Read text file backwards

[Ronzan]

Hello,

Im trying to find out how to read a text file backwards.

I want like a ReadLastLine() method of some kind, and then be able to step though the lines backwards.

I have some very large logfiles(2GB+) that I need to parse over time when new lines are added to those files.

Anyone got any ideas ?

Thanks

 

[Nerseus]

This is off the top of my head, but I would guess theres nothing to read a whole line from the end of file. You might be able to use a Seek type of method and manually find the EOL character to split the lines manually.

Sorry, not much help on this one.

-ner

 

[Ronzan]

Thanks for replying anyways Nerseus

Ive used the seek method to put the pointer at EOF and then step back 2 bytes with offset -2, that gets me to the last character of the last line with text(the very last is a crlf).

Then i tried something like this:

dim buffer(1) as char
dim temp as string

do while not buffer(0) = vbLf
oStream.Read(buffer,0,1)
temp = temp & buffer(o).ToString
oStream.BaseStream.Position = oStream.BaseStream.Position - 2
loop

but this gives me some very strange results, almost like the cursor random "jumps" to different positions ??!

Im puzzled

Any ideas will greatly appreciated, thanks

 

[peipei]

Have you found the solution?

Hi Ronzan,

I am also trying to read text file backward...
I need to read a log file backwards, using activeX script in DTS.
But still unable to find the solution.

Have you solve this problem?

Anyone have any idea?

Regards,
Peipei

 

[Ronzan]

Yes I found a way to do it.
Sorry for not posting it before..

    Private Function GetNextLineReversed(ByVal lngStartPos As Long) As String
        ************************************************************************
        ** GET NEXT LINE REVERSED                                             **
        ** ------------------------------------------------------------------ **
        ** Returns a text line backwards from a file, starting at lngStartPos **
        ** Returns "eof" if no lines found                                    **
        ************************************************************************
        Try
            Dim myReader As New System.IO.StreamReader(myFileStream, System.Text.Encoding.Default)
            Dim temp As String = ""
            Dim sTempLine As String = ""
            Dim TempBlock(0) As Char
            Dim cBuffer(0) As Byte
            Dim lngLastLineStartPos As Long
            Dim lngLastLineEndPos As Long
            Goto lngStartPos, step back to previous line
            Save     for end of line
            lngLastLineEndPos = lngStartPos - 2

            If lngLastLineEndPos < 1 Then no can do, this would get us to     -1 in next statement
                Return "eof"
            End If

            Loop to find first vbLF reversed
            myFileStream.Position = lngLastLineEndPos - 1
            myFileStream.Read(cBuffer, 0, 1)
            Do While Not ChrW(cBuffer(0)) = vbLf And myFileStream.Position > 1
                temp = temp & ChrW(cBuffer(0))
                myFileStream.Position = myFileStream.Position - 2
                myFileStream.Read(cBuffer, 0, 1)
            Loop
            Save     for start of line
            lngLastLineStartPos = myFileStream.Position

            Clear variables
            TempBlock = ""
            sTempLine = ""

            Get the line from lngLastLineEndPos to lngLastLineStartPos
            ReDim TempBlock(lngLastLineEndPos - lngLastLineStartPos)
            myFileStream.Position = lngLastLineStartPos
            myReader.ReadBlock(TempBlock, 0, lngLastLineEndPos - lngLastLineStartPos)
            sTempLine = TempBlock

            here you can save the start position of the last line parsed
save it in a global variable to use it as a new start posistion when calling the GetNextLineReversed() function again
            g_lngLastLineStartPosition = lngLastLineStartPos

            return the line
            Return sTempLine

        Catch
            insert error handling here
        End Try

End Function


To use the function:
----------------------------------------------------
Dim strFileName as string
Dim myFileStream as FileStream
Dim lngStartPos as long

file name and path to parse
strFileName = "c:\test.txt"

Create a global filestream object
myFileStream = New System.IO.FileStream(strFileName,System.IO.FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.ReadWrite)

go to end of file and save position (stream.length can also be used to find the last byte    )
myFileStream.Seek(0, SeekOrigin.End)
lngStartPos = myFileStream.Position

sLine = GetNextLineReversed(lngStartPos)

----------------------------------------------------

Im sure there is a better/easier way to do this, but the was what I could come up with for the time being due to a deadline for the project where I use this function

Let me know if you have any questions regarding the function.

 

[peipei]

Hi Ronzan,

Really thanks alot for helping and your fast reply. Ill try it.

Thanks
Peipei