Extract or Replace Parts of an Object

Description

Operators act on vectors, arrays, dataframes and lists to extract or replace subsets.

Usage

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

Details

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 [.data.frame is used for subsetting rows of a data.frame, it returns a dataframe with unique row names, using make.names( * , unique = TRUE), see the swiss example below.

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.

The operators $ and $<- do not evaluate their second argument. It is translated to a string and that string is used to locate the correct component of the first argument.

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

data(swiss)
swiss[ c(1, 1:2), ]   # duplicate row, unique row names