Changing a value of a cell based on another cell - excel

Question

What I want to do is if column O contains "weekend" then change the value of column M cells to "3".
Sub weekly_weekend()
lastrow = Sheet1.Cells(Rows.Count, 1).End(xlUp).Row
Application.ScreenUpdating = False
For x = 2 To lastrow
If InStr(1, Sheet1.Range("O" & x).Value, UCase("weekend"), 1) > 0 Then
Sheet1.Range("M" & x).Value = "3"
Next x
Application.ScreenUpdating = True
End Sub

Answer 1

The problem with your code is that you're getting the last row of the column A, and this will prevent the For to be executed. To fix your code, you can proceed in multiple ways.
Using Range
One is to use the Range property, so you can explicitly write your column name, like this:
Sub weekly_weekend()
lastrow = Sheet1.Range("O" & Sheet1.Rows.Count).End(xlUp).Row
Application.ScreenUpdating = False
For x = 2 To lastrow
If InStr(1, Sheet1.Range("O" & x).Value, UCase("weekend"), 1) > 0 Then Sheet1.Range("M" & x).Value = "3"
Next x
Application.ScreenUpdating = True
End Sub
Picking up the right column
Or you can simply pick the right number of the column you want (in this case column O is 15), like this:
Sub weekly_weekend()
lastrow = Sheet1.Cells(Sheet1.Rows.Count, 15).End(xlUp).Row
Application.ScreenUpdating = False
For x = 2 To lastrow
If InStr(1, Sheet1.Range("O" & x).Value, UCase("weekend"), 1) > 0 Then Sheet1.Range("M" & x).Value = "3"
Next x
Application.ScreenUpdating = True
End Sub
Note: Please note that if you add or remove columns, with the second method you'll need to remember to change the column index in your code accordingly.
Hope this helps.