var tab_arr = new Array("tab1","tab2");

function switch_tab (obj) {
  var content_elem;
  obj.className="tab_aktiv";
  content_elem = obj.id + "_content";
  document.getElementById(content_elem).className="tab_content";


  for(var i=0; i<tab_arr.length; i++) {
     if(tab_arr[i] == obj.id)
       continue;
     else {
       document.getElementById(tab_arr[i]).className="tab";
       content_elem = tab_arr[i] + "_content";
       document.getElementById(content_elem).className="tab_content_inaktiv";

     }
  }

}