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Languages :: Pascal :: Restrictions on "file of byte" type
By: VB guy Canada  Date: 06/02/2003 00:00:00  English  Points: 50 Status: Answered
Quality : Excellent
Hi,

I'm trying to write a sequence of 256 bytes, from 0 to 255, in any order, in a file of byte. Instead, after close the file I just got 64 bytes, but I was expecting 256. I've tried use Write, Blockwrite etc and the problem ever occurs. Can you help me?

Thanks.

VBguy
By: sumotimor Date: 06/02/2003 16:53:00 English  Type : Comment
Think above the file of byte like a PILE...
you must create the file and write a 256 "ceros" of byte...
and after that every byte can be access by position...
because the file is now 256 bytes on length...

Procedure Create25file;
Var I : Integer;
F : File of byte;
Begin
Reset(F,''); {<--- I forgot the command for create }
For A=1 to 256
Begin
Write(F,0);
End;
Close(F)
End;

Function ReadByteFile ( Poz : Integer ) : Byte;
Var Va : Byte;
Begin
Open(F,'');
Seek(F,Poz);
Read(F,Va);
Close(F);
ReadByteFile := Va;
End;

---------- This can helkp you
By: monange Date: 06/02/2003 17:02:00 English  Type : Comment
Post your code so we can look at it.
By: VGR Date: 07/02/2003 00:22:00 English  Type : Answer
I suspect that you read back your file of byte using an integer (or an other 4 bytes-format) because of the 64->256 discrepancy.

use the above comment from leirag, put
AssignFile(F,'yourfilenamehere');
ReWrite(F);

in stead of reset(); // forgot...

put := and "do" in "For a:=1 to 256 Do"

oh ok, better rewrite it myself :

Procedure Create25file;
Var I : Integer;
F : File of byte;
A : Byte;
Begin
AssignFile(F,'hereyourfilename');
ReWrite(F);
For A:=0 to 255 Do Write(F,0);
CloseFile(F);
End;

// use unsigned integers for type of "Poz" here. Byte would be enough, Word is better, you've LongWord too ;-)

Function ReadByteFile ( fn : String; Poz : Word ) : Byte;
Var Va : Byte;
Begin
AssignFile(F,fn);
{$I-} // or the reverse ;-)
ReSet(F);
{$I+}
if IOResult<>0 Then Begin Va:=0; WriteLn('invalid filename '+fn); End
Else Begin
Seek(F,Poz);
Read(F,Va);
Close(F);
End; // if, then, else
ReadByteFile := Va;
End;
By: VB guy Date: 07/02/2003 16:23:00 English  Type : Comment
Thank you all for the help.

But I want to do more explanations about the doubt.

I want to do the following:

var c1 : byte;
df : file of byte;
begin
assign(df,'Data.bin');

rewrite(df);

for c1:=0 to 255 do
write(df, c1);

closefile(df);
end;

Note that I want always write all 256 possible values of one byte.

And more. The simplification above don´t show, but the order of the 256 possible values of one byte may be random.

Thanks again.
By: sumotimor Date: 08/02/2003 18:04:00 English  Type : Comment
Yes after you have run the Create25File

you can read one byte from the file with function
ReadByteFile

And you can have a write changed a lien with Read--- for write... and add on argumments a byte to write and position

Ok... Thanks...VGR...

Tanks VGR again----->
By: VGR Date: 08/02/2003 18:22:00 English  Type : Comment
it worked ? :D
By: VB guy Date: 10/02/2003 11:42:00 English  Type : Comment
Yes, it works.
And all the others answers did that.
Thanks you all.

Regards.

VBguy

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