function flipQty(chkObj, qtyObj) {
  qtyObj.value = (chkObj.checked) ? '1' : '' ;
}

function flipCheck(qtyObj, chkObj) {
  var v = qtyObj.value;
  // if empty string or '0' or non numeric string, check box is off
  chkObj.checked = ( (v == '') || (v == '0') || (!isNumericString (v))) ? false : true ;
  
  // set qty to empty input if check box went off
  qtyObj.value = (chkObj.checked) ? qtyObj.value : '' ;
}