Delete lines starting from bottom - vbscript

Question

I got this code which deletes 10 lines starting from the top.
Is it possible to do the same but starting the delete from the bottom to the top of the txt file?
So if I have 30 lines, I want the last 10 or 20 lines to be deleted.
Const FOR_READING = 1
Const FOR_WRITING = 2
strFileName = "C:\scripts\delete.txt"
iNumberOfLinesToDelete = 10
Set objFS = CreateObject("Scripting.FileSystemObject")
Set objTS = objFS.OpenTextFile(strFileName, FOR_READING)
strContents = objTS.ReadAll
objTS.Close
arrLines = Split(strContents, vbNewLine)
Set objTS = objFS.OpenTextFile(strFileName, FOR_WRITING)
For i=0 To UBound(arrLines)
If i > (iNumberOfLinesToDelete - 1) Then
objTS.WriteLine arrLines(i)
End If
Next

Answer 1

If you read the entire file into an array of lines you'd use more or less the same approach for removing lines from beginning or end.
To remove lines from the beginning you start at an offset after the lines that you want removed:
filename = "C:\path\to\your.txt"
numLinesToRemove = 10
Set fso = CreateObject("Scripting.FileSystemObject")
txt = Split(fso.OpenTextFile(filename).ReadAll, vbNewLine)
Set f = fso.OpenTextFile(filename, 2)
For i = numLinesToRemove To UBound(txt)
f.WriteLine txt(i)
Next
f.Close
To remove lines from the end of the file you stop before the lines that you want removed:
filename = "C:\path\to\your.txt"
numLinesToRemove = 10
Set fso = CreateObject("Scripting.FileSystemObject")
txt = Split(fso.OpenTextFile(filename).ReadAll, vbNewLine)
Set f = fso.OpenTextFile(filename, 2)
For i = 0 To UBound(txt) - numLinesToRemove
f.WriteLine txt(i)
Next
f.Close
This approach only works for small files, though. If you need to process large files you usually can't read the entire file into memory. If you did your computer would start swapping data from memory to disk, causing the system to slow down to a crawl. To avoid this you normally read the file line by line in a loop and write to a temporary file, then replace the original file with the temp file after processing is complete.
Removing lines from the beginning of a file is still fairly trivial, because TextStream objects have a Line property that holds the current line number (i.e. the number of the line that the next ReadLine call would read).
Set f = fso.OpenTextFile(filename)
Set tmp = fso.OpenTextFile(filename & ".tmp", 2, True)
Do Until f.AtEndOfStream
If f.Line <= numLinesToRemove Then
f.SkipLine
Else
tmp.WriteLine f.ReadLine
End If
Loop
f.Close
tmp.Close
However, you can't do that for removing lines from the end of the file, because you don't know the number of lines beforhand. One way to deal with this is to create a ring buffer the size of the number of lines you want to remove, fill it as you read lines from the input file, and write lines to the output file when they are removed from the buffer. That way the last numLinesToRemove lines are still in the buffer (not written to the output file) when the loop terminates.
ReDim buf(numLinesToRemove) 'ring buffer
i = -1 'ring buffer pointer
Set f = fso.OpenTextFile(filename)
Set tmp = fso.OpenTextFile(filename & ".tmp", 2, True)
Do Until f.AtEndOfStream
i = (i + 1) Mod numLinesToRemove 'advance ring buffer pointer
'if current buffer slot is filled write it to the output file ...
If Not IsEmpty(buf(i)) Then tmp.WriteLine buf(i)
'... then put current line from input file into current buffer slot
buf(i) = f.ReadLine
Next
f.Close
tmp.Close
In both cases you'd replace the original file after processing is complete, e.g. like this:
fso.DeleteFile filename
fso.MoveFile filename & ".tmp", filename

Answer 2

just loop backwards in your for statement
For i=UBound(arrLines) To (UBound(arrLines) -10) step -1
Next