function frdtbl(){
  var Ptr=document.getElementById("tab").getElementsByTagName("tr"); 
  for (i=1;i<Ptr.length+1;i++) {  
    Ptr[i-1].className = (i%2>0)?"t1":"t2";  
  }
  for(var i=0;i<Ptr.length;i++) { 
    Ptr[i].onmouseover=function(){ 
      this.tmpClass=this.className; 
      this.className = "t3"; 
    }; 
    Ptr[i].onmouseout=function(){ 
      this.className=this.tmpClass; 
    };
  }
}