Change a loop to run more efficiently - excel

Question

I have a pretty basic loop that I am using to run some random scenarios. On one of my worksheets I am using the =Rand() function to generate random numbers/scenarios for my workbook. What I am trying to program from there is a macro that refreshes the workbook (and the random set of numbers) and then deposits my results onto my worksheet each time the scenario is run. Ultimately, I'd like to be able to run/generate 100,000 random scenarios and deposit the results. Here is what I've coded so far:
Sub Run()
Application.Calculation = xlCalculationManual
Application.ScreenUpdating = False
Application.DisplayAlerts = False
Dim wksInput As Worksheet, i As Integer
Set wksInput = Sheets("Input")
For i = 2 To 102
Application.Calculate
With wksInput
.Range("P" & i).Value = .Range("J35").Value
.Range("Q" & i).Value = .Range("K35").Value
.Range("R" & i).Value = .Range("L35").Value
.Range("S" & i).Value = .Range("G32").Value
End With
Next i
Application.ScreenUpdating = True
Application.DisplayAlerts = True
End Sub
What I have here works just fine, except that it takes 23 seconds to run just 100 scenarios. Based on my calculation that would require over 6 hours of run time to get my 100,000 random scenarios.
My question is does anyone know of a clever way to either change the code to run more efficiently or optimize what I already have. I done all the basic things like turning calculation to manual and turning off screenupdating.
Thanks for your ideas.

Answer 1

The problem is this line:
Application.Calculate
As it's placed inside the For i = 2 To 102, it means that for every row you add all the =rand() functions of the spreadsheet are recalculated.
May I suggest you to generate the random numbers within the code, using the Rnd() built-in function of VBA. Like that, you will generate only the random input you need avoiding to generate the N-1 other inputs that you will regenerate anyway at the Next loop when calling Application.Calculate once again.
what might that look like?
This is the logic (I cannot tell you specifically because you didn't show your whole code/spreadsheet nor the logic behind this random generation): let's say that you have three random numbers in the cells A1, A2 and A3. They are all calculated with a function =Rand() inside the cell.
Now, with your code, you want that in B1, B2 and B3 there is the sum of the random number in A + 1.
the Excel solution (i.e. Excel calculates the random inputs with the function =Rand()):
For j = 1 To 3
Application.Calculate '<-- this re-calculates 3 =rand(), but you need only the one you're going to use right after (==j)
Range("B" & j) = Range("A" & j) + 1
Next j
the VBA solution (i.e. the random numbers are not in the Excel spreadsheet but calculated through VBA):
Randomize
For j = 1 To 3
Range("A" & j) = Rnd() '<-- you insert the random value in A1, A2...
Range("B" & j) = Range("A" & j) + 1 '<-- you use it
Next j
The Excel solution calculates 3 random functions for 3 times, i.e. 9 iterations. The VBA solution calculates 1 random function for 3 times, i.e. 3 iterations.
I let you imagine the multiplication for 100,000 scenarios with 100 data each.