Using data.table package inside my own package

Here is the complete recipe:

1. Add data.table to Imports in your DESCRIPTION file.

2. Add @import data.table to your respective .R file (i.e., the .R file that houses your function that's throwing the error Error in [.data.frame(x, i, j) : object 'Val' not found).

3. Type library(devtools) and set your working directory to point at the main directory of your R package.

4. Type document(). This will ensure that your NAMESPACE file includes a import(data.table) line.

5. Type build()

6. Type install()

For a nice primer on what build() and install() do, see: http://kbroman.org/pkg_primer/.

Then, once you close your R session and login next time, you can immediately jump right in with:

1. Type library("my_R_package")

2. Type the name of your function that's housed in the .R file mentioned above.

3. Enjoy! You should no longer receive the dreaded Error in [.data.frame(x, i, j) : object 'Val' not found

Andrie's guess is right, +1. There is a FAQ on it (see vignette("datatable-faq")), as well as a new vignette on importing data.table:

FAQ 6.9: I have created a package that depends on data.table. How do I ensure my package is data.table-aware so that inheritance from data.frame works?

Either i) include data.table in the Depends: field of your DESCRIPTION file, or ii) include data.table in the Imports: field of your DESCRIPTION file AND import(data.table) in your NAMESPACE file.

Further background ... at the top of [.data.table (and other data.table functions), you'll see a switch depending on the result of a call to cedta(). This stands for Calling Environment Data Table Aware. Typing data.table:::cedta reveals how it's done. It relies on the calling package having a namespace, and, that namespace Import'ing or Depend'ing on data.table. This is how data.table can be passed to non-data.table-aware packages (such as functions in base) and those packages can use absolutely standard [.data.frame syntax on the data.table, blissfully unaware that the data.frame is() a data.table, too.

This is also why data.table inheritance didn't used to be compatible with namespaceless packages, and why upon user request we had to ask authors of such packages to add a namespace to their package to be compatible. Happily, now that R adds a default namespace for packages missing one (from v2.14.0), that problem has gone away :

CHANGES IN R VERSION 2.14.0
* All packages must have a namespace, and one is created on installation if not supplied in the sources.