Question

Given I have the following

<Sheet 1>
Item    QTY
A        5
B        1
C        3


<Sheet 2>
Item    QTY
A        15
B        4
C        1
D        8

What is the best way to generate a report showing the difference between sheet 1 and 2?

Like

<Difference>
Item    QTY
A        10
B        3
C       -2
D        8

Answer 1

In Excel VBA, use a Dictionary. Use your items from one of the sheets as keys, QTY as values. Put the item/QTY pairs of sheet 1 into the dictionary, then run through the items of sheet 2 update the dictionary accordingly to get the differences in there. Finally, put the result into sheet 3.

EDIT: here is a complete example in code (you have to set a reference to the Microsoft Scripting runtime to get it working this way):

Option Explicit
Sub CreateDiff()

    Dim dict As New Dictionary
    Dim sh1 As Worksheet, sh2 As Worksheet, sh3 As Worksheet
    Dim i As Long, v As String

    Set sh1 = ThisWorkbook.Sheets("Sheet1")
    Set sh2 = ThisWorkbook.Sheets("Sheet2")
    Set sh3 = ThisWorkbook.Sheets("Sheet3")
    For i = 2 To sh1.Cells.SpecialCells(xlCellTypeLastCell).Row
        v = Trim(sh1.Cells(i, 1).Value)
        dict(v) = -sh1.Cells(i, 2).Value
    Next
    For i = 2 To sh2.Cells.SpecialCells(xlCellTypeLastCell).Row
        v = Trim(sh2.Cells(i, 1).Value)
        If dict.Exists(v) Then
            dict(v) = dict(v) + sh2.Cells(i, 2).Value
        Else
            dict(v) = sh2.Cells(i, 2).Value
        End If
    Next
    For i = 0 To dict.Count - 1
        v = dict.Keys(i)
        sh3.Cells(i + 2, 1) = v
        sh3.Cells(i + 2, 2) = dict(v)
    Next

End Sub