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% File src/library/methods/man/StructureClasses.Rd

% Part of the R package, https://www.R-project.org

% Copyright 1995-2025 R Core Team

% Distributed under GPL 2 or later

\name{StructureClasses}

\title{Classes Corresponding to Basic Structures}

\docType{class}

%

\alias{structure-class}

\alias{matrix-class}

\alias{array-class}

\alias{ts-class}

% Group Methods of these

\alias{Math,structure-method}

\alias{Ops,structure,vector-method}

\alias{Ops,structure,structure-method}

\alias{Ops,structure,array-method}

\alias{Ops,vector,structure-method}

\alias{Ops,array,structure-method}

\alias{Ops,array,array-method}

\alias{initialize,array-method}

\alias{initialize,matrix-method}

\alias{initialize,ts-method}

\alias{initialize,mts-method}

\alias{show,ts-method}

%

\description{

  The virtual class \code{structure} and classes that

  extend it are formal classes analogous to S language structures such

  as arrays and time-series.

}

\usage{

## The following class names can appear in method signatures,

## as the class in as() and is() expressions, and, except for

## the classes commented as VIRTUAL, in calls to new()

"matrix"

"array"

"ts"

"structure" ## VIRTUAL

}

\section{Objects from the Classes}{

  Objects can be created by calls of the form \code{new(Class, ...)},

  where \code{Class} is the quoted name of the specific class (e.g.,

  \code{"matrix"}), and the other arguments, if any, are interpreted as

  arguments to the corresponding function, e.g., to function

  \code{matrix()}.  There is no particular advantage over calling those

  functions directly, unless you are writing software designed to work

  for multiple classes, perhaps with the class name and the arguments

  passed in.

  Objects created from the classes \code{"matrix"} and \code{"array"}

  are unusual, to put it mildly, and have been for some time.  Although

  they may appear to be objects from these classes, they do not have the

  internal structure of either an S3 or S4 class object.  In particular,

  they have no \code{"class"} attribute and are not recognized as

  objects with classes (that is, both \code{\link{is.object}} and

  \code{\link{isS4}} will return \code{FALSE} for such objects).

  However, methods (both S4 and S3) can be defined for these

  pseudo-classes and new classes (both S4 and S3) can inherit from them.

  That the objects still behave as if they came from the corresponding

  class (most of the time, anyway) results from special code

  recognizing such objects being built into the base code of \R.

  For most purposes, treating the classes in the usual way will work,

  fortunately.  One consequence of the special treatment is that these

  two classes\emph{may} be used as the data part of an S4 class; for

  example, you can get away with \code{contains = "matrix"} in a call

  to \code{\link{setGeneric}} to create an S4 class that is a subclass

  of \code{"matrix"}.  There is no guarantee that everything will work

  perfectly, but a number of classes have been written in this form

  successfully.

  Note that a class containing \code{"matrix"} or \code{"array"} will

  have a  \code{.Data} slot with that class.  This is the only use of

  \code{.Data} other than as a pseudo-class indicating the type of the

  object.  In this case the type of the object will be the type of the

  contained matrix or array. See \code{\link{Classes_Details}} for a general

  discussion.

  The class \code{"ts"}  is basically an S3 class

  that has been registered with S4, using the

  \code{\link{setOldClass}} mechanism.  Versions of \R through 2.7.0

  treated this class as a pure S4 class, which was in principal a good

  idea, but in practice did not allow subclasses to be defined and had

  other intrinsic problems.  (For example, setting the

  \code{"tsp"} parameters as a slot often fails because the built-in

  implementation does not allow the slot to be temporarily

  inconsistent with the length of the data. Also, the S4 class

  prevented the correct specification of the S3 inheritance for class

  \code{"mts"}.)

  Time-series objects, in contrast to matrices and arrays, have a valid

  S3 class, \code{"ts"}, registered  using an S4-style definition (see the

  documentation for \code{\link{setOldClass}} in the examples section

  for an abbreviated listing of how this is done).  The S3

  inheritance of \code{"mts"} in package \pkg{stats} is also

  registered.

  These classes, as well as \code{"matrix"} and \code{"array"} should

  be valid in most examples as superclasses for new S4 class

  definitions.

  All of these classes have special S4 methods for

  \code{\link{initialize}} that accept the same arguments as the basic

  generator functions, \code{\link{matrix}},

  \code{\link{array}}, and \code{\link{ts}}, in so far as possible.

  The limitation is that a class that has more than one non-virtual

  superclass must accept objects from that superclass in the call to

  \code{\link{new}}; therefore, a such a class (what is called a

  \dQuote{mixin} in some languages) uses the default method for

  \code{\link{initialize}}, with no special arguments.

}

\section{Extends}{

  The specific classes all extend class \code{"structure"}, directly, and

  class \code{"vector"}, by class \code{"structure"}.

}

\section{Methods}{

  \describe{

    \item{coerce}{Methods are defined to coerce arbitrary objects to

      these classes, by calling the corresponding basic function, for

      example, \code{as(x, "matrix")} calls \code{as.matrix(x)}.

      If \code{strict = TRUE} in the call to \code{as()}, the method

      goes on to delete all other slots and attributes other than the

      \code{dim} and \code{dimnames}.

    }

    \item{Ops}{Group methods (see, e.g., \code{\link{S4groupGeneric}})

      are defined for combinations of structures and vectors (including

      special cases for array and matrix), implementing the concept of

      vector structures as in the reference.  Essentially, structures

      combined with vectors retain the structure as long as the

      resulting object has the same length.  Structures combined with

      other structures remove the structure, since there is no

      automatic way to determine what should happen to the slots

      defining the structure.

      Note that these methods will be activated when a package is loaded

      containing a class that inherits from any of the structure

      classes or class \code{"vector"}.

    }

  }

}

\seealso{Class \linkS4class{nonStructure}, which enforces the

  alternative model, in which all slots are dropped if any math

  transformation or operation is applied to an object from a class

  extending one of the basic classes.

}

\references{

  \bibinfo{R:Becker+Chambers+Wilks:1988}{footer}{For the original vector

    structures.)}

  \bibinfo{R:Chambers:1998}{footer}{(For the original S4 version.)}

  \bibinfo{R:Chambers:2008}{footer}{(For the R version.)}

  \bibshow{R:Becker+Chambers+Wilks:1988,

    R:Chambers:1998,

    R:Chambers:2008}

}

\examples{

showClass("structure")

## explore a bit :

showClass("ts")

(ts0 <- new("ts"))

str(ts0)

showMethods("Ops") # six methods from these classes, but maybe many more

}

\keyword{classes}