VBA - check for duplicates while filling cells through a loop - vba

Question

I am writing a VBA code that goes through a defined matrix size and filling cells randomly within its limits.
I got the code here from a user on stackoverflow, but after testing it I realized that it does not fit for avoiding duplicate filling, and for instance when filling 5 cells, I could only see 4 cells filled, meaning that the random filling worked on a previously filled cell.
This is the code I'm working with:
Dim lRandom As Long
Dim sCells As String
Dim sRandom As String
Dim rMolecules As Range
Dim i As Integer, j As Integer
Dim lArea As Long
lArea = 400 '20x20
'Populate string of cells that make up the container so they can be chosen at random
For i = 1 To 20
For j = 1 To 20
sCells = sCells & "|" & Cells(i, j).Address
Next j
Next i
sCells = sCells & "|"
'Color the molecules at random
For i = 1 To WorksheetFunction.Min(5, lArea)
Randomize
lRandom = Int(Rnd() * 400) + 1
sRandom = Split(sCells, "|")(lRandom)
Select Case (i = 1)
Case True: Set rMolecules = Range(sRandom)
Case Else: Set rMolecules = Union(rMolecules, Range(Split(sCells, "|")(lRandom)))
End Select
sCells = Replace(sCells, "|" & sRandom & "|", "|")
lArea = lArea - 1
Next i
rMolecules.Interior.ColorIndex = 5
Using this same exact code which works perfectly, WHAT can I insert and WHERE do I do that so that the code would check if a cell is previously already filled with a string or a color?
I feel as though this code I'm looking for should be right before
rMolecules.Interior.ColorIndex = 5
But I'm not sure what to type.
EDIT
From the comments I realized that I should be more specific.
I am trying to randomly fill cells with the blue color (.ColorIndex = 5), but what I need to check first is if the randomizing hadn't marked a cell twice, so that for instance in this case, if I want to mark 5 different cells, it marks only 4 of them because of a duplicate and thus fills only 4 cells with the blue color. I need to avoid that and make it choose another cell to mark/fill.
I'd appreciate your help.

Answer 1

Keep the cells you use in a Collection and remove them as you fill the random cells:
Sub FillRandomCells(targetRange As Range, numberOfCells As Long)
' populate collection of unique cells
Dim c As Range
Dim targetCells As New Collection
' make sure arguments make sense
If numberOfCells > targetRange.Cells.Count Then
Err.Raise vbObjectError, "FillRandomCells()", _
"Number of cells to be changed can not exceed number of cells in range"
End If
For Each c In targetRange.Cells
targetCells.Add c
Next
' now pick random 5
Dim i As Long, randomIndex As Long
Dim upperbound As Long
Dim lowerbound As Long
For i = 1 To numberOfCells
lowerbound = 1 ' collections start with 1
upperbound = targetCells.Count ' changes as we are removing cells we used
randomIndex = Int((upperbound - lowerbound + 1) * Rnd + lowerbound)
Set c = targetCells(randomIndex)
targetCells.Remove randomIndex ' remove so we don't use it again!
c.Interior.Color = 5 ' do what you need to do here
Next
End Sub
Sub testFillRandomCells()
FillRandomCells ActiveSheet.[a1:t20], 5
FillRandomCells ActiveSheet.[b25:f30], 3
End Sub
EDIT: Changed to make the target range and number of changed cells configurable as arguments to a function. Also added error checking (always do that!).

Answer 2

Why not build a list of random numbers and place in a Scripting.Dictionary, one can use the Dictionary's Exist method to detect duplicates, loop through until you have enough then you can enter your colouring code confident that you have a unique list.