Sắp xếp danh sách sinh viên

uses crt;
type
  ht = record
    ho: string[7];
    dem: string[7];
    ten: string[7];
  end;
  danhsach = record
    hoten: ht;
    diemtb: real;
  end;
var
  n,chon: byte;
  a: array[1..100] of danhsach;
procedure nhap;
var i:byte;
begin
  write('n= '); readln(n);
  for i:=1 to n do
    begin
      writeln('** Nhap nguoi thu ',i);
      writeln('Ho ten   : ');
      write('- Ho : '); readln(a[i].hoten.ho);
      write('- Dem: '); readln(a[i].hoten.dem);
      write('- Ten: '); readln(a[i].hoten.ten);
      write('Diem TB  : '); readln(a[i].diemtb);
    end;
end;
procedure xuat;
var i:byte;
    st: string[21];
begin
  writeln('*** Danh sach ***':32);
  writeln('STT':5, 'Ho ten':22, 'Diem TB':10);
  for i:=1 to n do
    begin
      st := a[i].hoten.ho + ' '+ a[i].hoten.dem + ' '+ a[i].hoten.ten;
      writeln(i:5,st:22,a[i].diemtb:10:1);
    end;
end;
procedure tang;
var i,j:byte;
    tg: danhsach;
begin
  for i:=1 to n-1 do
    for j:=i+1 to n do
      if a[i].diemtb > a[j].diemtb then
        begin
          tg := a[i];
          a[i] := a[j];
          a[j] := tg;
        end;
end;
procedure giam;
var i,j:byte;
    tg: danhsach;
begin
  for i:=1 to n-1 do
    for j:=i+1 to n do
      if a[i].diemtb < a[j].diemtb then
        begin
          tg := a[i];
          a[i] := a[j];
          a[j] := tg;
        end;
end;
begin
  clrscr;
  nhap;
  writeln('Ban muon sap xep tang/giam ?');
  writeln(' - Chon 1 de tang');
  writeln(' - Chon 2 de giam');
  readln(chon);
  case chon of
    1: tang;
    2: giam;
  end;
  xuat;
  readln;
end.