Name in a hat suffle vb.net - vb.net

Question

I'm new to stack overflow and I'm learning a new language which is vb.net. I'm working on a first app that need the use of a database, and what i'm trying to do is taking every entries in a column, putting them in an array, suffle the array and then putting back the new array in the column of the database. Sort of a names in a hat application.. I don't have any code right now, I'm doing my lessons first and I do search many forums and I have already many pieces of the puzzle like the randomize array, database handling.. My problem is that I don't find any code to prevent data to get the same order after the shuffle for an example:
Let's pretend I have 4 entries in my database:
Sarah, james, alex, daniel.
When suffling the array, how can I prevent sarah to comes first, and or james second, etc..
If you could just give me a point to start.. As I told you I'm starting to learn this language and I don't want you guys to write the app for me but just having a little clue will be much apreciate. I check lessons online but I'm getting a little bored with begginers course and the "hello world" first app demonstration so.. I think I'm ready for the next step!

Answer 1

#Steven got a valid point. But I think you can even go without adding the SortIndex, because you can shuffle results directly in your query, like this (assuming your backend is MSSQL):
SELECT * FROM table1 ORDER BY NEWID()
If still necessary, here is how an array can be shuffled programmatically:
Dim a() As String = {"Sarah", "James", "Alex", "Daniel"}
Dim shuffledList As New List(Of String)
Randomize()
For i = 0 To UBound(a)
Dim allowedValues() As String = a.Except(shuffledList).ToArray
shuffledList.Add(allowedValues(Math.Floor(Rnd() * allowedValues.Count)))
Next
If you want to make sure original order is not repeated for any of the elements, use this code instead:
Dim a() As String = {"Sarah", "James", "Alex", "Daniel"}
Dim ub As Integer = UBound(a)
Dim shuffledList As New List(Of String)
Randomize()
For i = 0 To ub
Dim allowedValues() As String = a.Except(shuffledList).Except({a(i)}).ToArray
Dim randomValue As String
If i = ub - 1 And allowedValues.Contains(a(ub)) Then
randomValue = a(ub)
Else
randomValue = allowedValues(Math.Floor(Rnd() * allowedValues.Count))
End If
shuffledList.Add(randomValue)
Next
Here the most important part is where the item before the last one is forced to be the last item, if the last item was not already picked. This is to make sure last item goes somewhere except the last position, otherwise there is nothing to pick from in the last step. Overall, the sequence appears to be randomly sorted and it's guaranteed that item #1 will not be #1, #2 will not be #2 etc. The algorithm always ends in N steps, where N is the number of items.

Answer 2

In the first instance, why would the initial order be an invalid shuffling?
Putting that aside, the simplest method would be to clone your original list and element compare it with the result of your function and keep randomising until it isn't. Better check for 0 or 1 element lists or else you will get an infinite loop.

Answer 3

In SQL compliant databases, the physical order of the rows in a table is supposed to be largely irrelevant, since you can sort on anything when you select the data out of it (the physical order in SQL Server is determined/set by the clustered index). So, in your situation, where you want to persist the sort order to the database, I would recommend adding an additional column that stores the sort index of each row.
So, for instance, when sorted in alphabetical order, the data would look like this:
Name SortIndex
------ ---------
Sarah 3
James 2
Alex 0
Daniel 1
And when shuffled, the physical order wouldn't change, but the sort indexes would, for instance:
Name SortIndex
------ ---------
Sarah 2
James 0
Alex 1
Daniel 3

Answer 4

Thanks for your answers. I already did that script in another language, the way I achieve it is pretty simple:
Taking the same array: Sarah, James, Alex, Daniel;
I would duplicate the array;
Then every entries in the first array would pick a random entry in the second one; Sarah would go first - if she doe's pick her own name, cancel it then pick again until she pick another name;
Then retire the picked name from the second array;
Then repeat every step with the second name, third...
All this done by a loop of course.
Maybe all this could be achieve very simply in vb.net but I'm pretty sure ( without knowing much of it) that there is no "Ramdomize.PreventSameSortIndexOrder" kind of function in vb.net ;). I understand what Steven was demonstrating but could you explain me how this way it can prevent the Index IDs to come back to the same place after the suffle? I was learning to handle MS Access database with VB but if SQL is better for some reason then it won't be a problem to switch. #Neolisk Could you tell me more about this command you wrote:
SELECT * FROM table1 ORDER BY NEWID()
Thanks all!