manipulating binary data

[geezer34]

I'm a novice VB programmer and am having some issues with binary files (which I've never touched before) and I was hoping you may be able to help with...

I've read a binary file into a byte array and now want to pull out data from this.

I know the following about my format:

- byte position 0 is an ascii Char
- byte position 1,2,3 refers to an unsigned 24 bit (I want to put this in a long)
- bytes poistion 4,5 refers to a signed 16 bit numner ( I want to put this in an integer)

- byte 6 is an ascii char
etc etc

I believe data was written to the file using little endian if that helps.

I obvisouly need to loop through my array but how do I convert the data at the byte position into longs / integers??

Any code to illistrate would be great.

Thanks for you help

 

[JeffTullin]

This is top-of-the-head, as I am away from help files and Vb at the moment..

In general terms, the last time I tried this, I had to resort to using custom Types

I had one type like this:

Public Type MyLong
v as long
end type

and another like this:

Public Type My4Bytes
b1 as byte
b2 as byte
b3 as byte
b4 as byte
end type


declaring a variable of type My4Bytes, I set the values of b1,b2, b3 etc.

The using LSET (or RSET, can't recall off hand), I forced the value in that into a variable of type MyLong

dim v1 as MyLong
dim v2 as My4Bytes

v2.b2 = 76
LSET v1 = v2

debug.print v1


You can do it mathematically, by mutliplying the values by 1, 256, 256^2 etc, but that gets messy to read very quickly...

 

[geezer34]

Ok thanks for that.

Query re UDTs as I've got an error come up saying that LSET only allowed on strings and user defined data types...

Got the following code:

Private Type Test
a As String
b As Long
End Type

Dim record() As Test

Private Type My4Bytes
b1 As Byte
b2 As Byte
b3 As Byte
b4 As Byte
End Type

Dim v2 As My4Bytes

'after I have readin in my binary file I do

v2.b2 = myarray(counter + 1)
v2.b3 = myarray(counter + 2)
v2.b4 = myarray(counter + 3)
LSet record(index).b = v2 ** error here **

where index is the array index of the record I'm populating, and counter is the index where the binary data is stored in my data that I read in.

Any ideas what I've messed up here?

 

[geezer34]

OK got something going here. Basicaly I created a load of UDTs and then used these within another UDT!!

Seems to work OK.

Thanks for your pointer

 

[JeffTullin]

Glad to see you have it moving.

The line
LSet record(index).b = v2 ** error here **

is failing because the 'record(index).b' is not a UDT
Both sides of the equation have to be, but you have worked that out for yourself, I guess..