Highlight Overlap Days Excel VBA

Question

I have the following problem to solve to increase the speed at which the code performs the task.

I have a table with names of Hire Cars and two dates - From and To. I need to go through the range (say 10k rows) check and highlight all overlapping dates.

No Hire Car From To

1 ABC 01 Jan 12 12 Jan 12
2 ABC 14 Jan 12 15 Jan 12
3 ABC 25 Jan 12 02 Feb 12
4 DEF 01 Jan 12 12 Jan 12
5 DEF 12 Jan 12 02 Feb 12
6 DEF 14 Jan 12 15 Jan 12

For hire car DEF there are overlapping days, double counting in fact which i need to be able to highlight so that the user can quickly identify and correct.

This is the code that I have developed. The problem is that if you have a Range of 10k Rows it is extremely slow.

I am using Windows 7 Ultimate with Office/Excel 2010

    Function CheckOverlap(StartLine, EndLine, StartColumn)

Dim i As Integer, y As Integer
Dim DateToCompare
Dim HireCar
Dim Count As Integer
Dim Msg, Style, Title, Response

'Check StartDate Column
For i = StartLine To EndLine

    DateToCompare = Cells(i, StartColumn)
    HireCar = Cells(i, 2)
    For y = StartLine To EndLine
        'If we are at the same line with DateToCompare cell then we should not perform any check
        If i <> y Then    
            If DateToCompare >= Cells(y, StartColumn) And DateToCompare <= Cells(y, StartColumn + 1) And HireCar = Cells(y, 2) Then
                'We should highlight both cells that contain overlapping dates
                ActiveSheet.Cells(i, StartColumn).Interior.Color = 5296274
                ActiveSheet.Cells(y, StartColumn).Interior.Color = 5296274
            End If
        End If
    Next y
Next i

HireCar = 0

'Check EndDate Column
For i = StartLine To EndLine

    DateToCompare = Cells(i, StartColumn + 1)
    HireCar = Cells(i, StartColumn - 1)
    For y = StartLine To EndLine
        'If we are at the same line with DateToCompare cell then we should not perform any check
        If i <> y Then    
            If DateToCompare >= Cells(y, StartColumn) And DateToCompare <= Cells(y, StartColumn + 1) And HireCar = Cells(y, StartColumn - 1) Then
                'We should highlight both cells that contain overlapping dates
                ActiveSheet.Cells(i, StartColumn + 1).Interior.Color = 5296274
                ActiveSheet.Cells(y, StartColumn + 1).Interior.Color = 5296274
            End If
        End If
    Next y
Next i


'Last check: If the starting and ending date are the same
For i = StartLine To EndLine
    If Cells(i, StartColumn) - Cells(i, StartColumn + 1) = 0 Then
        ActiveSheet.Cells(i, StartColumn).Interior.Color = 5296274
        ActiveSheet.Cells(i, StartColumn + 1).Interior.Color = 5296274
    End If
Next i

' If there are no Overlap Days in Database skip filtering
' StartDate and EndDate Column
' Count Cells with Interior.Color = 5296274 (Green Colour)
Count = 0

For i = StartLine To EndLine
    If Cells(i, StartColumn).Interior.Color = 5296274 Then
        Count = Count + 1
    End If
Next i

' Msg if Database has no Overlap Days
Msg = "Validation check completed. There are 'NO' Overlap Days"
Style = vbOKOnly
Title = "Cash Flow"

' Require on Error Resume Next in case Database is NOT filtered
On Error Resume Next
If Count = 0 Then
    ActiveSheet.ShowAllData
    Response = MsgBox(Msg, Style, Title)
    Exit Function
Else
    Call Filter_Colour
End If

MsgBox "Any Green highlights indicate Overlap Days"

End Function

Answer 1

The fastest approach would be to sort the table (first order: cars, second order: from-date)

Then for each line: there is a collision iif the line above is the same car and the to-date from above is larger than the from-date of the current line.

You can do these steps either with VBA or Excel-Formulas.

Answer 2

Here is a simple algo to show you a blank when there's an overlap on the latter rows. To run this, it's strictly assumed that your CAR column is sorted as per sample shown in the question.

Option Explicit

'-- assuming the CAR names column is sorted
'-- so each car block in one place
'-- run on button click event

Sub FindOverlaps()
Dim i As Integer, j As Integer
Dim vInput As Variant
Dim rng As Range

Set rng = Sheets(2).Range("B2:E7")
vInput = WorksheetFunction.Transpose(WorksheetFunction.Transpose(rng))

For i = LBound(vInput) To UBound(vInput) - 1
    For j = LBound(vInput) + 1 To UBound(vInput)
        If vInput(i, 2) = vInput(j, 2) Then
            If vInput(i, 4) = vInput(j, 3) Then
                vInput(j, 3) = ""
                vInput(j, 4) = ""
            End If
        End If
    Next j
Next i

rng.Offset(0, 6).Resize(UBound(vInput), UBound(Application.Transpose(vInput))) = vInput

End Sub

Output:

---

EDIT AS PER OP'S COMMENT

1. Transpose the sorted data into the same range as per input data, so remove offset(0,4) :
2. Add conditiona formatting to highlight anyrow that's null within the specified range. (otherwise entire sheet will be coloured where empty cells are)

Code changes:

rng.Offset(0, 6).FormatConditions.Delete
rng.Offset(0, 6).FormatConditions.Add Type:=xlCellValue, Operator:=xlEqual, _
        Formula1:="="""""
rng.Offset(0, 6).FormatConditions(1).Interior.ColorIndex = 20