On 01/07/2009 9:34 PM, Rolf Turner wrote:
On 2/07/2009, at 12:20 PM, Hsiu-Khuern Tang wrote:
Is this expected behavior?
z <- 1:5
z[1] <<- 0
Error in z[1] <<- 0 : object "z" not found
The documentation seems to suggest that z will be found in the global
environment and modified accordingly.
I would agree that the documentation would *suggest* that, but it
doesn't explicitly say it, and no more it should, since the "[<<-"
operator does not exist. Unlike the "[<-" operator which does exist.
Moreover there is little reason for a "[<<-" operator to exist.
The basic raison-d'etre for the "<<-" operator is to allow one to
assign objects in the global environment from within a function.
But you shouldn't do it, even though it is possible!!!
No, I'd say that's an unfortunate compromise. The real use is for
nested evaluation: a little worker function defined within a bigger
function needs to access the parent's variables for reading (no problem,
lexical scope gives that) and writing (that's what <<- is for).
So what to do when the target of <<- doesn't exist? I'd say a better
solution than we have is for it to fail, rather than assign in the
global environment.
Duncan Murdoch
Modifying a component of an existing object in the global environment
from within a function is even more to be avoided, it seems to me. And
the designers of R seem to have drawn the line at this point.
There may be conceptual difficulties with implementing a "[<<-"
operator,
over and above the philosophical position that using such an operator
would be bad practice. I don't know. But I doubt that R Core will move
to build a "[<<-" operator, even should it prove to be possible.
The error message ``object "z" not found'' is certainly misleading.
I don't
know how hard it would be to change the code so that a more enlightening
error message gets issued. Probably not worth the effort since few
users
would ever entertain the idea of using "[<<-".
cheers,
Rolf Turner
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