Wroking with files ... fast

[plenderj]

Hi.

I need to swap every two characters in a file.
Eg :
abcdefgh -> badcfehg

The code I'm using at the moment is :

Option Explicit

Private var_line As String

Private Sub Command1_Click()
    If (Text1.Text <> "") And (Text2.Text <> "") Then
        Label1.Caption = "Working"
        Open Text1.Text For Binary Access Read As #1
            Open Text2.Text For Binary Access Write As #2
                Do While Loc(1) < LOF(1)
                    var_line = Input(2, #1)
                    DoEvents
                    var_line = Right(var_line, 1) & Left(var_line, 1)
                    Put #2, , var_line
                Loop
            Close #2
        Close #1
        Label1.Caption = "Finished"
    End If
End Sub

The code must work in such a way that if it is run again on the file, one gets the original file back.
Eg : f(f(x)) = x

Anyone have a better approach ?

- jamie

Read the entire file
Swap everything
Write the entire file

That should be a LOT faster.

Gerco.

[plenderj]

I was doing that originally, but the files are between 8 and 30 megs. So VB just runs out of memory.

- jamie

How about chunks? Read 1000 lines, swap, write in new file, repeat until done, delete old file, rename new file.

Gerco.

[plenderj]

Well, they are binary files, and each line can contain any number of characters.
So I need to open and write in binary mode to make sure that VB doesnt **** up the characters.

So I cant read in a line.
I can read in a certain number of characters (as below).

- jamie

Then read 32k or 1024k of the file and do as above..

Be careful by the way... seek will happily report a current position way beyond the end of file, forcing this:

Private Sub ReadBytes()
   Get #1,buffer
   If Seek(1)>=Lof(1) then 
      bytesread=lof(1)-previousseek
  else
      bytesread=currentseek-previousseek
  endif
End Sub

But hey, who cares about that little inconvenience.

Gerco.