# 代码清单7-7 使用by()分组计算描述性统计量
> vars dstats 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