var i=0;
$("#table tr").each(function(){
$("#table tr:eq(i) td:first").addClass("table_left");
alert(i+$("#table tr:eq(i) td:first").text());
i++;
});
$("#table tr").each(function(){
$("#table tr:eq(i) td:first").addClass("table_left");
alert(i+$("#table tr:eq(i) td:first").text());
i++;
});
$("table tr").each(function(i){
alert($(this).find("td").eq(0).text());
})<table>
<tr>
<td>1</td>
<td>2</td>
<td>3</td>
</tr>
<tr>
<td>4</td>
<td>5</td>
<td>6</td>
</tr>
</table>