上QQ阅读APP看书，第一时间看更新
Getting ready
Create a sample data frame:
> salary <- c(20000, 30000, 25000, 40000, 30000, 34000, 30000)
> family.size <- c(4,3,2,2,3,4,3)
> car <- c("Luxury", "Compact", "Midsize", "Luxury", "Compact", "Compact", "Compact")
> prospect <- data.frame(salary, family.size, car)