Full_Name: Keith Bare
Version: 2.7.1
OS: Linux
Submission from: (NULL) (128.2.134.48)
I observed unexpected behavior attempting to use lapply to vary parameters in
generated closures. All the generated closures ran with the last parameter
value in the list.
Here's a simple example:
> funcs <- lapply(c("alpha", "beta", "gamma", "delta"), function(x) function()
print(x))
> funcs[[1]]()
[1] "delta"
> funcs[[2]]()
[1] "delta"
> funcs[[3]]()
[1] "delta"
> funcs[[4]]()
[1] "delta"
What appears to be happening, is that the unevaluated promise for x is getting
stored in the generated closure. This promise references a variable in the
local environment for the lapply call. However, that variable gets clobbered by
subsequent processing by lapply.
This may be a language "feature" rather than a bug. But IMO, the observed
behavior is very non-intuitive. If so, maybe it deserves mention in the
documentation for lapply, "function", or the language reference section on
functions.
For now, I've found that adding a force in the function that generates the
returned closure is a workaround. E.g.:
> funcs <- lapply(c("alpha", "beta", "gamma", "delta"), function(x) { force(x);
function() print(x) })
> funcs[[1]]()
[1] "alpha"
> funcs[[2]]()
[1] "beta"
> funcs[[3]]()
[1] "gamma"
> funcs[[4]]()
[1] "delta"
And here's my version output (I'm running the version in Debian lenny):
> version
_
platform x86_64-pc-linux-gnu
arch x86_64
os linux-gnu
system x86_64, linux-gnu
status
major 2
minor 7.1
year 2008
month 06
day 23
svn rev 45970
language R
version.string R version 2.7.1 (2008-06-23)
--Keith
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
