[R] Antwort: Re: Visibility of libraries called from within functions

[G.Maubach at weinwolf.de]

Hi Duncan,

many thanks for your reply.

Your suggestion of using requireNamespace() together with explicit 
namespace calling using the "::" operator is what I was looking for:

-- cut --

f_test <- function() {
    requireNamespace("openxlsx")
    cat("Loaded packages AFTER loading library")
    print(search())
    xlsx::read.xlsx(file = "c:/temp/test.xlsx",
                    sheetName = "test")
}

cat("Loaded packages BEFORE function call ----------------------------")
search()

f_test()

cat("Loaded packages AFTER function call -----------------------------")
search()

-- cut  --

When reading ?requireNamespace I did not really get how R operates behind 
the scenes.

Using "library" attaches the namespace to the search path. Using 
"requireNamespace" does not do that.

But how does R find the namespace then? What kind of list or directory 
used R to to store the namespace and lookup the correct function or 
methods of this namespace?

Kind regards

Georg




Von:    Duncan Murdoch <murdoch.duncan at gmail.com>
An:     G.Maubach at weinwolf.de, r-help at r-project.org, 
Datum:  13.10.2016 10:43
Betreff:        Re: [R] Visibility of libraries called from within 
functions



On 13/10/2016 4:18 AM, G.Maubach at weinwolf.de wrote:
> Hi All,
>
> in my R programs I use different libraries to work with Excel sheets, i.
> e. xlsx, excel.link.
>
> When running chunks of code repeatedly and not always in the order the
> program should run for development purposes I ran into trouble. There 
were
> conflicts between the methods within these functions causing R to crash.
>
> I thought about defining functions for the different task and calling 
the
> libraries locally to there functions. Doing this test
>
> -- cut --
>
> f_test <- function() {
>     library(xlsx)
>     cat("Loaded packages AFTER loading library")
>     print(search())
> }
>
> cat("Loaded packages BEFORE function call ----------------------------")
> search()
>
> f_test()
>
> cat("Loaded packages AFTER function call -----------------------------")
> search()
>
> -- cut --
>
> showed that the library "xlsx" was loaded into the global environment 
and
> stayed there although I had expected R to unload the library when 
leaving
> the function. Thus confilics can occur more often.
>
> I had a look into ?library and saw that there is no argument telling R 
to
> hold the library in the calling environment.
>
> How can I load libraries locally to the calling functions?

You can detach at the end of your function, but that's tricky to get 
right:  the package might have been on the search list before your 
function was called.  It's better not to touch the search list at all.

The best solution is to use :: notation to get functions without putting 
them on the search list.  For example, use

xlsx::write.xlsx(data, file)

If you are not sure if your user has xlsx installed, you can use 
requireNamespace() to check.

Duncan Murdoch