silently testing for data from another package for .Rd examples

.packages(all = TRUE) will give you a list of all available packages
without really loading them like require().

Regards,
Yihui
--
Yihui Xie<xieyihui at gmail.com>
Phone: 515-294-2465 Web: http://yihui.name
Department of Statistics, Iowa State University
2215 Snedecor Hall, Ames, IA



On Wed, Aug 24, 2011 at 9:28 AM, Michael Friendly<friendly at yorku.ca>  wrote:

In an .Rd example for a package, I want to use data from another package,
but avoid loading the entire
package and avoid errors/warnings if that other package is not available.

If I don't care about loading the other package, I can just do:

if (require("ElemStatLearn", quietly=TRUE)) {
    data(prostate)
  #  rest of example
}

I'd rather just be able to do something like:

if (data(prostate, package="ElemStatLearn")) {
  #  rest of example
}

but it appears that data() doesn't return anything useful (like FALSE or
NULL) in case the named data
set doesn't exist, or the package cannot be found.  Below are some test
cases in a fresh R 2.13.1 session.

Is there someway I can incorporate such a data example silently without
errors or warnings if the
package doesn't exist, as is the case with require()?

data(prostate, package="ElemStatLearn")
dd<- data(prostate, package="ElemStatLearn")
dd

[1] "prostate"

dd2<- data(xxxxx, package="ElemStatLearn")

Warning message:
In data(xxxxx, package = "ElemStatLearn") : data set 'xxxxx' not found

dd2

[1] "xxxxx"

dd2<- data(xxxxx, package="ElemStatLearn", verbose=FALSE)

Warning message:
In data(xxxxx, package = "ElemStatLearn", verbose = FALSE) :
  data set 'xxxxx' not found

dd3<- data(zzzzz, package="foobar")

Error in find.package(package, lib.loc, verbose = verbose) :
  there is no package called 'foobar'

dd3

Error: object 'dd3' not found

try() doesn't seem to help here:

ddtry<- try(data(zzzzz, package="foobar"))

Error in find.package(package, lib.loc, verbose = verbose) :
  there is no package called 'foobar'

ddtry

[1] "Error in find.package(package, lib.loc, verbose = verbose) : \n  there
is no package called 'foobar'\n"
attr(,"class")
[1] "try-error"

--
Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA