by(data, INDICES, FUN, …)

Apply a function across subsets of data, where the subsets are defined by the INDICES argument. The by function returns a list, where each list item represents the results for a particular subset of the data.

- data – The full set of data, e.g. a vector or data.frame.
- INDICES – A vector describing how the data subsets are to be constructed (e.g. a factor vector). Multiple columns in a data frame may also be given, where each combination of the variables defines a new subset of data.
- FUN – A function to apply to each subset of data.
- … – Any additional arguments to pass to FUN.

Example. The textbooks data from the openintro package is loaded. Then we examine a summary of the price differences between new textbook prices at the UCLA bookstore and on Amazon for two subsets: courses where one book is required and courses where more than one book is required.

> > library(openintro) Loading required package: Cairo Please visit openintro.org for free statistics materials Attaching package: ‘openintro’ The following object(s) are masked from ‘package:datasets’: cars > data(textbooks) > by(textbooks$diff, textbooks$more, summary) textbooks$more: N Min. 1st Qu. Median Mean 3rd Qu. Max. -0.250 9.262 14.530 17.460 28.260 40.940 ------------------------------------------ textbooks$more: Y Min. 1st Qu. Median Mean 3rd Qu. Max. -9.530 1.750 4.960 9.836 9.480 66.000 >

It seems college students are wise to shop online!