Extract                 package:base                 R Documentation

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

Arguments:

i,j,name: elements to extract or replace

    drop: For data frames, matrices, and arrays.  If `TRUE' the result
          is coerced to the lowest possible dimension (see examples
          below).

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

