Question

I know I could do this with loops (as a matter of fact, I currently am, but I am trying to learn / imporve my Linq skills and i also hope it will provide a more efficent solution. So, here’s my scenario:

Say I have the following 3 lists (I’m just making up a similar example, so forgive it’s stupidity):

Dim lstTeachers as New List(of string)
Dim lstStudentsSex as New List(of string)
Dim lstStudentName as New List(of string)

And they are, for example, as follows:

lstTeachers:     lstStudentsSex:     lstStudentName:
Teacher 1        Male                Whatever Name 1
Teacher 2        Female              Whatever Name 2 
Teacher 1        Female              Whatever Name 3
Teacher 1        Female              Whatever Name 4
Teacher 2        Male                Whatever Name 5
Teacher 3        Male                Whatever Name 6
Teacher 3        Female              Whatever Name 7
Teacher 1        Male                Whatever Name 8
Teacher 1        Female              Whatever Name 9
Teacher 2        Male                Whatever Name 10

Where each entry in each list matches the other entries with the same index – Basically like a datatable, just stored in separate lists.

Now, suppose I want to create the following structure with the following values:

Dim dictTeacherSexName as New Dictionary(Of String, Dictionary(Of String, List(of String)))

Dict1_Key:        Dict1_Value / Dict2_Key:         Dict2_Value:
Teacher 1         Male                             Whatever Name 1
                                                   Whatever Name 8
                  Female                           Whatever Name 3
                                                   Whatever Name 4
                                                   Whatever Name 9
Teacher 2 ...

… I hope this explains what I’m looking to accomplish.

Now, again, I know this may be a stupid idea, but I’m also asking it because I want to improve my Linq skills – especially Group By and multiple Selects which still catches me, so please help.

Thanks so much!

Answer 1

The three lists make things a bit difficult, but not impossible. Either you first zip the lists together as suggested by @fsimonazzi, or you base your query on the list indices rather than the list entries themselves. The result could look like this:

_(C#)

var result = Enumerable
    .Range(0, lstTeachers.Count)
    .GroupBy(i => lstTeachers[i])
    .ToDictionary(g => g.Key, g => g
        .GroupBy(i => lstStudentsSex[i])
        .ToDictionary(h => h.Key, h => h
            .Select(i => lstStudentName[i])
            .ToList()));

// result is Dictionary<string, Dictionary<string, List<string>>>