library(ggplot2)
set.seed(1)
train.ind <- sample(1:nrow(mpg), round(nrow(mpg)/2))
lm_mod <- lm(displ ~ ., data = mpg[train.ind, ])
lm_pred <- predict(lm_mod, mpg[-train.ind, ])
运行最后一行给出了错误：
Error in model.frame.default(Terms, newdata, na.action = na.action, xlev = object$xlevels) :
factor model has new levels land cruiser wagon 4wd
我很困惑，因为我在mpg数据中没有任何因子变量。
> str(mpg)
Classes ‘tbl_df’, ‘tbl’ and 'data.frame': 234 obs. of 11 variables:
$ manufacturer: chr "audi" "audi" "audi" "audi" ...
$ model : chr "a4" "a4" "a4" "a4" ...
$ displ : num 1.8 1.8 2 2 2.8 2.8 3.1 1.8 1.8 2 ...
$ year : int 1999 1999 2008 2008 1999 1999 2008 1999 1999 2008 ...
$ cyl : int 4 4 4 4 6 6 6 4 4 4 ...
$ trans : chr "auto(l5)" "manual(m5)" "manual(m6)" "auto(av)" ...
$ drv : chr "f" "f" "f" "f" ...
$ cty : int 18 21 20 21 16 18 18 18 16 20 ...
$ hwy : int 29 29 31 30 26 26 27 26 25 28 ...
$ fl : chr "p" "p" "p" "p" ...
$ class : chr "compact" "compact" "compact" "compact" ...
我该如何解决这个问题？