Add dependencies for my Rcpp package

Question

Rcpp beginner's question:

I want to improve my execution efficiency in R. So I write some code in cpp and use Rcpp to help me compile them.

Question is that I use some other R packages in my .cpp files and I want those packages to be installed and imported automatically when a user installs my package.

e.g. If I use the R package 'gtools' in my files, I don't want the error:

* installing to library 'C:/Program Files/R/R-3.4.1/library'
* installing *source* package 'pkgname' ...
make: Nothing to be done for `all`.
** libs
installing to C:/Program Files/R/R-3.4.1/library/pkgname/libs/i386
** R
** preparing package for lazy loading
Error in library(gtools) : there is no package called 'gtools'
Error : unable to load R code in package 'pkgname'
ERROR: lazy loading failed for package 'pkgname'
* removing 'C:/Program Files/R/R-3.4.1/library/pkgname'

Exited with status 1.

I tried to add depended package name to the DESCRIPTION file. i.e.

Imports: Rcpp (>= 0.12.12),gtools
LinkingTo: Rcpp, gtools

But it gives me following error:

ERROR: dependency 'gtools' is not available for package 'pkgname'

I don't find any similar questions and please tell me if there are.

Answer 1

First, you should probably make sure gtools is installed on your system. I say this because of the following error:

Error in library(gtools) : there is no package called 'gtools'

With this being said, the main issue you are running into is uncertainty between the LinkingTo: and Imports: fields in the DESCRIPTION file. This is covered in Section 1.1.3: Package Dependencies of Writing R Extensions.

Specifically, we have:

The ‘Imports’ field lists packages whose namespaces are imported from (as specified in the NAMESPACE file) but which do not need to be attached. Namespaces accessed by the ‘::’ and ‘:::’ operators must be listed here, or in ‘Suggests’ or ‘Enhances’ (see below). Ideally this field will include all the standard packages that are used, and it is important to include S4-using packages (as their class definitions can change and the DESCRIPTION file is used to decide which packages to re-install when this happens). Packages declared in the ‘Depends’ field should not also be in the ‘Imports’ field. Version requirements can be specified and are checked when the namespace is loaded (since R >= 3.0.0).

And the LinkingTo field:

A package that wishes to make use of header files in other packages needs to declare them as a comma-separated list in the field ‘LinkingTo’ in the DESCRIPTION file. For example

LinkingTo: link1, link2

The ‘LinkingTo’ field can have a version requirement which is checked at installation.

Specifying a package in ‘LinkingTo’ suffices if these are C++ headers containing source code or static linking is done at installation: the packages do not need to be (and usually should not be) listed in the ‘Depends’ or ‘Imports’ fields. This includes CRAN package BH and almost all users of RcppArmadillo and RcppEigen.

For another use of ‘LinkingTo’ see Linking to native routines in other packages.

So, the Imports: is meant to specify packages that contain R functions that you wish to import. In particular, the function from a given package or the entire package itself must be specified in the NAMESPACE file. For packages that use Rcpp, you can typically expect R functions to be available if the author has exported the routine from C++.

Now, regarding the LinkingTo:, this is a bit more specific. If an author wishes to make available a C++ API via header files they must explicitly declare the statements as is given in native methods of Writing R Extensions. Generally, packages that proceed in this manner are "header-only". These packages place the header definitions under inst/include, e.g.

|- pkgname
   |- inst/
      |- include/
         |- pkgname.h
   |- R/
   |- man/
   |- DESCRIPTION
   |- NAMESPACE

However, another trend is to allow for "non-header" packages. This leads to a bit more complicated of topic as you have to understand shared objects and dynamic libraries. CRAN presents an overview of how to "Link" packages in Section 5.8: Linking to other packages of Writing R Extensions

If the author does not make available a C++ API, then there are four options:

1. Ask the author nicely to support calling the C++ API or submit a patch that enables access to the C++ API.
2. Call an R function from C++. (This negates any performance gain from writing your code in C++ though.)
3. Copy the implementation from the author's package while respecting their intellectual property.
4. Implement the desired functionality from scratch to avoid licensing issues.

Unfortunately, this is the case for gtools. As the author(s) do not provide a means to "link" to the C++ version of package's code.