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confusion about evaluation.

2 messages · James Bullard, Brian Ripley

Original
James Bullard
# 
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. 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
Brian Ripley
#
On Sun, 20 Jul 2008, James Bullard wrote:

            

      
      
      
    

        
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).