manipulating bits

[fenris]

I have a custom file format that I need to be able to read and write single bits from. I know the positions of these bits and can read them by:

dim fileName as string 'path to the file
dim fil as integer 'holds the first available file number
dim x as byte

fil = FreeFile

open fileName for Binary as #fil
get #fil, 55,x 'the first bit is located in byte 55 and is the third bit
close fil

'after retrieving the byte, I have a conversion routine that converts the byte to a bit string. The function ByteToBit converts a byte a bit string.

'------------------------
Private Function ByteToBit(ByteArray) As String
Dim i As Integer

ByteToBit = ""
For i = 7 To 0 Step -1
If Int(ByteArray / (2 ^ i)) = 1 Then
ByteToBit = ByteToBit & "1"
ByteArray = ByteArray - (2 ^ i)
Else
If ByteToBit <> &quot;&quot; Then
ByteToBit = ByteToBit & &quot;0&quot;
End If
End If
Next i
End Function
'------------------------

AFter converting the Byte to a bit string I can use the Mid$ function to pull out the specific bit that I need.

First Question, is there an easier way to read a specific bit in a file?

How do I write the specific bit back to the file? For example, if I flip the bit how do I write the changes back?
Are there intrinsic functions in VB that would assist with this or am I reading the bits the best way?



Troy Williams B.Eng.
fenris@hotmail.com

 

[MichaelRed]

I'm not sure they are BETTER than what you have 'grown', but bitwise operators exist. To select specific bits, you need to develop the &quot;Masks&quot; for the specific bits. These are 'generally' just a set of hex value constants. To Set / Reset (&quot;Flip&quot bits, just XOR the 'variable' with the mask for the bit. In binary files, I believe the Put operation is the 'compliment' of the get operation you are already using, so it is available for the 'replacement' of the value.

MichaelRed
mred@att.net

There is never time to do it right but there is always time to do it over

 

[fenris]

Michael, thanks for the response. It's given me some new directions to look in.
Troy Williams B.Eng.
fenris@hotmail.com