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% File src/library/base/man/NumericConstants.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2008 R Core Development Team
% Distributed under GPL 2 or later

\name{NumericConstants}
\alias{NumericConstants}
\title{Numeric Constants}
\description{
  How \R parses numeric constants.
}
\details{
  \R parses numeric constants in its input in a very similar way to C99
  floating-point constants.

  \code{\link{Inf}} and \code{\link{NaN}} are numeric constants (with
  \code{\link{typeof}(.) "double"}).  These are recognized ignoring case,
  as is \code{infinity} as an alternative to \code{Inf}.
  \code{\link{NA_real_}} and \code{\link{NA_integer_}} are constants of
  types \code{"double"} and \code{"integer"} representing missing
  values.  All other numeric constants start with a digit or period and
  are either a decimal or hexadecimal constant optionally followed by
  \code{L}.

  Hexadecimal constants start with \code{0x} or \code{0X} followed by
  a nonempty sequence from \code{0-9 a-f A-F .} which is interpreted as a
  hexadecimal number, optionally followed by a binary exponent.  A binary
  exponent consists of an \code{P} or \code{p} followed by an optional
  plus or minus sign followed by a non-empty sequence of (decimal)
  digits, and indicates multiplication by a power of two.  Thus
  \code{0x123p456} is \eqn{291 \times 2^{456}}{291 * 2^456}.

  Decimal constants consist of a nonempty sequence of digits possibly
  containing a period (the decimal point), optionally followed by a
  decimal exponent.  A decimal exponent consists of an \code{E} or
  \code{e} followed by an optional plus or minus sign followed by a
  non-empty sequence of digits, and indicates multiplication by a power
  of ten.

  Values which are too large or too small to be representable will
  overflow to \code{Inf} or underflow to \code{0.0}.

  A numeric constant immediately followed by \code{i} is regarded as an
  imaginary \link{complex} number.

  An numeric constant immediately followed by \code{L} is regarded as an
  \code{\link{integer}} number when possible (and with a warning if it
  contains a \code{"."}).

  Only the ASCII digits 0--9 are recognized as digits, even in languages
  which have other representations of digits.  The \sQuote{decimal
    separator} is always a period and never a comma.

  Note that a leading plus or minus is not regarded by the parser as
  part of a numeric constant but as a unary operator applied to the constant.
}
\seealso{
  \code{\link{Syntax}}.

  \code{\link{Quotes}} for the parsing of character constants,
}
\examples{
2.1
typeof(2)
sqrt(1i) # remember elementary math?
utils::str(0xA0)
identical(1L, as.integer(1))

## You can combine the "0x" prefix with the "L" suffix :
identical(0xFL, as.integer(15)) # (with a regard to Fritz :-)
}
\keyword{documentation}