BufDataSet - LoadFromFile problem
lazarus / 63-Database

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HappyLarry · 2016-08-08 08:51

In BufDataSet, I can not get LoadFromFile to work.
I have tried both
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
 T.LoadFromFile('Names.bds');
and
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i         
 T.Edit;     T.LoadFromFile('Names.bds');     T.Post;but neither shows
the record.

All help gratefully received.

Here is my test program
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Loadtest;uses  db,BufDataset; var  T:TBufDataSet; begin     //Create a
dataset, add a record and save it     T:=TBufDataset.Create(nil);   
 T.FieldDefs.Add('Name',ftString,20);     T.CreateDataset;   
 T.Append;     T.Fields[0].value:='Fred';     T.Post;    
 writeln('Before Save :',T.Fields.FieldByName('Name').AsString);    
 T.SaveToFile('Names.bds');     T.Free;      //Create a dataset, load
the file and show it     T:=TBufDataset.Create(nil);   
 T.FieldDefs.Add('Name',ftString,20);     T.CreateDataset;   
 //T.Edit;     T.LoadFromFile('Names.bds');     //T.Post;   
 writeln('After Load :',T.Fields.FieldByName('Name').AsString);    
 T.free;    writeln('------ done ----------');   readln;end.

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howardpc · 2016-08-08 09:06

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

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i //Create a
dataset, load the file and show it  T:=TBufDataset.Create(nil); 
T.LoadFromFile('Names.bds');  writeln('After Load
:',T.Fields.FieldByName('Name').AsString);  T.free;

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ttomas · 2016-08-08 09:06

This work OK
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   //Create
a dataset, load the file and show it 
T:=TBufDataset.Create(nil);//---->
 T.FieldDefs.Add('Name',ftString,20);   //---->  T.CreateDataset; 
//T.Edit;  T.LoadFromFile('Names.bds');

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wp · 2016-08-08 09:27

Or do this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i T :=
TBufDataset.Create(nil);T.FileName := 'Names.bds';T.Open;Having
specified the filename will cause automatic saving of the file if the
dataset is closed later.

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HappyLarry · 2016-08-08 12:22

@ 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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
LoadTextFile(Filename:string; var T:TBufDataSet);var  MyFile:Text; 
C1:string;  P1:integer;  A1:real; begin     
Assignfile(MyFile,FileName);      Reset(MyFile);      While not
System.EOF(MyFile) do      begin        readln(MyFile,C1);       
readln(MyFile,P1);        readln(MyFile,A1);       
T.AppendRecord([C1, P1, A1]);      end;      CloseFile(MyFile);end; 
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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
Loadtest;uses  db,BufDataset; var  T:TBufDataSet; procedure
SaveTextFile(Filename:string; T:TBufDataSet);var  j:integer; 
MyFile:text;begin  Assignfile(MyFile,FileName);  Rewrite(MyFile); 
T.First;  While not T.EOF do  begin    For j:= 0 to T.FieldCount-1 do 
  begin        writeln(MyFile,T.Fields[j].Value);    end;    T.next; 
end;  CloseFile(MyFile);end; procedure LoadTextFile(Filename:string;
var T:TBufDataSet);var  MyFile:Text;  C1:string;  P1:integer; 
A1:real; begin      Assignfile(MyFile,FileName);      Reset(MyFile); 
   While not System.EOF(MyFile) do      begin       
readln(MyFile,C1);        readln(MyFile,P1);       
readln(MyFile,A1);        T.AppendRecord([C1, P1, A1]);      end;    
CloseFile(MyFile);end;    begin     //Create a dataset,   
 T:=TBufDataset.Create(nil);     T.FieldDefs.Add('C',ftString,20);   
 T.FieldDefs.Add('P',ftInteger);     T.FieldDefs.Add('A',ftFloat);   
 //add a record     T.CreateDataset;     T.Append;   
 T.Fields[0].value:='Belgium';     T.Fields[1].value:=11;   
 T.Fields[2].value:=32.5;     T.Post;      //Save     writeln('Before
Save :',T.Fields.FieldByName('C').AsString);   
 SaveTextFile('Countries.txt', T);      //Clear Table     T.Delete;  
 writeln('After Delete :',T.RecordCount,' records');      //Reload   
 LoadTextFile('Countries.txt',T);     writeln('After Load
:',T.Fields.FieldByName('C').AsString);      T.free;  
 writeln('------ done ----------');   readln;end.

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howardpc · 2016-08-08 12:52

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.