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;
    ch:char;
begin
  n:=0;
  repeat
  n:=n+1;
    writeln('** Nhap nguoi thu ',n);
    writeln('Ho ten   : ');
    write('- Ho : '); readln(a[n].hoten.ho);
    write('- Dem: '); readln(a[n].hoten.dem);
    write('- Ten: '); readln(a[n].hoten.ten);
    write('Diem TB  : '); readln(a[n].diemtb);
    repeat
      writeln('Ban co muon tiep tuc nhap them khong? c/k ');
      readln(ch);
    until (ch='c') or (ch='C') or (ch='k') or (ch='K');
  until (ch='k') or (ch='K');
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 dtang/giam ?');
  writeln(' - Chon 1 de tang');
  writeln(' - Chon 2 de giam');
  readln(chon);
  case chon of
    1: tang;
    2: giam;
  end;
  xuat;
  readln;
end.