For Loop VBA Speed up by putting into an array - vba

Question

I am trying to go through about 2000 lines worth of data and do a for loop and several if statements within each loop. It works, but right now it is painfully slow. I understand from my research that if I can put the data into an array and manipulate it there and then put the data back into the cells would be much faster but I could use some help on the coding to do so. Here is my code.
Sub EliminateVariance()
Dim Old As Long
Dim Older As Long
Dim Oldest As Long
Dim Current As Long
Dim VarianceOld As Long
Dim VarianceNew As Long
Dim VarianceNew1 As Long
Dim VarianceNew2 As Long
Dim Month1 As Variant
Dim SheetName As Variant
Dim LastRow As Long
Dim i As Long
Month1 = InputBox("What is this month?")
SheetName = Month1 & " SummaryByCust"
Worksheets(SheetName).Activate
LastRow = Cells(Rows.count, "B").End(xlUp).row
For i = 3 To LastRow
VarianceOld = Range("V" & i)
Oldest = Range("I" & i)
Older = Range("H" & i)
Old = Range("G" & i)
Current = Range("F" & i)
If VarianceOld > Oldest Then
VarianceNew = VarianceOld - Oldest
Range("I" & i) = 0
If VarianceNew > Older Then
VarianceNew1 = VarianceNew - Older
Range("H" & i) = 0
If VarianceNew1 > Old Then
VarianceNew2 = VarianceNew1 - Old
Range("G" & i) = 0
If VarianceNew2 > Current Then
MsgBox ("Error: Deferred is greater than what it should be. Verify your numbers")
Else
Range("F" & i) = Current - VarianceNew2
End If
Else
Range("G" & i) = Old - VarianceNew1
End If
Else
Range("H" & i) = Older - VarianceNew
End If
Else
Range("I" & i) = Oldest - VarianceOld
End If
Next i
End Sub

Answer 1

Here is an example on how to use Arrays:
Sub arrayEx()
'Set the range
Dim rng As Range
Set rng = Worksheets("Sheet1").Range("A1:B20000")
'Bulk load the values from the range into an array
'Even if a single column this will create a 2D array
Dim rngArr As Variant
rngArr = rng.Value
'Loop the "Rows" of the array
Dim i As Long
For i = LBound(rngArr, 1) To UBound(rngArr, 1)
'Do something with that array
'when loaded from a range it is similar nomenclature to Cells: array(row,column)
If rngArr(i, 1) = "A" Then
rngArr(i, 2) = "B"
End If
Next i
'overwrite the values in range with the new values from the array.
rng.Value = rngArr
End Sub
Try to adapt to fit your needs.