// 根据指定长度截取字符串 
function getStringByNumber(source,num)
{	
   if ((source == null)||(source.length)<=0) return "";	
   if (num == null) num = 15;						
   if (source.length <= num) return source;
   var tmp = source.substring(0,num);
	for (var i = 0; i < tmp.length; i++)
	{
		var ch = tmp.charAt(i);
		if ((ch <= "\x7f")){
	  			num = num + 0.5;
	  		}
	}							
   return source.substring(0,Math.round(num)) + "...";
}