## Extract or Replace Parts of an Object

## Usage

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

## 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.

## 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

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