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best way to isolate record # in a flat file.

Ok folks. I'm writing a program that reads data from a datafile that is generated within the system. Since it's a standard delphi typed file handle, I am using filepos, filesize, and seek to move to record numbers. However, for some reason, my code keeps seeking me to record #0 or #1 or #2. I can never get to #2 or above. Here's an illustration of what I'm doing:

type datafilerecord = record
deleted : boolean;
first_name: string[30];
last_name: string[30];
end;

In my function to move to the next record, I have this:

procedure TDvdCatForm.NextRecBtnClick(Sender: TObject);
var thefilepos : integer; // for debugging
begin
VerifySaveRec;
TheFilePos := filepos(moviefile); // for debugging
LoadRecord(filepos(moviefile)+1);
end;

The first time I press button named NextRecBtn, I get record #1 (ok), Next time I press the button, I could get #2 (ok) or I could get #1 again (WHAT!?)

Please don't tell me to use BDE or ODBC or anything. I have no experience in them and frankly I don't want to use them.

What am I doing wrong?

Thanks!

kretzschmar

you may seeking somewhere to another pos,
or you close and reopen your file somewhere

kretzschmar

maybe you should use a var for storing your current pos,
instead of seeking relative

rondi

Hi,

Please show the code for VerifySaveRec and LoadRecord

rondi.

wimmeyvaert

Is this flat file a line sequential file.
If so, then you could make a Schema-File (=recordlayout)
and use the file like a DB-Table (with the TTable-Comp).
You can then write Queries, Locate records, Move to records, Append Records, ... (Deletion of records is then not possible).

If you need a demo project, let me know.

Best regards,
The Mayor.

DanEgli

ASKER

Ok. To answer the questions. It is not a line sequential. It is a file of records.

As far as storing current position in a var, thats fine, but I still need an efficent way to seek a particular record and then overwrite it.

Code for VerifySaveRec and LoadRecord:

procedure TDvdCatForm.VerifySaveRec;
begin
if (itemunsaved) then
if (Application.MessageBox('You have not saved this entry. Do you wish to save it now?', 'Exit Confirmation', MB_YESNO+MB_ICONQUESTION) = 6) then
SaveScreenRecord;
end;

procedure TDvdCatForm.savescreenrecord;
begin
movie.deleted := dvdcatform.CheckBox1.Checked;
movie.Movie_ID := dvdcatform.MovieID.text;
movie.Title := dvdcatform.MovieName.Text;
movie.Format := dvdcatform.MovieFormat.ItemIndex;
movie.Genere := dvdcatform.MovieGenere.ItemIndex;
movie.Location := dvdcatform.MovieLocation.text;
if not filepos(moviefile) = filesize(moviefile) then
seek(moviefile, filepos(moviefile));
WRITE(moviefile, movie);
itemunsaved := false;
end;

procedure tDvdCatForm.loadrecord(recno: integer);
begin
if (recno < 0) then
begin
ShowMessage('Cannot Move beyond top of file.');
exit;
end
else if (recno > filesize(moviefile)-1) then
begin
ShowMessage('Cannot move beyond end of file.');
exit;
end;
seek(moviefile, recno);
read(moviefile, movie);
dvdcatform.MovieID.text := movie.Movie_ID;
dvdcatform.MovieName.text := movie.Title;
dvdcatform.MovieFormat.ItemIndex := movie.Format;
dvdcatform.MovieLocation.Text := movie.location;
dvdcatform.MovieGenere.ItemIndex := movie.Genere;
dvdcatform.OtherGenere.Text := movie.alt_genere;
dvdcatform.CheckBox1.checked := movie.deleted;
itemunsaved := false;
if (length(movie.movie_id) < 6) then
NewEntryMenuItemClick(Application);
end;

kretzschmar

your savescreenrecord do already overwrite the next record,
after the load the filecursor is behind the loaded record,
maybe you do duplicate/overwrite records unknown, which may cause the effect the rec#1 is near identical to rec#2

try this change(don't know if this applyable to your logic you use)

if not filepos(moviefile) = filesize(moviefile) then
//seek(moviefile, filepos(moviefile)); this may cause no seek
seek(moviefile, filepos(moviefile)-1);
WRITE(moviefile, movie);

i may also wroing in this case

meikl ;-)

DanEgli

ASKER

Worth a shot. Thanks! I'll try it tonight.

ASKER CERTIFIED SOLUTION

kretzschmar

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