var lastSelectedCity = '';
function setCityValue(textField, dropdown){
   var cityDD = document.getElementsByName(dropdown);
   var cityText = document.getElementsByName(textField);
   var selectIndex = parseInt(cityDD[0].selectedIndex);
   if (selectIndex > 0) {
      cityText[0].value =  cityDD[0].options[selectIndex].value;
      lastSelectedCity =  cityDD[0].options[selectIndex].value;
   } else if ((selectIndex == 0) 
                 && (lastSelectedCity == cityText[0].value)) {
      cityText[0].value = '';
      lastSelectedCity ='';
   } 
}