Extract or Replace Parts of an Object

Description

These operators act on vectors, arrays, dataframes and lists. If one of these expressions appears on the left side of an assignment then that part of x is set to the value of the right hand side of the assignment.

These operators are generic. You can write methods to handle subsetting of specific classes of data.

The [[ operator requires all relevant subscripts be supplied. With the [ operator a comma separated blank indicates that all entries in that dimension are selected.

When operating on a list, the [[ operator gives the specified element of the list while the [ operator returns a list with the specified element(s) in it.

Usage

x[i]
x[i, j, ...]
x[i, j, ... , drop=TRUE]
x[[i]]
x[[i, j, ...]]
x$name

See Also

list, array, matrix.

Examples

x <- 1:12; m <- matrix(1:6,nr=2); li <- list(pi=pi, e = exp(1))
x[10]		      # the tenth element of x
m[1,]		      # the first row of matrix m
m[1, , drop = FALSE]  # is a 1-row matrix
li[[1]]		      # the first element of list li
y <- list(1,2,a=4,5)
y[c(3,4)]	      # a list containing elements 3 and 4 of y
y$a		      # the element of y named a