random and shuffle questions vb - vb.net

Question

i'm puzzled in the logic of shuffling the questions that i'll display from my database.
my current code displays random questions to the textbox but it also display empty and duplicate entries.
Dim r As MySqlDataReader
Dim i As Integer = 0
Dim temp() As Integer = {}
Dim txtQ() As TextBox = {txtQ1, txtQ2, txtQ3, txtQ4, txtQ5, txtQ6, txtQ7, txtQ8, txtQ9, txtQ10}
Dim cbA() As CheckedListBox = {cbA1, cbA2, cbA3, cbA4, cbA5, cbA6, cbA7, cbA8, cbA9, cbA10}
con.Open()
cmd = New MySqlCommand("select * from tbexam", con)
r = cmd.ExecuteReader
While r.Read
If i <= 9 Then
Randomize()
Dim j As Integer = CInt(Int(9 * Rnd()))
txtQ(j).Text = r.GetString("exam_question")
cbA(j).Items.Clear()
cbA(j).Items.Add("a) " & r.GetString("exam_ans_a"))
cbA(j).Items.Add("b) " & r.GetString("exam_ans_b"))
cbA(j).Items.Add("c) " & r.GetString("exam_ans_c"))
cbA(j).Items.Add("d) " & r.GetString("exam_ans_d"))
i = i + 1
End If
End While

Answer 1

When you place a random number in j, it is (very nearly) a random number between 0 and 9. This is equivalent to rolling a ten-sided dice. What do you think the chances are that if you were to roll it ten times you would get exactly one of each of the values 0 to 9? Pretty low. What you will actually get is something like: 2, 0, 8, 2, 5, 1, 7, 6, 1, 4. This sequence contains duplicates of 2 and 1 and the values 3, and 9 do not appear. This is where your duplicates and gaps come from.
One option you could try is to fill the array sequentially and then shuffle it.
Dim r As MySqlDataReader
Dim i As Integer = 0
Dim temp() As Integer = {}
Dim txtQ() As TextBox = {txtQ1, txtQ2, txtQ3, txtQ4, txtQ5, txtQ6, txtQ7, txtQ8, txtQ9, txtQ10}
Dim cbA() As CheckedListBox = {cbA1, cbA2, cbA3, cbA4, cbA5, cbA6, cbA7, cbA8, cbA9, cbA10}
con.Open()
cmd = New MySqlCommand("select * from tbexam", con)
r = cmd.ExecuteReader
' Read the Q&As into the arrays in the order they appear in the DB
While r.Read
If i <= 9 Then
txtQ(i).Text = r.GetString("exam_question")
cbA(i).Items.Clear()
cbA(i).Items.Add("a) " & r.GetString("exam_ans_a"))
cbA(i).Items.Add("b) " & r.GetString("exam_ans_b"))
cbA(i).Items.Add("c) " & r.GetString("exam_ans_c"))
cbA(i).Items.Add("d) " & r.GetString("exam_ans_d"))
i = i + 1
End If
End While
' Re-order the arrays by swapping each element with another, randomly selected element.
' Start with i at the count of the elements read in the last loop and work down.
Dim n as Integer = i - 1
While i > 0
Dim j As Integer = CInt(Int(n * Rnd()))
i = i - 1
' Swap elements i and j of the arrays
Dim tmpQ As TextBox = txtQ(i)
txtQ(i) = txtQ(j)
txtQ(j) = tmpQ
Dim tmpA() As CheckedListBox = cbA(i)
cbA(i) = cbA(j)
cbA(j) = tmpA
End While
This could be simplified and abstracted but you get the idea.

Answer 2

It is hard to avoid duplicate entry using random number generation, but you can avoid empty entries. You get empty entries because you generates random number and fetch question with that id, and unfortunately that id is not available in database always. SO first, fetch all the ids from database in an array and generate random number from 0 to array length. and select id value of array key.