library(rpart)

attach(olives)

n<-length(olives[,1])
fold <- 15
chunk <- trunc(n/fold)
nn <- chunk*fold
runs <- 20

Area <- as.factor(Area)

CV <- matrix(0, runs, 1)

for(i in 1:runs) {
   for(j in 1:fold) {
      test <- (1:chunk)*fold-fold+j
      train <- (1:nn)[-test]
      rpt <- rpart(Area ~ palmitic + palmitoleic + stearic + 
                          oleic + linoleic + linolenic + 
                          arachidic + eicosenoic, subset=train, 
                          cp=0.000001, minsplit=i, minbucket=i)
      conf <- as.matrix(table(predict(rpt, 
                                      newdata=olives[test,4:11], 
                                      type="class") , 
                              Area[test]))
      CV[i] <- CV[i] + sum(conf - diag(diag(conf), 9,9))
   }
}
plot(CV, xlab="Minbucket", pch="o")
lines(lowess(CV ~ (1:runs)))