Enclosing variables within for loop

Question

So consider the following chunk of code which does not work as most people might expect it to

#cartoon example
a <- c(3,7,11)
f <- list()

#manual ini         


        

Answer 1

This isn't a complete answer, partly because I'm not sure exactly what the question is (even though I found it quite interesting!).

Instead, I'll just present two alternative for-loops that do work. They've helped clarify the issues in my mind (in particular by helping me to understand for the first time why force() does anything at all in a call to lapply()). I hope they'll help you as well.

First, here is one that's a much closer equivalent of your properly function lapply() call, and which works for the same reason that it does:

a <- c(3,7,11)
f <- list()

## `for` loop equivalent of:
## f <- lapply(1:3, function(i) {force(i); function(x) a[i]+x})
for(i in 1:3) {
    f[[i]] <- {X <- function(i) {force(i); function(x) a[i]+x}; X(i)}
}
f[[1]](1)
# [1] 4

Second, here is one that does use local() but doesn't (strictly- or literally-speaking) rename i. It does, though, "rescope" it, by adding a copy of it to the local environment. In one sense, it's only trivially different from your functioning for-loop, but by focusing attention on i's scope, rather than its name, I think it helps shed light on the real issues underlying your question.

a <- c(3,7,11)
f <- list()

for(i in 1:3) {
   f[[i]] <- local({i<-i; function(x) a[i]+x})
}
f[[1]](1)
# [1] 4

Answer 2

Will this approach work for you?

ff<-list()
for(i in 1:3) {
    fillit <- parse(text=paste0('a[',i,']+x' ))
   ff[[i]] <- function(x) ''
   body(ff[[i]])[1]<-fillit
}

It's sort of a lower-level way to construct a function, but it does work "as you want it to."

Answer 3

An alternative for force() that would work in a for-loop local environment is

capture <- function(...) {
    vars<-sapply(substitute(...()), deparse); 
    pf <- parent.frame(); 
    Map(assign, vars, mget(vars, envir=pf, inherits = TRUE), MoreArgs=list(envir=pf))
 }

Which can then be used like

for(i in 1:3) {
    f[[i]] <- local({capture(i); function(x) a[i]+x})
}

(Taken from the comments in Josh's answer above)