Muncul undefined offset: 1, undefined offset: 2 , undefined offset: 3

Saya menggunakan rumus WP yang ada di google untuk ranking, setelah dijalankan muncul pesan Undefined offset: 1 in D:\kkp\Level\Admin\Hitung.php on line 77 sampe line 78 dengan masalan OFFSET 1, 2, 3

codenya

 <?php
    $sql="SELECT * FROM totalnilai ORDER BY nisnip,idbobot";
    $result=$db->query($sql);
    $X=array();
    $alternative='';
    while($row=$result->fetch_object()){
      if($row->nisnip!=$alternative){
        $X[$row->nisnip]=array();
        $alternative=$row->nisnip;
      }
      $X[$row->nisnip][$row->idbobot]=$row->nilai;
    }
    $S=array();
    foreach($X as $alternative=>$x){
      $S[$alternative]=1;
      echo "S<sub>{$alternative}</sub> = ";
      foreach($x as $namabobot=>$nilai){
         $S[$alternative]*=pow($nilai,-$W[$namabobot]);
         echo "({$nilai}<sup>".($namabobot-$W[$namabobot])."</sup>)";
      };
      echo " = {$S[$alternative]}<br>";
    }
    ?>

ada yang bisa bantu? terimakasih