On Sun, 20 Jul 2008, James Bullard wrote:
Hi All, I am confused about the following code. I thought that the problem
stemmed from lazy evaluation and the fact that 'i' is never evaluated within
the first lapply. However, I am then confused as to why it gets bound to the
final element of the lapply.
Correct. 'i' in the environments is a promise, and it gets the value at
the time the promise is evaluated from the environment in which the
promise is created. Since you don't evaluate it until after the lapply,
the environment in which the promise is evaluated has the final value at
that time. BTW, the following variant
x <- vector("list", 4)
for(i in 1:3) x[[i]] <- function() i^2
sapply(1:3, function(i) x[[i]]())
works the same way.
Lazy evaluation works somewhat differently in R and in S -- I've never
seen a comprehensive discussion as to why (nor exactly how it works in
either system, although there is a lot about promises in the 'R Internals'
manual).
The environments of the returned functions are
indeed different in both cases and each environment has a local binding for
'i' it just happens to be 3 -- which I will say is wrong, but I await
reeducation.
I looked for documentation concerning this, but I wasn't able to find
anything -- I imagine that this must be documented somewhere as it appears
like a reasonable thing to do. Thank you as always for any insight. -- jim
## doesn't do what I think it should
x <- lapply(1:3, function(i) {
function() { i^2 }
})
sapply(1:3, function(i) x[[i]]())
[1] 9 9 9
## does what I expect
x <- lapply(1:3, function(i) {
print(i)
function() { i^2 }
})
[1] 1
[1] 2
[1] 3
sapply(1:3, function(i) x[[i]]())
[1] 1 4 9
sessionInfo()
R version 2.7.1 Patched (2008-07-20 r46088)
i386-apple-darwin9.4.0
locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
