add head() and tail(), improved from code by Pat Burns

git-svn-id: https://svn.r-project.org/R/trunk@28111 00db46b3-68df-0310-9c12-caf00c1e9a41

NEWS

@@ -211,6 +211,9 @@ NEW FEATURES
     o	princomp() now warns if both `x' and `covmat' are supplied,
 	and returns scores only if the centring used is known.
 
+    o	New functions head() and tail() in package `utils'.
+	(Based on a contribution by Patrick Burns.)
+
 
 UTILITIES
 

src/library/utils/R/head.R

@@ -0,0 +1,33 @@
+### placed in the public domain 2002
+### Patrick Burns patrick@burns-stat.com
+
+head <- function(x, ...) UseMethod("head")
+
+head.default <- function(x, n=6)
+{
+    ans <- x[seq(len=min(n, length(x)))]
+    if(length(dim(x)) == 1) array(ans, n, list(names(ans))) else ans
+}
+
+head.data.frame <- head.matrix <- function(x, n=6)
+    x[seq(len=min(n, nrow(x))), , drop=FALSE]
+
+head.function <- function(x, n=6) head(deparse(x), n=n)
+
+tail <- function(x, ...) UseMethod("tail")
+
+tail.default <- function(x, n=6)
+{
+    xlen <- length(x)
+    n <- min(n, xlen)
+    ans <- x[seq(to=xlen, length=n)]
+    if(length(dim(x)) == 1) array(ans, n, list(names(ans))) else ans
+}
+
+tail.data.frame <- tail.matrix <- function(x, n=6)
+{
+    nrx <- nrow(x)
+    x[seq(to=nrx, length=min(n, nrx)), , drop=FALSE]
+}
+
+tail.function <- function(x, n=6) tail(deparse(x), n=n)

src/library/utils/man/head.Rd

@@ -0,0 +1,56 @@
+\name{head}
+\alias{head}
+\alias{head.default}
+\alias{head.data.frame}
+\alias{head.function}
+\alias{head.matrix}
+\alias{tail}
+\alias{tail.default}
+\alias{tail.data.frame}
+\alias{tail.function}
+\alias{tail.matrix}
+
+\title{
+  Return the First or Last Part of an Object
+}
+\description{
+  Returns the first or last parts of a vector, matrix, data frame or function.
+}
+\usage{
+head(x, \dots)
+\method{head}{default}(x, n=6)
+\method{head}{data.frame}(x, n=6)
+
+tail(x, \dots)
+\method{tail}{default}(x, n=6)
+\method{tail}{data.frame}(x, n=6)
+}
+\arguments{
+  \item{x}{an object}
+  \item{n}{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.}
+}
+\details{
+  For matrices and data frames, the first/last \code{n} rows are returned.
+  For functions, the first/last \code{n} lines of the deparsed function are
+  returned as character strings.
+}
+\value{
+  An object (usually) like \code{x} but generally smaller.
+}
+\author{
+  Patrick Burns, improved and corrected by R-core
+}
+
+\examples{
+data(freeny)
+head(freeny.x, n=10)
+head(freeny.y)
+
+tail(freeny.x)
+tail(freeny.y)
+
+tail(library)
+}
+\keyword{ manip }