library                 package:base                 R Documentation

Loading and Listing of Packages

Description:

     'library' and 'require' load add-on packages.

     '.First.lib' is called when a package is loaded; '.Last.lib' is
     called when a package is detached.

     '.packages' returns information about package availability.

     '.path.package' returns information about where a package was
     loaded from.

     '.find.package' returns the directory paths of installed packages.

Usage:

     library(package, help, pos = 2, lib.loc = NULL, character.only = FALSE,
             logical.return = FALSE, warn.conflicts = TRUE,
             keep.source = getOption("keep.source.pkgs"),
             verbose = getOption("verbose"), version)
     require(package, quietly = FALSE, warn.conflicts = TRUE,
             keep.source = getOption("keep.source.pkgs"),
             character.only = FALSE, version, save = TRUE)

     .First.lib(libname, pkgname)
     .Last.lib(libpath)

     .packages(all.available = FALSE, lib.loc = NULL)
     .path.package(package = .packages(), quiet = FALSE)
     .find.package(package, lib.loc = NULL, quiet = FALSE,
                   verbose = getOption("verbose"))
     .libPaths(new)

     .Library
     .Autoloaded

Arguments:

package, help: the name of a package, given as a name or literal
          character string, or a character string, depending on whether
          'character.only' is 'FALSE' (default) or 'TRUE').

     pos: the position on the search list at which to attach the loaded
          package.  Note that '.First.lib' may attach other packages,
          and 'pos' is computed _after_ '.First.lib' has been run.  Can
          also be the name of a position on the current search list as
          given by 'search()'.

 lib.loc: a character vector describing the location of R library trees
          to search through, or 'NULL'.  The default value of 'NULL'
          corresponds to all libraries currently known.

character.only: a logical indicating whether 'package' or 'help' can be
          assumed to be character strings.

 version: A character string denoting a version number of the package
          to be loaded.  If no version is given, a suitable default is
          chosen.

logical.return: logical.  If it is 'TRUE',  'FALSE' or 'TRUE' is
          returned to indicate success.

warn.conflicts: logical.  If 'TRUE', warnings are printed about
          'conflicts' from attaching the new package, unless that
          package contains an object '.conflicts.OK'.

keep.source: logical.  If 'TRUE', functions "keep their source"
          including comments, see argument 'keep.source' to 'options'.

 verbose: a logical.  If 'TRUE', additional diagnostics are printed.

 quietly: a logical.  If 'TRUE', no message confirming package loading
          is printed.

    save: logical or environment.  If 'TRUE', a call to 'require' from
          the source for a package will save the name of the required
          package in the variable '".required"', allowing function
          'detach' to warn if a required package is detached. See
          section 'Packages that require other packages' below.

 libname: a character string giving the library directory where the
          package was found.

 pkgname: a character string giving the name of the package.

 libpath: a character string giving the complete path to the package.

all.available: logical; if 'TRUE' return a character vector of all
          available packages in 'lib.loc'.

   quiet: logical. For '.path.package', should this not give warnings
          or an error if the package(s) are not loaded? For
          '.find.package', should this not give warnings or an error if
          the package(s) are not found?

     new: a character vector with the locations of R library trees.

Details:

     'library(package)' and 'require(package)' both load the package
     with name 'package'.  'require' is designed for use inside other
     functions; it returns 'FALSE' and gives a warning (rather than an
     error as 'library()' does) if the package does not exist.  Both
     functions check and update the list of currently loaded packages
     and do not reload code that is already loaded.

     For large packages, setting 'keep.source = FALSE' may save quite a
     bit of memory.

     If 'library' is called with no 'package' or 'help' argument, it
     lists all available packages in the libraries specified by
     'lib.loc', and returns the corresponding information in an object
     of class '"libraryIQR"'.  The structure of this class may change
     in future versions.  In earlier versions of R, only the names of
     all available packages were returned; use '.packages(all = TRUE)'
     for obtaining these.  Note that 'installed.packages()' returns
     even more information.

     'library(help = somename)' computes basic information about the
     package 'somename', and returns this in an object of class
     '"packageInfo"'.  The structure of this class may change in future
     versions.  When used with the default value ('NULL') for
     'lib.loc', the loaded packages are searched before the libraries.

     '.First.lib' is called when a package is loaded by 'library'.  It
     is called with two arguments, the name of the library directory
     where the package was found (i.e., the corresponding element of
     'lib.loc'), and the name of the package (in that order).  It is a
     good place to put calls to 'library.dynam' which are needed when
     loading a package into this function (don't call 'library.dynam'
     directly, as this will not work if the package is not installed in
     a "standard" location).  '.First.lib' is invoked after the search
     path interrogated by 'search()' has been updated, so
     'as.environment(match("package:name", search()))' will return the
     environment in which the package is stored.  If calling
     '.First.lib' gives an error the loading of the package is
     abandoned, and the package will be unavailable.  Similarly, if the
     option '".First.lib"' has a list element with the package's name,
     this element is called in the same manner as '.First.lib' when the
     package is loaded.  This mechanism allows the user to set package
     "load hooks" in addition to startup code as provided by the
     package maintainers.

     '.Last.lib' is called when a package is detached.  Beware that it
     might be called if '.First.lib' has failed, so it should be
     written defensively. (It is called within 'try', so errors will
     not stop the package being detached.)

     '.packages()' returns the "base names" of the currently attached
     packages _invisibly_ whereas '.packages(all.available = TRUE)'
     gives (visibly) _all_ packages available in the library location
     path 'lib.loc'.

     '.path.package' returns the paths from which the named packages
     were loaded, or if none were named, for all currently loaded
     packages. Unless 'quiet = TRUE' it will warn if some of the
     packages named are not loaded, and given an error if none are. 
     This function is not meant to be called by users, and its
     interface might change in future versions.

     '.find.package' returns the paths to the locations where the given
     packages can be found.  If 'lib.loc' is 'NULL', then then attached
     packages are searched before the libraries.  If a package is found
     more than once, the first match is used.  Unless 'quiet = TRUE' a
     warning will be given about the named packages which are not
     found, and an error if none are.  If 'verbose' is true, warnings
     about packages found more than once are given.  This function is
     not meant to be called by users, and its interface might change in
     future versions.

     '.Autoloaded' contains the "base names" of the packages for which
     autoloading has been promised.

     '.Library' is a character string giving the location of the
     default library, the 'library' subdirectory of 'R_HOME'.
     '.libPaths' is used for getting or setting the library trees that
     R knows about (and hence uses when looking for packages).  If
     called with argument 'new', the library search path is set to the
     existing files in 'unique(new, .Library)' and this is returned. 
     If given no argument, a character vector with the currently known
     library trees is returned.

     The library search path is initialized at startup from the
     environment variable 'R_LIBS' (which should be a colon-separated
     list of directories at which R library trees are rooted) by
     calling '.libPaths' with the directories specified in 'R_LIBS'.

Value:

     'library' returns the list of loaded (or available) packages (or
     'TRUE' if 'logical.return' is 'TRUE'). 'require' returns a logical
     indicating whether the required package is available.

Packages that require other packages:

     The source code for a package that requires one or more other
     packages should have a call to 'require', preferably near the
     beginning of the source, and of course before any code that uses
     functions, classes or methods from the other package.  The default
     for argument 'save' will save the names of all required packages
     in the environment of the new package.  The saved package names
     are used by 'detach' when a package is detached to warn if other
     packages still require the detached package.  Also, if a package
     is installed with saved image (see INSTALL), the saved package
     names are used to require these packages when the new package is
     attached.

Formal methods:

     'library' takes some further actions when package 'methods' is
     attached (as it is by default).  Packages may define formal
     generic functions as well as re-defining functions in other
     packages (notably 'base') to be generic, and this information is
     cached whenever such a package is loaded after 'methods' and
     re-defined functions are excluded from the list of conflicts.  The
     check requires looking for a pattern of objects; the pattern
     search may be avoided by defining an object '.noGenerics' (with
     any value) in the package.  Naturally, if the package _does_ have
     any such methods, this will prevent them from being used.

Note:

     'library' and 'require' can only load an _installed_ package, and
     this is detected by having a 'DESCRIPTION' file containing a
     'Built:' field.  Packages installed prior to 1.2.0 (released in
     December 2000) will need to be re-installed.

     Under Unix-alikes, the code checks that the package was installed
     under a similar operating system as given by 'R.version$platform'
     (the canonical name of the platform under which R was compiled),
     provided it contains compiled code.  Packages which do not contain
     compiled code can be shared between Unix-alikes, but not to other
     OSes because of potential problems with line endings and
     OS-specific help files.

     'library' and 'require' use the underlying file system services to
     locate the libraries, with the result that on case-sensitive file
     systems package names are case-sensitive (i.e., 'library(foo)' is
     different from 'library(Foo)'), but they are not distinguished on
     case-insensitive file systems such as MS Windows.  A warning is
     issued if the user specifies a name which isn't a perfect match to
     the package name, because future versions of R will require exact
     matches.

Author(s):

     R core; Guido Masarotto for the 'all.available=TRUE' part of
     '.packages'.

References:

     Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) _The New S
     Language_. Wadsworth & Brooks/Cole.

See Also:

     'attach', 'detach', 'search', 'objects', 'autoload',
     'library.dynam', 'data', 'install.packages' and
     'installed.packages'; 'INSTALL', 'REMOVE'.

Examples:

     (.packages())               # maybe just "base"
     .packages(all = TRUE)       # return all available as character vector
     library()                   # list all available packages
     library(lib = .Library)     # list all packages in the default library
     library(help = eda)         # documentation on package 'eda'
     library(eda)                # load package 'eda'
     require(eda)                # the same
     (.packages())               # "eda", too
     detach("package:eda")

     # if the package name is in a character vector, use
     pkg <- "eda"
     library(pkg, character.only = TRUE)
     detach(pos = match(paste("package", pkg, sep=":"), search()))

     require(pkg, character.only = TRUE)
     detach(pos = match(paste("package", pkg, sep=":"), search()))

     .path.package()
     .Autoloaded                 # maybe "ctest"

     .libPaths()                 # all library trees R knows about

     require(nonexistent)        # FALSE
     ## Not run: 
     ## Suppose a package needs to call a shared library named 'fooEXT',
     ## where 'EXT' is the system-specific extension.  Then you should use
     .First.lib <- function(lib, pkg) {
       library.dynam("foo", pkg, lib)
     }

     ## if you want to mask as little as possible, use
     library(mypkg, pos = "package:base")
     ## End(Not run)

