Loading an array of records back into a program from a file?

Question

I am creating a program that moves through an array of records and save these student records to a file.

However I now wish to reload the data (StudentName,Class,Grade) back into the array and subsequently display them in a list box on another form.

I have tried a few methods but with no success.

This is the code that wrote the file:

unit NewStudent;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls,
  ExtCtrls, studentdata;


  { TFormNewStudent }
Type
  TFormNewStudent = class(TForm)
    Button1: TButton;
    ButtonAddStudent: TButton;
    Button3: TButton;
    ComboBoxPredictedGrade: TComboBox;
    EditClass: TEdit;
    EditName: TEdit;
    procedure Button1Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure ButtonAddStudentClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);

  private
    { private declarations }
  public
    { public declarations }
  end;

Type
  TPupil = Record
    Name:String[30];
    ClassGroup:String;
    ComboBoxPredictedGrade:Integer;
  end;

var
  FormNewStudent: TFormNewStudent;
  StudentRecArray : Array[1..30] of TPupil;
  StudentNo:integer;
  studentFile:TextFile;
implementation

{$R *.lfm}

{ TFormNewStudent }

procedure TFormNewStudent.Button1Click(Sender: TObject);
begin
   FormStudentData.visible:=true;
  FormNewStudent.visible:=false;
end;

procedure TFormNewStudent.Button3Click(Sender: TObject);
begin
  FormStudentData.visible:=False;
  FormNewStudent.visible:=True;
end;

procedure TFormNewStudent.ButtonAddStudentClick(Sender: TObject);
var
   newStudent:string;
Begin
   assignfile(studentFile,'G:\ExamGen\studentfile.txt');
   StudentRecArray[StudentNo].Name:=EditName.text;
   StudentRecArray[StudentNo].ClassGroup:=EditClass.text;
   StudentRecArray[StudentNo].ComboBoxPredictedGrade:=ComboBoxPredictedGrade.ItemIndex;
   append(studentFile);
   newStudent:=(StudentRecArray[StudentNo].Name)+','+(StudentRecArray[StudentNo].ClassGroup)+','+(IntToStr(StudentRecArray[StudentNo].ComboBoxPredictedGrade));
   writeln(studentFile,newStudent);
   closefile(StudentFile);
   StudentNo := StudentNo + 1;
end;

procedure TFormNewStudent.FormCreate(Sender: TObject);
begin
  ComboBoxPredictedGrade.Items.Add('A');
  ComboBoxPredictedGrade.Items.Add('B');
  ComboBoxPredictedGrade.Items.Add('C');
  ComboBoxPredictedGrade.Items.Add('D');
  ComboBoxPredictedGrade.Items.Add('E');
  ComboBoxPredictedGrade.Items.Add('U');
end;



end.                    

ScreenShot 1: StudentFile
ScreenShot 2: AddStudent Form

Answer 1

Answer given by Zamrony P. Juhara is correct, but your approach here may be not the most convenient. You define record which contains information about each student, then you write procedures to write this record to file and another one to read it. If you'll eventually change format of your record, you'll have to rewrite this code also. There are better ways, in my opinion.

You can define record containing only simplest members, like Ken White suggested:

  TPupil = Record
    Name:String[30];
    ClassGroup:String[20]; //some convenient value
    ComboBoxPredictedGrade:Integer;
  end;

Such a record have fixed size and contains all needed information in itself (version with ClassGroup:String actually stores pointer to another area in memory where your string is), and then you can save and load it extremely easy:

var
  myFile : File of TPupil;

procedure TFormNewStudent.ButtonAddStudentClick(Sender: TObject);
Begin
   assignfile(studentFile,'G:\ExamGen\studentfile.txt');
   StudentRecArray[StudentNo].Name:=EditName.text;
   StudentRecArray[StudentNo].ClassGroup:=EditClass.text;
   StudentRecArray[StudentNo].ComboBoxPredictedGrade:=ComboBoxPredictedGrade.ItemIndex;
   append(studentFile);

   Write(studentFile,StudentRecArray[StudentNo]); //THAT'S IT!

   closefile(StudentFile);
   inc(StudentNo);
end;

procedure TFormNewStudent.ReadFromFile;
begin
  AssignFile(myFile,'G:\ExamGen\studentfile.txt');
  Reset(studentFile);

  StudentNo:=1;
  while not Eof(studentFile) do begin
    Read(studentFile,StudentRecArray[i]);
    inc(StudentNo);
  end;
end;

There is little drawback: file is not so readable as it was before, because Integer is saved exactly as 4-byte value, not its decimal representation.

There is much more interesting possibilities if you move from record to class, in that case you can use streaming system in a way as IDE saves forms to disc, in .dfm or .lfm files, so you'll be able to automatically save complex ierarchies of objects and load them back.

Answer 2

var
  myFile : TextFile;
  text   : string;
  lines  : TStringList;
  i      : integer;
...

lines := TStringList.Create();
AssignFile(studentFile,'G:\ExamGen\studentfile.txt');
Reset(studentFile);

i:=1;
while not Eof(studentFile) do
begin
  ReadLn(studentFile, text);
  lines.CommaText := text;
  studentRecArray[i].Name := lines[0];
  studentRecArray[i].ClassGroup := lines[1];
  studentRecArray[i].ComboBoxPredictedGrade := StrToInt(lines[2]);
  inc(i);
end;

CloseFile(studentFile);

lines.Free();