Read text file backwards

[Ronzan]

Hello,

 

I'm 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 there's 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 :)

 

I've 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 ??!

 

I'm puzzled :confused:

 

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:confused:

[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. I'll try it.

 

Thanks :D

Peipei