\Header{Constants}{Built-in Constants}
\alias{LETTERS}{Constants}
\alias{letters}{Constants}
\alias{month.abb}{Constants}
\alias{month.name}{Constants}
\alias{pi}{Constants}
\keyword{sysdata}{Constants}
\begin{Description}\relax
Constants built into \R.\end{Description}
\begin{Usage}
\begin{verbatim}
LETTERS
letters
month.abb
month.name
pi
\end{verbatim}
\end{Usage}
\begin{Details}\relax
\R{} has a limited number of built-in constants (there is also a rather
larger library of data sets which can be loaded with the function
\code{\Link{data}}).

The following constants are available:
\Itemize{
\item \code{LETTERS}: the 26 upper-case letters of the Roman
alphabet;
\item \code{letters}: the 26 lower-case letters of the Roman
alphabet;
\item \code{month.abb}: the three-letter abbreviations for the
English month names;
\item \code{month.name}: the English names for the months of the
year;
\item \code{pi}: the ratio of the circumference of a circle to its
diameter.
}\end{Details}
\begin{SeeAlso}\relax
\code{\Link{data}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
stopifnot(
 nchar(letters) == 1,
 month.abb == substr(month.name, 1, 3)
)
eps <- .Machine$double.eps
all.equal(pi, 4*atan(1), tol= 2*eps)

# John Machin (1705) computed 100 decimals of pi :
all.equal(pi/4, 4*atan(1/5) - atan(1/239), 4*eps)
\end{ExampleCode}
\end{Examples}

