Using css to alternate dynamic table row colors

[ibiangalex]

I have a problem with alternating the rows of my php dynamic table. I saw an example here which I was able to use but I dont understand how to switch between between html and the php on the <tr class= <?php "' . $class .'" ?>' altrow is my css class definition. below is my mark up:

 
<tr>
    <th scope="col">Trackid</th>
       <th scope="col">NMEA</th>
      <th scope="col">Date</th>
 
</tr>
    $result = mysql_query($sql, $link);
    $i = 0;
    if (!$result) {
        die("Query to show averages from table failed");
    }
    while ($row = mysql_fetch_assoc($result)){
        $class = ($i++ % 2) ? 'altrow' : '';
?>
    <tr class= <?php "' . $class .'" ?>>';
    <td><?php echo $row['Trackid']; ?></td>
      <td><?php echo $row['NMEA']; ?></td>
      <td><?php echo $row['Date']; ?></td>
 
</tr>
<?php } ?>
</table>

Can someone please show how to achieve this alternation? Thanks

[Dormilich]

you have the alteration on line 14, you only need to echo it.