c1<-cbind(rnorm(5,sd=5),rnorm(5,sd=5), rep(1,5))
c2<-cbind(rnorm(5,sd=5),rnorm(5,sd=5), rep(2,5))
cx<-rbind(c1,c2)
plot(cx,col=cx[,3])
write.table(c1[,-3],"ex23a.dat",col.names=F,row.names=F,quote=F)
write.table(c2[,-3],"ex23b.dat",col.names=F,row.names=F,quote=F)