Split a List into parts - VB conversion fails

Question

Trying to write a method that splits a list into sublists.

Private Function SplitIdsIntoChunks(ByVal keys As List(Of String)) As List(Of List(Of String))
    Return keys _
        .Select(Function(x, i) New With {Key .Index = i, Key .Value = x}) _
        .GroupBy(Function(x) (x.Index / 10000)) _
        .Select(Function(x) x.Select(Function(v) v.Value).ToList()) _
        .ToList()
End Function

I used C# solution from here. C# solution works fine.

My version written in VB returns a collection of lists with one element instead of 10000. Where did I go wrong?

Thanks in advance.

Edit 1:

Usage:

Dim chunks As List(Of List(Of String)) = SplitIdsIntoChunks(keys)

'Keys' content:

My method returns a lists of lists with one item inside:

Expected result: list of two lists - 10000 items in first and 6256 in second.

Answer 1

The main 'problem' is that in c# dividing an int by an int, x.Index / 3, results in an integer. Standard division / in VB always results in a Double and therefore a unique Group for each item. e.g.: {.33, .333333, .333334} etc.

The 'trick' is to use integer division \ to get an integer result. I also added a chunksize argument to make it more flexible and reusable:

Private Function SplitIntoChunks(keys As List(Of String), chunkSize As Integer) As List(Of List(Of String))
    Return keys.
            Select(Function(x, i) New With {Key .Index = i, Key .Value = x}).
            GroupBy(Function(x) (x.Index \ chunkSize)).
            Select(Function(x) x.Select(Function(v) v.Value).ToList()).
            ToList()
End Function

One other difference is that c# is very flexible regarding line continuation, so the dot can be at the start or end of a line. VB (since VS2010) supports implicit line continuation after a great many things including member qualifier character (the dot .) so you can get rid of the explicit line continuation (" _").

Usage:

' split list of 1000 into 300 chunks
Dim splits = SplitIntoChunks(myList, 300)

For n As Integer = 0 To splits.Count - 1
    Console.WriteLine("Split {0} has {1} items", n.ToString(), splits(n).Count)
Next

Output/Result:

Split 0 has 300 items
Split 1 has 300 items
Split 2 has 300 items
Splits 3 has 100 items

Note that there is a lot of commentary on the linked C# post about performance of various methods. This just gets it working, some of the alternatives might perform better depending on what is actually being split. The Skip and Take approach used here is interesting too.

Answer 2

In the linked answer the First Select is like this

.Select((x, i) => new { Index = i, Value = x })

You have in your code

.Select(Function(x, i) New With {Key .Index = i, Key .Value = x})

Why did you repeat Key, I think you only need to specify the Properties of the anonymous type

Answer 3

The correct translation of that code is:

Private Function SplitIdsIntoChunks(ByVal keys As List(Of String)) As List(Of List(Of String))
    Return keys _
        .Select(Function(x, i) New With {Key .Index = i, Key .Value = x}) _
        .GroupBy(Function(x) (x.Index \ 10000)) _
        .Select(Function(x) x.Select(Function(v) v.Value).ToList()) _
        .ToList()
End Function

Note the difference between / (which returns a floating-point) and \ (which returns an integer).