BufDataSet - LoadFromFile problem

[HappyLarry]

In BufDataSet, I can not get LoadFromFile to work.
I have tried both

Code: Pascal  [Select]

1.      T.LoadFromFile('Names.bds');

and

Code: Pascal  [Select]

1.      
2.      T.Edit;
3.      T.LoadFromFile('Names.bds');
4.      T.Post;

but neither shows the record.

All help gratefully received.

Here is my test program

Code: Pascal  [Select]

1. program Loadtest;
2. uses
3.   db,BufDataset;
4.  
5. var
6.   T:TBufDataSet;
7.  
8. begin
9.      //Create a dataset, add a record and save it
10.      T:=TBufDataset.Create(nil);
11.      T.FieldDefs.Add('Name',ftString,20);
12.      T.CreateDataset;
13.      T.Append;
14.      T.Fields[0].value:='Fred';
15.      T.Post;
16.  
17.      writeln('Before Save :',T.Fields.FieldByName('Name').AsString);
18.  
19.      T.SaveToFile('Names.bds');
20.      T.Free;
21.  
22.      //Create a dataset, load the file and show it
23.      T:=TBufDataset.Create(nil);
24.      T.FieldDefs.Add('Name',ftString,20);
25.      T.CreateDataset;
26.      //T.Edit;
27.      T.LoadFromFile('Names.bds');
28.      //T.Post;
29.      writeln('After Load :',T.Fields.FieldByName('Name').AsString);
30.  
31.      T.free;
32.  
33.    writeln('------ done ----------');
34.    readln;
35. end.
36.  

[howardpc]

LoadFromFile creates the data set.
All you want after saving the data set to file is this:

Code: Pascal  [Select]

1. //Create a dataset, load the file and show it
2.   T:=TBufDataset.Create(nil);
3.   T.LoadFromFile('Names.bds');
4.   writeln('After Load :',T.Fields.FieldByName('Name').AsString);
5.   T.free;    

[ttomas]

This work OK

Code: Pascal  [Select]

1.   //Create a dataset, load the file and show it
2.   T:=TBufDataset.Create(nil);
3. //---->  T.FieldDefs.Add('Name',ftString,20);  
4. //---->  T.CreateDataset;
5.   //T.Edit;
6.   T.LoadFromFile('Names.bds');
7.  

[wp]

Or do this:

Code: Pascal  [Select]

1. T := TBufDataset.Create(nil);
2. T.FileName := 'Names.bds';
3. T.Open;

Having specified the filename will cause automatic saving of the file if the dataset is closed later.

[HappyLarry]

@ All
Great answers - Thanks

Another problem: I have tried to write a procedure to load a text file. The text file record structure is (String, Integer, Real). I have written:

Code: Pascal  [Select]

1. procedure LoadTextFile(Filename:string; var T:TBufDataSet);
2. var
3.   MyFile:Text;
4.   C1:string;
5.   P1:integer;
6.   A1:real;
7.  
8. begin
9.       Assignfile(MyFile,FileName);
10.       Reset(MyFile);
11.       While not System.EOF(MyFile) do
12.       begin
13.         readln(MyFile,C1);
14.         readln(MyFile,P1);
15.         readln(MyFile,A1);
16.         T.AppendRecord([C1, P1, A1]);
17.       end;
18.       CloseFile(MyFile);
19. end;
20.  

This works but is not generally usable, because it relies on the record structure being(String, Integer, Real),
Is there a way of writing it so that it works whatever the record structure, for instance if the record structure was (String, Integer, Integer, String);?

If it helps, here is the full program code.

Code: Pascal  [Select]

1. program Loadtest;
2. uses
3.   db,BufDataset;
4.  
5. var
6.   T:TBufDataSet;
7.  
8. procedure SaveTextFile(Filename:string; T:TBufDataSet);
9. var
10.   j:integer;
11.   MyFile:text;
12. begin
13.   Assignfile(MyFile,FileName);
14.   Rewrite(MyFile);
15.   T.First;
16.   While not T.EOF do
17.   begin
18.     For j:= 0 to T.FieldCount-1 do
19.     begin
20.         writeln(MyFile,T.Fields[j].Value);
21.     end;
22.     T.next;
23.   end;
24.   CloseFile(MyFile);
25. end;
26.  
27. procedure LoadTextFile(Filename:string; var T:TBufDataSet);
28. var
29.   MyFile:Text;
30.   C1:string;
31.   P1:integer;
32.   A1:real;
33.  
34. begin
35.       Assignfile(MyFile,FileName);
36.       Reset(MyFile);
37.       While not System.EOF(MyFile) do
38.       begin
39.         readln(MyFile,C1);
40.         readln(MyFile,P1);
41.         readln(MyFile,A1);
42.         T.AppendRecord([C1, P1, A1]);
43.       end;
44.       CloseFile(MyFile);
45. end;
46.  
47.  
48.  
49.  
50. begin
51.      //Create a dataset,
52.      T:=TBufDataset.Create(nil);
53.      T.FieldDefs.Add('C',ftString,20);
54.      T.FieldDefs.Add('P',ftInteger);
55.      T.FieldDefs.Add('A',ftFloat);
56.  
57.      //add a record
58.      T.CreateDataset;
59.      T.Append;
60.      T.Fields[0].value:='Belgium';
61.      T.Fields[1].value:=11;
62.      T.Fields[2].value:=32.5;
63.      T.Post;
64.  
65.      //Save
66.      writeln('Before Save :',T.Fields.FieldByName('C').AsString);
67.      SaveTextFile('Countries.txt', T);
68.  
69.      //Clear Table
70.      T.Delete;
71.      writeln('After Delete :',T.RecordCount,' records');
72.  
73.      //Reload
74.      LoadTextFile('Countries.txt',T);
75.      writeln('After Load :',T.Fields.FieldByName('C').AsString);
76.  
77.      T.free;
78.  
79.    writeln('------ done ----------');
80.    readln;
81. end.
82.  

[howardpc]

For persistent data saved to a textfile you can organise your file so that the first line saved with SaveToText represents the field structure (fieldname, type, and length). Your LoadTextFile procedure would then parse the first line read as a special composite value whose individual parts enable you to reconstruct the correct TFieldDefs for your BufDataSet. Or you can save this information as a separate schema file which LoadTextFile opens and reads first.

Either way, once the correct database structure is determined, LoadTextFile can continue to read individual records using ReadLn, parsing each record into its constituent fields which are appended to the dataset in memory.

Note that best practice is to use TextFile (rather than Text) as the file type. If you ever migrate your code to a GUI app, use of Text will most likely produce  unwelcome name clashes with the Text properties of numerous classes.