Reporting services difference between rows

[FishGuy]

I have a datatable in reporting services report with a single column which holds integer values.

I want to work out the integer difference + or - between the row and the previous row and display that value in a second column.

i.e
Col1 Col2
1 0
3 2
5 2
1 -4
8 7

I think it may be possible usine the code behind the report using a function but am unsure how to make it work.

I have made a start but am struggling with what im actually trying to do in code.

Any help please.

Function CalcChangeInAvailable(ByVal RecsTot As integer, ByVal RecsAvail As integer, ByVal rowNum as Integer) As Decimal

if rowNum <=0 then 
CalcChangeInAvailable = 0
else


redim RecordsAvailable(rowNum)

RecordsAvailable(rowNum) = RecsAvail
 
Recs = RecsAvail(rowNum-1) - rowNum
CalcChangeInAvailable = Recs
end if


End Function

[FishGuy]

Dim RecordsAvailable() As integer
Dim Recs As integer = 0
Dim LastRecs As integer = 0
Dim NowRecs As integer = 0
Dim CurrentRow As integer = 0
Dim PreviousRow As integer = 0
Function CalcChangeInAvailable(ByVal RecsAvail As integer, ByVal rowNum as Integer) As integer
CurrentRow = rowNum
PreviousRow = CurrentRow -1
if CurrentRow <=0 then 
CalcChangeInAvailable = 0
else


redim RecordsAvailable(CurrentRow)

RecordsAvailable(CurrentRow) = RecsAvail

LastRecs = RecordsAvailable(PreviousRow) 
NowRecs = RecordsAvailable(CurrentRow) 
Recs = LastRecs - NowRecs
CalcChangeInAvailable = LastRecs
end if


End Function

I think im nearly there but the LastRecs value always seems to be 0