> dstats <- function(x)(c(mean=mean(x), sd=sd(x)))
 > by(mtcars[vars],mtcars$am, dstats)
 Error in is.data.frame(x) : 
   (list) object cannot be coerced to type 'double'
 In addition: Warning message:
 In mean.default(x) : argument is not numeric or logical: returning NA
 我看R语言实战上面也这么写的。能运行出结果，后来翻阅书籍帮我解决了这个，参考如下：