数据梗概：
> str(train)
'data.frame':        72194 obs. of  8 variables:
$ building_area     : num  14.4 10.2 9 8.6 8.3 8.5 26.2 16.8 21 8.3 ...
$ city              : Factor w/ 9 levels "北京","成都",..: 9 7 1 1 1 1 8 1 1 1 ...
$ bedroom_num       : num  4 3 3 3 3 4 3 4 4 3 ...
$ hall_num          : num  1 1 1 1 1 1 1 1 1 1 ...
$ price             : num  1690 1460 1630 2290 2790 2460 2160 3060 99 1390 ...
$ orientation       : Factor w/ 11 levels "","北","东","东北",..: 7 7 2 2 7 2 7 2 7 2 ...
$ private_bathroom  : Factor w/ 2 levels "独卫","无独卫": 2 2 2 2 2 2 1 2 2 2 ...
$ independent_balcon: Factor w/ 2 levels "独立阳台","无独立阳台": 1 2 2 2 1 2 2 1 1 1 ...
- attr(*, "na.action")= 'omit' Named int  4515 19640 19970 21744 21966 21997 23152 28446 41437 41441 ...
  ..- attr(*, "names")= chr  "5020" "21913" "22268" "24103" ...
#出错提示：
Error in object$var.levels[[i]] : subscript out of bounds