The R Project SVN R

\name{Extract.data.frame}

\alias{[.data.frame}

\alias{[[.data.frame}

\alias{[<-.data.frame}

\alias{[[<-.data.frame}

\alias{$<-.data.frame}

\title{Extract or Replace Parts of a Data Frame}

\description{

  Extract or replace subsets of data frames.

}

\usage{

x[i]

x[i] <- value

x[i, j, drop = TRUE]

x[i, j] <- value

x[[i]]

x[[i]] <- value

x[[i, j]]

x[[i, j]] <- value

x$name

x$name <- value

}

\arguments{

  \item{x}{data frame.}

  \item{i, j}{elements to extract or replace. \code{i, j} are

    \code{numeric} or \code{character} or, for \code{[} only, empty.

    Numeric values are coerced to integer as if by \code{\link{as.integer}}.

    For replacement by \code{[}, a logical matrix is allowed.

  }

  \item{drop}{logical.  If \code{TRUE} the result is coerced to the

    lowest possible dimension: however, see the Warning below.}

  \item{value}{A suitable replacement value: it will be repeated a whole

    number of times if necessary and it may be coerced: see the

    Coercion section.  If \code{NULL}, deletes the column if a single

    column is selected.}

  \item{name}{name or literal character string.}

}

\details{

  Data frames can be indexed in several modes.  When \code{[} and

  \code{[[} are used with a single index, they index the data frame

  as if it were a list.  In this usage a \code{drop} argument is

  ignored, with a warning.  Using \code{$} is equivalent to using

  \code{[[} with a single index.

  When \code{[} and \code{[[} are used with two indices they act

  like indexing a matrix:  \code{[[} can only be used to select one element.

  If \code{[} returns a data frame it will have unique (and non-missing)

  row names, if necessary transforming the row names using

  \code{\link{make.unique}}.  Similarly, column names

  will be transformed (if columns are selected more than once).

  When \code{drop =TRUE}, this is applied to the subsetting of any

  matrices contained in the data frame as well as to the data frame itself.

  The replacement methods can be used to add whole column(s) by specifying

  non-existent column(s), in which case the column(s) are added at the

  right-hand edge of the data frame and numerical indices must be

  contiguous to existing indices.  On the other hand, rows can be added

  at any row after the current last row, and the columns will be

  in-filled with missing values.

  For \code{[} the replacement value can be a list: each element of the

  list is used to replace (part of) one column, recycling the list as

  necessary.  If the columns specified by number are created, the names

  (if any) of the corresponding list elements are used to name the

  columns.  If the replacement is not selecting rows, list values can

  contain \code{NULL} elements which will cause the corresponding

  columns to be deleted.

  Matrixing indexing using \code{[} is not recommended, and barely

  supported.  For extraction, \code{x} is first coerced to a matrix.

  For replacement a logical matrix (only) can be used to select the

  elements to be replaced in the same ways as for a matrix.

  Missing values in the matrix are treated as false, unlike S which

  does not replace them but uses up the corresponding values in \code{value}.

}

\section{Coercion}{

  The story over when replacement values are coerced is a complicated

  one, and one that has changed during \R's development.  This section

  is a guide only.

  When \code{[} and \code{[[} are used to add or replace a whole column,

  no coercion takes place but \code{value} will be

  replicated (by calling the generic function \code{\link{rep}}) to the

  right length if an exact number of repeats can be used.

  When \code{[} is used with a logical matrix, each value is coerced to

  the type of the column in which it is to be placed.

  When  \code{[} and \code{[[} are used with two indices, the

  column will be coerced as necessary to accommodate the value.

  Note that when the replacement value is an array (including a matrix)

  it is \emph{not} treated as a series of columns (as

  \code{\link{data.frame}} and \code{\link{as.data.frame}} do) but

  inserted as a single column.

}

\section{Warning}{

  Although the default for \code{drop} is \code{TRUE}, the default

  behaviour  when only one \emph{row} is left is equivalent to

  specifying \code{drop = FALSE}.  To drop from a data frame to a list,

  \code{drop = FALSE} has to specified explicitly.

}

\value{

  For \code{[} a data frame, list or a single column (the latter two

  only when dimensions have been dropped).  If matrix indexing is used for

  extraction a matrix results.

  For \code{[[} a column of the data frame (extraction with one index)

  or a length-one vector (extraction with two indices).

  For \code{[<-}, \code{[[<-} and \code{$<-}, a data frame.

}

\seealso{

  \code{\link{subset}} which is often easier for extraction,

  \code{\link{data.frame}}, \code{\link{Extract}}.

}

\examples{

data(swiss)

sw <- swiss[1:5, 1:4]  # select a manageable subset

sw[1:3]      # select columns

sw[, 1:3]    # same

sw[4:5, 1:3] # select rows and columns

sw[1]        # a one-column data frame

sw[, 1, drop = FALSE]  # the same

sw[, 1]      # a (unnamed) vector

sw[[1]]      # the same

sw[1,]       # a one-row data frame

sw[1,, drop=TRUE]  # a list

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

sw[sw <= 6] <- 6  # logical matrix indexing

sw

## adding a column

sw["new1"] <- LETTERS[1:5]   # adds a character column

sw[["new2"]] <- letters[1:5] # ditto

sw[, "new3"] <- LETTERS[1:5] # ditto

                             # but this got converted to a factor in 1.7.x

sw$new4 <- 1:5

sapply(sw, class)

sw$new4 <- NULL              # delete the column

sw

sw[6:8] <- list(letters[10:14], NULL, aa=1:5) # delete col7, update 6, append

sw

## matrices in a data frame

A <- data.frame(x=1:3, y=I(matrix(4:6)), z=I(matrix(letters[1:9],3,3)))

A[1:3, "y"] # a matrix, was a vector prior to 1.8.0

A[1:3, "z"] # a matrix

A[, "y"]    # a matrix

}

\keyword{array}