R: Recipes for Analysis, Visualization and Machine Learning

Applying functions to subsets of a vector

The tapply function applies a function to each partition of the dataset. Hence, when we need to evaluate a function over subsets of a vector defined by a factor, tapply comes in handy.

Getting ready

Download the files for this chapter and store the auto-mpg.csv file in your R working directory. Read the data and create factors for the cylinders variable:

> auto <- read.csv("auto-mpg.csv", stringsAsFactors=FALSE)
> auto$cylinders <- factor(auto$cylinders, levels = c(3,4,5,6,8), labels = c("3cyl", "4cyl", "5cyl", "6cyl", "8cyl"))

How to do it...

To apply functions to subsets of a vector, follow these steps:

1. Calculate mean mpg for each cylinder type:

   > tapply(auto$mpg,auto$cylinders,mean) 3cyl 4cyl 5cyl 6cyl 8cyl ...