[R] Help with non standard evaluation and require function

[Luca Cerone]

Hi David and Duncan,
thanks for your answers!

I think what is not clear to me is actually how "substitute" works.

If I run require (dplyr) or require("dplyr") in the R console everything
works as I expect even without the character.only=T (actually because of
this I always interpreted that character.only=F means you can either use
nse or strings while with character.only=T you can only use strings).

What I don't understand is why in the require function

as.character (substitute(package))

returns "pkgname" (the name of the variable I use in my function) rather
than substituting the value of pkgname i.e. dplyr in my example.

I have no access to my laptop so I can't double check but I think in one of
Wickham's book there was an example like

f <- function (y) {
  substitute (x + y)
}

f(4)
[1] x + 4

i.e. where substitute inside a function was substituting the value of y and
returned the expression replacing y with 4, which is what I would expect to
happen.

It is probaby a very trivial problem but I find hard to figure out ho
substitute works.

Thanks a lot again for the help!
Cheers,
Luca

On Aug 12, 2016 20:14, "David Winsemius" <dwinsemius at comcast.net> wrote:

>
> > On Aug 12, 2016, at 8:57 AM, Luca Cerone <luca.cerone at gmail.com> wrote:
> >
> > Hi everybody,
> > I am having a hard time in understanding how to deal with non standard
> > evaluation and the require function.
> >
> > I asked about it on Stackoverflow at
> > http://stackoverflow.com/questions/38922012/r-function-
> to-install-missing-packages,
> > below you can find my question.
>
> It was already explained and teh code of `require provided: `substitute`
> does not lookup values in the symbol table so the symbol: `pkgname` is
> converted by `as.character` to "pkgname", .... unless 'character.only' is
> TRUE.
>
> What part of that is not understood?
>
> --
> David.
> >
> > Thanks a lot for the help!
> > Cheers,
> > Luca
> >
> > For one of my scripts I want to write an R function that checks if a
> > package is already installed: if so it should use library() to import
> > it in the namespace, otherwise it should install it and import it.
> >
> > I assumed that pkgname is a string and tried to write something like:
> >
> > ensure_library <- function(pkgname) {
> >  if (!require(pkgname)) {
> >    install.packages(pkgname, dependencies = TRUE)
> >  }
> >  require(pkgname)
> > }
> >
> > As simple as is this function does not work. If I try to run it like
> > ensure_library("dplyr") it installs the package dplyr but then it
> > fails because it trys to import pkgname rather than dplyr in the
> > namespace.
> >
> > ensure_library("dplyr")
> > Loading required package: pkgname
> > Installing package into ‘/home/luca/R-dev’
> > (as ‘lib’ is unspecified)
> > trying URL 'https://cran.rstudio.com/src/contrib/dplyr_0.5.0.tar.gz'
> > Content type 'application/x-gzip' length 708476 bytes (691 KB)
> > ==================================================
> > downloaded 691 KB
> >
> > * installing *source* package ‘dplyr’ ...
> > ** package ‘dplyr’ successfully unpacked and MD5 sums checked
> > ** libs
> >
> > .... a lot of compiling here....
> >
> > installing to /home/luca/R-dev/dplyr/libs
> > ** R
> > ** data
> > *** moving datasets to lazyload DB
> > ** inst
> > ** preparing package for lazy loading
> > ** help
> > *** installing help indices
> > ** building package indices
> > ** installing vignettes
> > ** testing if installed package can be loaded
> > * DONE (dplyr)
> >
> > The downloaded source packages are in
> >    ‘/tmp/Rtmpfd2Lep/downloaded_packages’
> > Loading required package: pkgname
> > Warning messages:
> > 1: In library(package, lib.loc = lib.loc, character.only = TRUE,
> > logical.return = TRUE,  :
> >  there is no package called ‘pkgname’
> > 2: In library(package, lib.loc = lib.loc, character.only = TRUE,
> > logical.return = TRUE,  :
> >  there is no package called ‘pkgname’
> >
> > Also, if I now re-run it it will install dplyr once again.
> >
> > I realize this is probably due to R non-standard-evaluation and I have
> > tried several combination of eval/substitute/quote in order to make it
> > work with require but I couldn't succeed.
> >
> > Can somebody help me understanding what is going on and if there is
> > some easy-fix?
> >
> > If a function already implementing this exists I would like to know,
> > but what I am really interested is understanding why my code does not
> > work as intended.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius
> Alameda, CA, USA
>
>

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