Automatically run VBA

I have the following VBA code:

Sub copycells()
ActiveWorkbook.Sheets("1-a").Select
ActiveSheet.Range("a1:c20000").Select
With Selection
.Copy
End With
ActiveWorkbook.Sheets("2-a").Select
ActiveSheet.Range("a1").Select
With Selection
.PasteSpecial (xlValues)
.PasteSpecial (xlFormats)
End With
End Sub

I want Sheet2-A to update in real time. That is, whenever I add something to 1-A, I want it to automatically execute without me having to click "Run."

Is this possible?

Go to VBA Editor. Press ctrl+R to make the project explorer visible. In the project explorer, find the VBA Project(your workbook). There, double click the entry with "1-A". It will look like Sheet1 (1-A).

In the editor, paste the code :

Private Sub Worksheet_Change(ByVal Target As Range)
     copycells
End Sub

HTH,
Vikas

I insterted what you wrote in Sheet1

However, I'm getting the following error:

Compile Error:
Sub or Function not defined.

Thoughts?

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Why do you need to duplicate the data?

Use the worksheet_change event

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Count > 1 Or Target.Column > 3 Then Exit Sub
    Sheets("2-a").Cells(Target.Row, Target.Column).Value = Target.Value
End Sub

Copy the code
Select the worksheet in which you want the code to run
Right click on the sheet tab and choose View Code, to open the Visual Basic Editor.
Where the cursor is flashing, choose Edit | Paste