[R] by function error

[苏文龙]

hi, I practice R programing by the step in <<R In Action>>; 
when i test by function, i get error like these;
call for help!


dstats <- function(x){c(mean=mean(x), sd=sd(x))}
> by(mtcars[,c('mpg', 'hp')], mtcars$am, dstats)
 Show Traceback
 
 Rerun with Debug
 Error in is.data.frame(x) : 
  (list) object cannot be coerced to type 'double'



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BestRegards
  
 Su Wenlong
 Phone:13408010439
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