VBA - Tree visualisation. Possibly the hardest code to date - vba

Question

I've been stuck here for hours trying to come up with a VBA excel logic for this problem. Think of this as a tree problem, in which the number of branches/nodes is determined by the user and inputted at the start.
For every node, there is 3 possible outcomes, staying flat, increasing by 1% or decreasing by 1%. i.e. for 5 nodes, you will get 5^3 nodes and so on...
How do i code this in vba so that it auto-populates the spreadsheet?
I am trying to model a population birth rate in a city where after every year, T+0 to T+n, where n is the number of nodes. So every +1 year, there could either be 101% of the population, 100% or 99% of the population remaining in the city.
To add to the complexity, the number of people in the city has to be dynamic at any time. So for example we know the path in the 5th year where we get +1% of population every year. so at 5th year we should have (1.01)^5 people. However, since this city is dynamic, there could be people leaving or entering the city, so manual adjustments to the city population has be to catered for.
E.G. At 5th year, 5000 left the city to another place. So the spreadsheet has to be dynamic enough to adjust the 5th level node from (1.01)^5 to (1.01)^5 - 5000. And the 6th node carries on from there.... branching out again.
Not sure if i explained this clearly enough. But this seems to be really hard to code with my amateur vba skills. Is this possible?
Also, the spreadsheet seems to crash when i simulate 10 levels and above
Sub test()
Dim startvalue As Double, levels As Integer, j As Long, i As Long, k As Long
Application.ScreenUpdating = False
startvalue = Sheets("Sheet1").Range("A2")
levels = Sheets("Sheet1").Range("B2")
Sheets("Sheet2").Activate
Cells.ClearContents
Range("A1") = startvalue
For j = 2 To levels
For k = Cells(Rows.Count, j - 1).End(xlUp).Row To 1 Step -1
If Cells(k, j - 1) <> "" Then
Rows(k + 1).Insert shift:=xlDown
Cells(k + 1, j) = Cells(k, j - 1).Value * 0.99
Cells(k, j) = Cells(k, j - 1).Value
Rows(k).Insert shift:=xlDown
Cells(k, j) = Cells(k + 1, j - 1).Value * 1.1
End If
Next k
Next j
End Sub

Answer 1

First and foremost, I'd suggest you store your results on internal memory, then display them on your spreadsheet once it's done. Inserting rows within a loop is a major performance killer.
As for the data structure, you could use a two-dimensional array storing the results of each year
e.g.
year 1 : array(1,1) to array(1,3)
year 2 : array(2,1) to array(2,9)
year 3 : array(3,1) to array(3,27)
...
Once this structure is valued, use a 2nd loop to display it on the spreadsheet "plainly", without the pain of inserting those lines everywhere
It goes like that
Dim values() As Long
Sub main()
Dim startvalue As Double, levels As Integer
Dim i, j, k As Long
startvalue = 2000
levels = 3
ReDim values(levels, 3 ^ levels)
' == Calculate pop evolution for every year ==
values(1, 1) = startvalue
For i = 2 To levels
k = 1
For j = 1 To 3 ^ (levels - 2)
values(i, k) = values(i - 1, j)
values(i, k + 1) = values(i - 1, j) * 0.99
values(i, k + 2) = values(i - 1, j) * 1.01
k = k + 3
Next j
Next i
' == Display in on spreadsheet ==
Sheets("Sheet2").Activate
For i = 1 To levels
Cells(4, i) = i
k = 1
For j = 1 To 3 ^ (i - 1)
Cells(4 + k, i) = values(i, k)
k = k + 1
Next j
Next i
End Sub