Редактирование типизированного файла

unit Unit1;
 
interface
 
uses
  SysUtils, Types, Classes, Variants, QTypes, QGraphics, QControls, QForms, 
  QDialogs, QStdCtrls, QMenus;
 
type
  tfile=file of char;
  TForm1 = class(TForm)
    Memo1: TMemo;
    Memo2: TMemo;
    Label1: TLabel;
    Label2: TLabel;
    MainMenu1: TMainMenu;
    N1: TMenuItem;
    N2: TMenuItem;
    N3: TMenuItem;
    N4: TMenuItem;
    N5: TMenuItem;
    procedure N2Click(Sender: TObject);
    procedure N3Click(Sender: TObject);
    procedure N4Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
  f:tfile;
  c,k:char;
  i:integer;
 
implementation
 
{$R *.xfm}
procedure replace(c:char; var r:tfile);
  begin
  assignfile(r,'r.dat');
  assignfile(f,'f.dat');
  reset(f);
  rewrite(r);
  for i:=0 to filesize(f)-1 do
    begin
      read(f,k);
      write(r,k);
      end;
    closefile(f);
    seek(r,filesize(r));
    write(r,c);
    closefile(r);
    end;
 
procedure TForm1.N2Click(Sender: TObject);
var
  s:string;
  i:integer;
begin
  s:=Memo1.Text;
  assignfile(f,'f.dat');
  rewrite(f);
  for i:=1 to length(s) do
    write(f,s[i]);
    closefile(f);
 
end;
 
procedure TForm1.N3Click(Sender: TObject);
begin
  Memo2.Clear;
  Memo2.Lines.LoadFromFile('r.dat');
end;
 
procedure TForm1.N4Click(Sender: TObject);
begin
replace(c,f);
end;
 
end.

assignfile(f,'r.dat');
rewrite(f);
for i:=1 to Memo1.Text do
write(f,s[i]);
closefile(f);

reset(f);
seek(f,filesize(f)-1);
write(f,c);

reset(f);
while not eof(f) do
 begin
  read(f,c);
  Memo2.Text:=Memo2.Text+c;
 end;
closefile(f);