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% File src/library/stats/man/isoreg.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2024 R Core Team
% Distributed under GPL 2 or later

\name{isoreg}
\title{Isotonic / Monotone Regression}
\alias{isoreg}
\concept{monotonic regression}
\description{
  Compute the isotonic (monotonically increasing nonparametric) least
  squares regression which is piecewise constant.
}
\usage{
isoreg(x, y = NULL)
}
\arguments{
  \item{x, y}{%in \code{isoreg},
    coordinate vectors of the regression points.  Alternatively a single
    plotting structure can be specified: see \code{\link{xy.coords}}.
    The y values, and even \code{sum(y)} must be finite, currently.
  }
}
\details{
  The algorithm determines the convex minorant \eqn{m(x)} of the
  \emph{cumulative} data (i.e., \code{cumsum(y)}) which is piecewise
  linear and the result is \eqn{m'(x)}, a step function with level
  changes at locations where the convex \eqn{m(x)} touches the
  cumulative data polygon and changes slope.\cr
  \code{\link{as.stepfun}()} returns a \code{\link{stepfun}}
  object which can be more parsimonious.
}
\value{
  \code{isoreg()} returns an object of class \code{isoreg} which is
  basically a list with components
  \item{x}{original (constructed) abscissa values \code{x}.}
  \item{y}{corresponding y values.}
  \item{yf}{fitted values corresponding to \emph{ordered} x values.}
  \item{yc}{cumulative y values corresponding to \emph{ordered} x values.}
  \item{iKnots}{integer vector giving indices where the fitted curve jumps,
    i.e., where the convex minorant has kinks.}
  \item{isOrd}{logical indicating if original x values were ordered
    increasingly already.}
  \item{ord}{\code{if(!isOrd)}: integer permutation \code{\link{order}(x)} of
    \emph{original} \code{x}.}
  \item{call}{the \code{\link{call}} to \code{isoreg()} used.}
}
\note{
  The inputs can be long vectors, but \code{iKnots} will wrap around at
  \eqn{2^{31}}{2^31}.
  
  The code should be improved to accept \emph{weights} additionally and
  solve the corresponding weighted least squares problem.\cr
  \sQuote{Patches are welcome!}
}
\references{
  \bibshow{R:Barlow+Bartholomew+Bremner:1972,
    R:Robertson+Wright+Dykstra:1988}
}
%%\author{Original C code by Brian Ripley; all else: Martin Maechler}
\seealso{the plotting method \code{\link{plot.isoreg}} with more examples;
  \code{\link[MASS]{isoMDS}()} from the \CRANpkg{MASS} package internally
  uses isotonic regression.
}
\examples{
require(graphics)

(ir <- isoreg(c(1,0,4,3,3,5,4,2,0)))
plot(ir, plot.type = "row")

(ir3 <- isoreg(y3 <- c(1,0,4,3,3,5,4,2, 3))) # last "3", not "0"
(fi3 <- as.stepfun(ir3))
(ir4 <- isoreg(1:10, y4 <- c(5, 9, 1:2, 5:8, 3, 8)))
cat(sprintf("R^2 = \%.2f\n",
            1 - sum(residuals(ir4)^2) / ((10-1)*var(y4))))

## If you are interested in the knots alone :
with(ir4, cbind(iKnots, yf[iKnots]))

## Example of unordered x[] with ties:
x <- sample((0:30)/8)
y <- exp(x)
x. <- round(x) # ties!
plot(m <- isoreg(x., y))
stopifnot(all.equal(with(m, yf[iKnots]),
                    as.vector(tapply(y, x., mean))))
}
\keyword{regression}
\keyword{smooth}