function alternate(id){ 

 if(document.getElementsByTagName){  
   var table = document.getElementById(id);  
   var rows = table.getElementsByTagName("tr");   
   for(var i = 0; i < rows.length; i++){     
     if(i % 2 == 0){ 
       rows[i].className = "treven"; 
     }else{ 
       rows[i].className = "trodd"; 
     }       
   }
 } 
}