On 04/04/2023 9:56 a.m., akshay kulkarni wrote:
Dear Members,
I have the following code typed at the console
prompt:
y <- x*10
X has not been defined and the above code throws an object not found error.
That is, the global environment does not contain x. Why doesn't it look further
in the environment stack, like that of packages? There are thousands of
packages that contain the variable named x. Of course, that happens if the
above code is in a function (or does it?).
What concept of R is at work in this dichotomy?
First, some background:
Packages are associated with multiple environments. There is the
internal one that the package sees, and the external one that contains
just the exports.
These are sometimes called the "package" environment and the "namespace"
environment, but I don't think those names are used consistently.
(There's another one containing the imports, but for these purposes,
it's indistinguishable from the internal one.)
When a package is loaded by loadNamespace("pkg"), nothing happens in the
global environment: no new variables are visible.
When it is attached by library("pkg"), a lot more happens. First, it is
loaded as above, then the search list is modified. The global
environment is entry 1 in the search list; it stays there, but its
"parent" is set to a copy of the external environment from the new
package, and the parent of that environment is set to the previous
parent, the second entry in the search list.
Okay, so now you search for "x" in the global environment, and it's not
there. It then goes to the other entries in the search list, which are
typically external environments from various packages. None of those
packages export "x", so it is not found.
It doesn't matter if those packages use "x" without exporting it,
because R won't look at internal environments in this kind of search.
And it doesn't matter what happens in other packages that are not on the
search list (i.e. not "attached" because you never called library() or
require() on them), because they just aren't in the chain of
environments where R looks.
Duncan Murdoch
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