\Header{log}{Logarithms and Exponentials}
\alias{exp}{log}
\alias{expm1}{log}
\alias{log10}{log}
\alias{log1p}{log}
\alias{log2}{log}
\alias{logb}{log}
\keyword{math}{log}
\begin{Description}\relax
\code{log} computes natural logarithms,
\code{log10} computes common (i.e., base 10) logarithms, and
\code{log2} computes binary (i.e., base 2) logarithms.
The general form \code{logb(x, base)} computes logarithms with base
\code{base} (\code{log10} and \code{log2} are only special cases).

\code{log1p(x)} computes \eqn{\log(1+x)}{log(1+x)} accurately also for
\eqn{|x| \ll 1}{|x| << 1} (and less accurately when \eqn{x \approx
-1}{x is approximately -1}).

\code{exp} computes the exponential function.

\code{expm1(x)} computes \eqn{\exp(x) - 1}{exp(x) - 1} accurately also for
\eqn{|x| \ll 1}{|x| << 1}.
\end{Description}
\begin{Usage}
\begin{verbatim}
log(x, base = exp(1))
logb(x, base = exp(1))
log10(x)
log2(x)
exp(x)
expm1(x)
log1p(x)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] a numeric or complex vector.
\item[\code{base}] positive number.  The base with respect to which
logarithms are computed.  Defaults to \eqn{e}{}=\code{exp(1)}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{exp} and \code{log} are generic functions: methods can be defined
for them individually or via the \code{\Link{Math}} group generic.
\end{Details}
\begin{Value}
A vector of the same length as \code{x} containing the transformed
values.  \code{log(0)} gives \code{-Inf} (when available).
\end{Value}
\begin{Note}\relax
\code{log} and \code{logb} are the same thing in \R, but \code{logb}
is preferred if \code{base} is specified, for S-PLUS compatibility.
\end{Note}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
(for \code{log}, \code{\\log10} and \code{exp}.)

Chambers, J. M. (1998)
\emph{Programming with Data. A Guide to the S Language}.
Springer. (for \code{logb}.)
\end{References}
\begin{SeeAlso}\relax
\code{\Link{Trig}},
\code{\Link{sqrt}},
\code{\Link{Arithmetic}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
log(exp(3))
log10(1e7)# = 7

x <- 10^-(1+2*1:9)
cbind(x, log(1+x), log1p(x), exp(x)-1, expm1(x))
\end{ExampleCode}
\end{Examples}

