mtcars
y<-quantile(mtcars$mpg,c(.8,.6,.4,.2))  #对mpg的数值取分位数
y
mtcars$group[mtcars$mpg<y[1]&mtcars$mpg>=y[2]]<-"A"  
mtcars$group[mtcars$mpg<y[2]&mtcars$mpg>=y[3]]<-"B"
mtcars$group[mtcars$mpg<y[3]&mtcars$mpg>=y[4]]<-"C"
mtcars$group[mtcars$mpg<y[4]]<-"D"
mtcars$group<-factor(mtcars$group)#将要预测的变量定义为因子，否则J48()函数无法识别
a<-round(3/4*sum(mtcars$group=="A"))
b<-round(3/4*sum(mtcars$group=="B"))
c<-round(3/4*sum(mtcars$group=="C"))
d<-round(3/4*sum(mtcars$group=="D"))

install.packages("sampling")
library(sampling)
sub<-strata(mtcars,stratanames="group",size=c(a,b,c,d),method="srswor")

然后报错了
sub<-strata(mtcars,stratanames="group",size=c(a,b,c,d),method="srswor")
Error in if (method != "srswr" & length(y) < size[i]) { :
  missing value where TRUE/FALSE needed
有大神遇到过类似问题吗？