The R environment has a rich set of options available for performing operations on data within the various data structures. These operations can be performed in a variety of ways and can be restricted according to various criteria. The focus of this section is the purpose and formats of the various apply commands.

The apply commands are used to instruct R to use a given command on specific parts of a list, vector, or array. Each data type has different versions of the apply commands that are available. Before discussing the different commands, it is important to define the notion of the margins of a table or array. The margins are defined along any dimension, and the dimension used must be specified. The margin command can be used to determine the sum of the row, columns, or the entire column of an array or table:

> A <- matrix(1:12,nrow=3,byrow=TRUE)
> A
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    5    6    7    8
[3,]    9   10   11  ...