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\name{first}
\alias{first}
\alias{last}
\alias{first<-}
\alias{last<-}
\title{Return first or last element of an object}
\description{
Return first or last element of an object. These functions are convenience
wrappers for \code{head(x, n=1, \dots)} and \code{tail(x, n=1, \dots)}.
}
\usage{
first(x, n=1, \dots)
last(x, n=1, \dots)
first(x, n=1, \dots) <- value
last(x, n=1, \dots) <- value
}
\arguments{
\item{x}{data object}
\item{n}{a single integer. If positive, size for the resulting object:
number of elements for a vector (including lists), rows for a
matrix or data frame or lines for a function. If negative,
all but the 'n' last/first number of elements of 'x'.}
\item{\dots}{arguments to be passed to or from other methods.}
\item{value}{a vector of values to be assigned
(should be of length \code{n})}
}
\value{
An object (usually) like 'x' but generally smaller.
}
\author{
Gregory R. Warnes \email{greg@warnes.net}
}
\seealso{
\code{\link[utils]{head}},
\code{\link[utils]{tail}},
\code{\link{left}},
\code{\link{right}}
}
\examples{
## Vector
v <- 1:10
first(v)
last(v)
first(v) <- 9
v
last(v) <- 20
v
## List
l <- list(a=1, b=2, c=3)
first(l)
last(l)
first(l) <- "apple"
last(l) <- "banana"
l
## Data frame
df <- data.frame(a=1:2, b=3:4, c=5:6)
first(df)
last(df)
first(df) <- factor(c("red","green"))
last(df) <- list(c(20,30)) # note the enclosing list!
df
## Matrix
m <- as.matrix(df)
first(m)
last(m)
first(m) <- "z"
last(m) <- "q"
m
}
\keyword{manip}
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