TBufDataset.First issue.

[TRon]

Not sure if there is any documentation for this 

https://www.freepascal.org/docs-html/fcl/db/tfield.fieldno.html ("It is a 1-based index")

Ok, I came in from another end: https://www.freepascal.org/docs-html/fcl/db/tfielddef.fieldno.html

[Zvoni]

Result

RecCount=0
RecCount=1
BOF=FALSE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG
BOF=FALSE - EOF=TRUE
recNo=1 - s=ABCDEFG
BOF=TRUE - EOF=FALSE
recNo=1 - s=ABCDEFG

Still doesn't explain the BOF/EOF confusion

What confusion?
They are correct in this example. (or am I missing something?)

BOF and EOF are not set until they are actually encountered.
With a POST they are not set. Only with Last, First, Next and Prior (and only in the direction you where going).

For a RecordCount = 1 i would expect BOF and EOF to be True.
Or at least EOF to be True from the get-go, since OP used Append

[rvk]

For a RecordCount = 1 i would expect BOF and EOF to be True.

No. BOF and EOF are only set on movement of the cursor.

1. The cursor is on the last record, and the TDataset.Next method is called.

https://www.freepascal.org/docs-html/fcl/db/tdataset.eof.html

So, although you are on the last record after Append with one record, Next isn't called, so EOF is false.

Although the documentation page of BOF isn't that clear, the same as for EOF goes for BOF.

Only exception is when there is an empty dataset. Then both BOF and EOF are true.

Also note:

In all other cases, EOF is False. Note: when the cursor is on the last-but-one record, and Next is called (moving the cursor to the last record), EOF will not yet be True.

So, EOF will only trigger if you are trying to go past the last record. Not when you reach that last record.
(That's why you do a while EOF loop with Next on the end, otherwise this would go wrong)

[wp]

Ok, I came in from another end: https://www.freepascal.org/docs-html/fcl/db/tfielddef.fieldno.html

I agree that it is "poorly documented". I just know this because I once was involved in a bug fix related to ZEOS components which changed their RecNo to begin at 0, and this caused some trouble (https://sourceforge.net/p/zeoslib/tickets/539/).

[Zvoni]

For a RecordCount = 1 i would expect BOF and EOF to be True.

No. BOF and EOF are only set on movement of the cursor.

1. The cursor is on the last record, and the TDataset.Next method is called.

https://www.freepascal.org/docs-html/fcl/db/tdataset.eof.html

So, although you are on the last record after Append with one record, Next isn't called, so EOF is false.

Although the documentation page of BOF isn't that clear, the same as for EOF goes for BOF.

Only exception is when there is an empty dataset. Then both BOF and EOF are true.

Also note:

In all other cases, EOF is False. Note: when the cursor is on the last-but-one record, and Next is called (moving the cursor to the last record), EOF will not yet be True.

So, EOF will only trigger if you are trying to go past the last record. Not when you reach that last record.
(That's why you do a while EOF loop with Next on the end, otherwise this would go wrong)

Ah... OK.
I've just read something on Microsofts website, and it is consistent
BOF/EOF will only be set to True if you try to MOVE BEYOND the First/Last record

[TRon]

BOF/EOF will only be set to True if you try to MOVE BEYOND the First/Last record

Ah, yes !!. That was the conclusion from the conversation I remembered as well. Thx for the refresh guys/galls !

[Thaddy]

An old example from 2016 written by me and already on this forum:

Code: Pascal  [Select][+][-]

1. {$apptype console}{$mode objfpc}
2. uses
3.   db,BufDataset;
4. var
5.   BufDb:TBufDataset;
6. begin
7.  BufDb:=TBufDataset.Create(nil);
8.  try
9.    BufDb.FieldDefs.Add('NAME',ftString,20);
10.    BufDb.FieldDefs.Add('NUM',ftinteger);
11.    BufDb.FieldDefs.Add('NUM2',ftInteger);
12.    BufDb.CreateDataSet;
13.    BufDb.Open;
14.    BufDb.Append;
15.    BufDb.FieldByName('NAME').Value:='Free';
16.    BufDb.Post;
17.    BufDb.Append;
18.    BufDb.FieldByName('NAME').Value:='Pascal';
19.    BufDb.Post;
20.    BufDb.SaveToFile('BufDb.txt');
21.  finally
22.    BufDb.Close;
23.    BufDb.Free;
24.  end;
25.  // now, open..
26.  BufDb := TBufDataset.Create(nil);
27.  try
28.    BufDb.LoadFromFile('BufDb.txt');
29.    BufDb.Open;
30.    BufDb.First;
31.    Writeln(BufDb.FieldByName('NAME').Value);
32.  finally
33.    BufDb.Close;
34.    BufDb.Free;
35.  end;
36.  
37. end.

It seems to me that just the call to open is the culprit.

https://forum.lazarus.freepascal.org/index.php/topic,33508.msg217317.html#msg217317

[Zvoni]

An old example from 2016 written by me and already on this forum:

Code: Pascal  [Select][+][-]

1. {$apptype console}{$mode objfpc}
2. uses
3.   db,BufDataset;
4. var
5.   BufDb:TBufDataset;
6. begin
7.  BufDb:=TBufDataset.Create(nil);
8.  try
9.    BufDb.FieldDefs.Add('NAME',ftString,20);
10.    BufDb.FieldDefs.Add('NUM',ftinteger);
11.    BufDb.FieldDefs.Add('NUM2',ftInteger);
12.    BufDb.CreateDataSet;
13.    BufDb.Open;
14.    BufDb.Append;
15.    BufDb.FieldByName('NAME').Value:='Free';
16.    BufDb.Post;
17.    BufDb.Append;
18.    BufDb.FieldByName('NAME').Value:='Pascal';
19.    BufDb.Post;
20.    BufDb.SaveToFile('BufDb.txt');
21.  finally
22.    BufDb.Close;
23.    BufDb.Free;
24.  end;
25.  // now, open..
26.  BufDb := TBufDataset.Create(nil);
27.  try
28.    BufDb.LoadFromFile('BufDb.txt');
29.    BufDb.Open;
30.    BufDb.First;
31.    Writeln(BufDb.FieldByName('NAME').Value);
32.  finally
33.    BufDb.Close;
34.    BufDb.Free;
35.  end;
36.  
37. end.

It seems to me that just the call to open is the culprit.

https://forum.lazarus.freepascal.org/index.php/topic,33508.msg217317.html#msg217317

No, it was the FieldNo in the (overloaded) Add-Function. OP (and myself) missed that FieldNo is 1-based. OP used for that sample FieldNo=0

[Thaddy]

I wouldn't call it no!, because if you want to read the first entry, first will do nicely. Hence that old example.
It is cleaner and much easier than the bull dung above.

[egsuh]

In Zvony’s example, calling RecNo might have moved cursor beyond last record.