield return partition.ToList();
                partition.Clear();
                counter = 0;
            }
        }

        if (counter != 0)
            yield return partition;
    }
}

usage:

var splitStudentList = studentList.Partition(numFormTeachers)
                                  .Select(x => x.ToList())
                                  .ToList();