Sunday, 17 May 2015

How to Optimize the VBA Code to Improve the Speed of the Macros

Techniques to Optimize the VBA Code to Improve the Speed of the Macros
Analyze the Code logic:
Before optimizing the syntax, pay more attention in optimizing the logic. Without a good logic, a good written VBA macro program has no value. So streamline your program logic and get the best performance of macros.

Avoid 'Screen Flickering' or 'Screen Repainting':
Application.ScreenUpdating = False 'To Turn Off at the start of code.
Application.ScreenUpdating = False 'To Turn on at the end of the code.

The ScreenUpdating property controls most display changes on the monitor while a procedure is running. When screen updating is turned off, toolbars remain visible and Word still allows the procedure to display or retrieve information using status bar prompts, input boxes, dialog boxes, and message boxes.

You can increase the speed of some procedures by keeping screen updating turned off.
You must set the ScreenUpdating property to True when the procedure finishes or when it stops after an error.

Turn off automatic calculations:
Whenever content(s) of a cell or range of cells are changed, the formulas dependent on them and Volatile functions are recalculated. You may turn off the automatic calculation using 

Application.Calculation = xlCalculationManual 'To turn off the automatic calculation
Application.Calculation = xlCalculationAutomatic 'To turn On the automatic calculation

Now, whenever due to the program logic(that due to macros dependent on existing formulas) you need to calculate the formulas, you may use the following code accordingly.

ActiveSheet.Calculate ' To calculate the formulas of Active Worksheet
Application.Calculate ' To calculate the formulas of Active workbook or all workbooks in current application.

Stop Events: 
Use Application.EnableEvents to tell VBA processor whether to fire events or not. We rarely fire an event for each cell we're changing via code. Hence, turning off events will speed up our VBA code performance.

Hide Page Breaks:
When we run a Microsoft VBA macro in a later version of Microsoft Excel, the macro may take longer to complete than it does in earlier versions of Excel. 
For example, a macro that required several seconds to complete in an earlier version of Excel may require several minutes to complete in a later version of Excel. This problem may occur if the following conditions are true:
The VBA macro modifies the properties of many rows or columns.
An operation has been performed that forced Excel to calculate page breaks. Excel calculates page breaks when we perform any of the following operations:
  • We display a print preview of your worksheet.
  • In Microsoft Office Excel 2003 and in earlier versions of Excel, we click Page Setup on the File menu.
  • We modify any of the properties of the PageSetup object in a VBA macro.
In Excel 2003 and in earlier versions of Excel, we selected the Page breaks check box on the View tab of the Options dialog box.

Disable Page breaks using ActiveSheet.DisplayPageBreaks = False

Use 'WITH' statement when working with objects: 

If we have to access an object's properties and methods in several lines, we must avoid using object's name or fully qualified object path again and again. It is annoying for VBA processor as it needs to fully qualify the object each time.


Sheets(1).Range("A1:K1").Font.Italic = True
K1").Font.Interior.Color = vbBlue
K1").MergeCells = True
With Sheets(1).Range("A1:K1")
    .Font.Italic = True
    .Font.Interior.Color = vbBlue
    .MergeCells = True

End With

The point here to understand is minimum qualifying of an object by VBA processor. i.e. using minimum dots/periods(.) in the code. This concept tells us to use [A1] rather than Range("A1") and Range("StockRange")(3,4) rather than Range("StockRange").Cells(3,4)

Use vbNullString instead of ""(2 double quotes) :
vbNullString is slightly faster than "", since vbNullString is not actually a string, but a constant set to 0 bytes, whereas "" is a string consuming at least 4-6 bytes for just existence.

For example: Instead of strVariable = "", use strVariable = vbNullString.

Release memory from object variables: 

Whenever we create an object in VBA, we actually create two things -- an object, and a pointer (called an object reference). We might say, "VB does not use pointers", but it is not true. "VB does not let you manipulate pointers" is more precise. Behind the scenes, VB still makes extensive use of pointers. 

To destroy an object in VB, you set it to Nothing. But wait a minute. If all we ever use are object pointers, how can we set the object itself to Nothing? The answer is: We can't.

When we set a reference to Nothing, something called the garbage collector kicks in. This little piece of software tries to determine whether or not an object should be destroyed. There are many ways to implement a garbage collector, but Visual Basic uses what is called the reference count method.

 Set objXL=Nothing.

When VB interprets the last line(where we generally sets our objects to Nothing), it will remove the existing reference. At this point, if the object has no more references, the garbage collector will destroy the object and de-allocate all its resources. If any other references point to the same object, the object will not be destroyed. 

Reduce the number of Lines: 
Avoid multiple statements especially when they can be clubbed into one line. For example - See these 2 macros

    With Selection
        .WrapText = True
        .ShrinkToFit = False

    End With

    With Selection
        .WrapText = True: .ShrinkToFit = False

    End With

As you can see, you can club multiple statements into one using colon character(:). When you do this with multiple statements, it will decrease the readability but will increase the speed.

Compiler Logic: 

When we save the macro, it is virtually compiled and unlike it's human readable form as we saw in VBE(Visual Basic Editor), keywords(the dark blue words which you cannot use as variable) are saved as three-byte token which are faster to process as machine understand them better and variables, comments and literal strings which are not either keyword or directive are saved "as is". 

However VBA compiler tokenizes the word but it does not compress the lines and each line is maintained as is ending with the 'Carriage Return'. When the VBA macro is executed, VBA processor fetched single line at a time.

The tokens of the fetched line saved by virtual compiler are now interpreted and executed then next line is fetched and so on. When we combine multiple lines using colon into one line, we're reducing the number of fetch cycles the processor must go through.

This change will bring minor difference in time due to faster processors today. Moreover, you cannot have more than 255 characters in a line and you won't be able to debug your code using F8 efficiently. So it's a kind of useless, there is no reason to trade-off with readability with such a minor change in time.

Declare variable as Variable and constant as Constant:
Many of us don't follow it. Like 
      Dim Pi As Double
      Pi = 3.14
instead use
      Const Pi As Double
      Pi = 3.14
Since, its value is never changed so it will be evaluated once during compilation unlike variable which are evaluated many times during the run-time.

Avoid Unnecessary Copy and Paste: 

Follow this table rules:
Avoid this Prefer this:
Application.CutCopyMode = False   
'Clear Clipboard
'Bypass the Clipboard
Sheet1.Range("A1:A300").Copy Destination:= Sheet2.Range("B1")
Sheet2.Range("B1").PasteSpecial xlPasteValues

'Clear Clipboard
'Bypass the Clipboard if only values are required
Sheet2.Range("B1:B300").Value = Sheet1.Range("A1:A300").Value
Sheet2.Range("B1").PasteSpecial xlPasteFormulas

'Clear Clipboard
'Bypass the Clipboard if only formulas are required
Sheet2.Range("B1:B300").Formula = Sheet1.Range("A1:A300").Formula

'Same can be done with FormulaR1C1 and Array Formulas.

Use Worksheet Functions rather developing own logic: 

By using Application.WorkSheetFunction, we tell VBA processor to use native code rather than interpreted code as VBA understands the worksheet functions better than your algorithm. So, for example use :
      tProduct = Application.WorkSheetFunction.Product(Range("C5:C10"))
rather than defining your own logic like this:
      mProduct = 1
      For i = 5 to 20
            tProduct = tProduct * Cells(3,i)


Use 'For Each' rather than 'indexed For'

We can avoid using Indexed For when looping through collections. For example, take the code just before this tip. It can be modified to:
      For Each myCell in Range("C5:C20")
tProduct tProduct * myCell.Value
This is in relation to qualifying object again and again as using "WITH" statements.

Avoid using 'Macro Recorder' style code:Ya, the code will look genius and eventually perform like Genius too ! You'll better catch it with example, so use:
      [A1].Interior.Color = vbBlue
rather than
      Selection.Interior.Color = 
Using too many Select and Selection effects the performance drastically. 

Avoid using Variant and Object in declaration statements: 

Do not use Dim As Variant or Dim mCell As Object. By trying to be specific,we will save a lot of system memory this way, particularly in case of large projects. We may not remember which has been declared variant above and misuse a variable assigning any value to it which will be type-casted without errors. 

A variant's descriptor is 16 bytes long while double is 8 bytes, long is 4 bytes and integer is 2 bytes. Hence use Variant cautiously. As an example, use:
      Dim As Long rather than Dim As Variant
Similarly use:
      Dim tCell As Range 'or
      Dim tSheet As Worksheet
rather than
tSheet As Object 'or
tSheet As Object

Declare OLE objects directly: 

Declaring and defining the OLE objects in declaration statement is called 'Early Binding' but declaring an object and defining it later is called 'Late Binding'. Always prefer 'Early Binding' over 'Late Binding'. Now for example use:
objXL As Excel.Application
rather than
      Dim objXL As Object
      Set objXL = CreateObject("Excel.Application")

Ref :

Thanks .,

          BI-Reporting Analyst

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