[R-pkg-devel] using package data in package code

[Linus Chen]

Dear all,

I am trying to understand the usage of package data.
Suppose I create an object and save it
    x <- 3
    save(x, 'sysdata.rda')
and then put it into mypac/data/ directoy.
And suppose in my packge code I have
    f <- function(x) x+1
Then, when the package has been built, installed, and loaded,
the user get access to mypac::f and mypac::x, and
    y <- mypac::f(mypac::x)
will give 4.

However, if I put the same code into the package, with the purpose to
create an object mypac::y, the package would not build, with the error
message: object 'x' not found.

So my question is: Is there a mechanism for the package data to be accessed by
the package code?
I have tried to look for answer in "Writing R Extensions",
and especially, I have read section 1.1.6 "Data in packages" carefully,
but no answer for my question...

In my real package, certainly x and f is non-trivial. Basically x is
kind of a look-up table.
The 'y' that I wish to provide with the package to the user is a S4 object.
And 'f', the function am using to create 'y', is itself a non-trivial
function defined in the package.

The work-around I can figure out are the following two:
1. put 'x' in a separate package, which 'mypac' will import.
    Then the package code "y <- f(x)".
    But it feels bad to have two packages for one integral set of information.

2. Do the whole thing in two pass, in the first round the package
contain only x anf f.
    Then, load the package, run y<-f(x), and add it to 'sysdata.rda'.
    But this way, the two-pass operation has to be repeated each time
the package is modified,
    even if the change does not affect 'x', and only affect 'f'.

So is there a more elegant way to deal with this?

I would be grateful to any hints, or useful links. Thank you!

Best regards,
Lei Chen