Đọc ghi file đơn giản trong Pascal

uses crt;
type
  mang = array[1..100] of byte;
var
  f,f2:text;
  i,j,k:byte;
  a:mang;
  n:byte;
procedure sxtang(var m:mang; n:byte);
var i,j,temp:byte;
begin
  for i:=1 to n-1 do
    for j:=i+1 to n do
      if (m[i]>m[j]) then
        begin
          temp:=m[i];
          m[i]:=m[j];
          m[j]:=temp;
        end;
end;
begin
  clrscr;
  {$I-}
  assign(f,'input.txt');
  reset(f); {f dung de doc file}
  {$I+}

  {Kiem tra ton tai file khong}
  if IOResult<>0 then
    begin
      write('Khong tim thay file input.txt');
      readln;
      exit;
    end;

  assign(f2,'output.txt');
  rewrite(f2); {f2 dung de ghi file}

  readln(f,n); writeln(f2,n);
  for i:=1 to n do
    begin
      j:=0;
      {Doc input.txt}
      while (not eoln(f)) do
        begin
          j:=j+1;
          read(f,a[j]);
        end;
      readln(f);
      {Xuat ra man hinh}
      for k:=1 to j do write(a[k]:2);
      write(' -> ');
      sxtang(a,j);
      for k:=1 to j do write(a[k]:2);
      writeln;
      {Ghi output.txt}
      for k:=1 to j do write(f2,a[k]:2);
      writeln(f2);
    end;
  readln;
  close(f);
  close(f2);
end.