Random numbers in array without any duplicates - vb.net

Question

I'm trying to randomize an array from numbers 0 to 51 using loops but I just cannot seem to pull it off. My idea was that
Generate a Random Number
Check if this random number has been used by storing the previous in an array
If this random number has been used, generate new random number until it is not a duplicate
If it's not a duplicate, store it
My attempt:
Dim list(51) As Integer
Dim templist(51) As Integer
For i As Integer = 0 To 51 Step 1
list(i) = i
Next i
Do While counter <= 51
p = rand.Next(0, 52)
templist(counter) = p
For n As Integer = 0 To 51 Step 1
p = rand.Next(0, 52)
If templist(n) = p Then
Do While templist(n) = p
p = rand.Next(0, 52)
Loop
templist(n) = p
Else
templist(n) = p
End If
Next
counter += 1
Loop
For n As Integer = 0 To 51 Step 1
ListBox1.Items.Add(templist(n))
Next

Answer 1

It will be a lot easier if you just have a list of all of the possible numbers (0 to 51 in your case), then remove the number from the list so it can't be picked again. Try something like this:
Dim allNumbers As New List (Of Integer)
Dim randomNumbers As New List (Of Integer)
Dim rand as New Random
' Fill the list of all numbers
For i As Integer = 0 To 51 Step 1
allNumbers.Add(i)
Next i
' Grab a random entry from the list of all numbers
For i As Integer = 0 To 51 Step 1
Dim selectedIndex as Integer = rand.Next(0, (allNumbers.Count - 1) )
Dim selectedNumber as Integer = allNumbers(selectedIndex)
randomNumbers.Add(selectedNumber)
allNumbers.Remove(selectedNumber)
' Might as well just add the number to ListBox1 here, too
ListBox1.Items.Add(selectedNumber)
Next i
If your goal is to get the numbers into ListBox1, then you don't even need the "randomNumbers" list.
EDIT:
If you must have an array, try something like this:
Function RandomArray(min As Integer, max As Integer) As Integer()
If min >= max Then
Throw New Exception("Min. must be less than Max.)")
End If
Dim count As Integer = (max - min)
Dim randomNumbers(count) As Integer
Dim rand As New Random()
' Since an array of integers sets every number to zero, and zero is possibly within our min/max range (0-51 here),
' we have to initialize every number in the array to something that is outside our min/max range.
If min <= 0 AndAlso max >= 0 Then
For i As Integer = 0 To count
randomNumbers(i) = (min - 1) ' Could also be max + 1
Next i
End If
Dim counter As Integer = 0
' Loop until the array has count # of elements (so counter will be equal to count + 1, since it is incremented AFTER we place a number in the array)
Do Until counter = count + 1
Dim someNumber As Integer = rand.Next(min, max + 1)
' Only add the number if it is not already in the array
If Not randomNumbers.Contains(someNumber) Then
randomNumbers(counter) = someNumber
counter += 1
End If
Loop
Return randomNumbers
End Function
This is good enough for your assignment, but the computer scientist in my hates this algorithm.
Here's why this algorithm is much less desirable. If zero is in your range of numbers, you will have to loop through the array at least 2N times (so 104+ times if you are going from 0 to 51). This is a best case scenario; the time complexity of this algorithm actually gets worse as the range of numbers scales higher. If you try running it from 0 to 100,000 for example, it will fill the first few thousand numbers very quickly, but as it goes on, it will take longer and longer to find a number that isn't already in the list. By the time you get to the last few numbers, you could potentially have randomly generated a few trillion different numbers before you find those last few numbers. If you assume an average complexity of 100000! (100,000 factorial), then the loop is going to execute almost ten to the half-a-millionth power times.
An array is more difficult to "shuffle" because it is a fixed size, so you can't really add and remove items like you can with a list or collection. What you CAN do, though, is fill the array with your numbers in order, then go through a random number of iterations where you randomly swap the positions of two numbers.

Answer 2

Do While counter <= 51
p = rand.Next(0, 52)
While Array.IndexOf(list, p) = -1
p = rand.Next(0, 52)
End While
counter += 1
Loop

Answer 3

Haven't written VB in about 5 years, but try this out:
Function GetRandomUniqueNumbersList(ByVal fromNumber As Integer, ByVal toNumber As Integer) As List(Of Integer)
If (toNumber <= fromNumber) Then
Throw New ArgumentException("toNumber must be greater than fromNumber", toNumber)
End If
Dim random As New Random
Dim randomNumbers As New HashSet(Of Integer)()
Do
randomNumbers.Add(random.Next(fromNumber, toNumber))
Loop While (randomNumbers.Count < toNumber - fromNumber)
Return randomNumbers.ToList()
End Function
Ok, that was painful. Please someone correct it if I made any mistakes. Should be very quick because it's using a HashSet.

Answer 4

First response to forum on stackoverflow - be gentle.
I was looking for a way to do this but couldn't find a suitable example online.
I've had a go myself and eventually got this to work:
Sub addUnique(ByRef tempList, ByVal n, ByRef s)
Dim rand = CInt(Rnd() * 15) + 1
For j = 0 To n
If tempList(j) = rand Then
s = True
End If
Next
If s = False Then
tempList(n) = rand
Else
s = False
addUnique(tempList, n, s)
End If
End Sub
Then call the sub using:
Dim values(15) As Byte
Dim valueSeen As Boolean = False
For i = 0 To 15
addUnique(values, i, valueSeen)
Next
This will randomly add the numbers 1 to 16 into an array. Each time a value is added, the previous values in the array are checked and if any of them are the same as the randomly generated value, s is set to true. If a value is not found (s=false), then the randomly generated value is added. The sub is recursively called again if s is still true at the end of the 'For' loop. Probably need 'Randomize()' in there somewhere.
Apologies if layout is a bit wobbly.