In the documentation of sapply and replicate there is a warning regarding using ...
Now, I can accept it as such, but would like to understand what is behind it. So I've created this little contrived example:
innerfunction<-function(x, extrapar1=0, extrapar2=extrapar1)
{
cat("x:", x, ", xp1:", extrapar1, ", xp2:", extrapar2, "\n")
}
middlefunction<-function(x,...)
{
innerfunction(x,...)
}
outerfunction<-function(x, ...)
{
cat("Run middle function:\n")
replicate(2, middlefunction(x,...))
cat("Run inner function:\n")
replicate(2, innerfunction(x,...))
}
outerfunction(1,2,3)
outerfunction(1,extrapar1=2,3)
outerfunction(1,extrapar1=2,extrapar2=3)
Perhaps I've done something obvious horribly wrong, but I find the result of this rather upsetting. So can anyone explain to me why, in all of the above calls to outerfunction, I get this output:
Run middle function:
x: 1 , xp1: 0 , xp2: 0
x: 1 , xp1: 0 , xp2: 0
Run inner function:
x: 1 , xp1: 0 , xp2: 0
x: 1 , xp1: 0 , xp2: 0
Like I said: the docs seem to warn for this, but I do not see why this is so.
?replicate, in the Examples section, tells us explicitly that what you are trying to do does not and will not work. In the Note section of ?replicate we have:
If ‘expr’ is a function call, be aware of assumptions about where
it is evaluated, and in particular what ‘...’ might refer to. You
can pass additional named arguments to a function call as
additional named arguments to ‘replicate’: see ‘Examples’.
And if we look at Examples, we see:
## use of replicate() with parameters:
foo <- function(x=1, y=2) c(x,y)
# does not work: bar <- function(n, ...) replicate(n, foo(...))
bar <- function(n, x) replicate(n, foo(x=x))
bar(5, x=3)
My reading of the docs is that they do far more than warn you about using ... in replicate() calls; they explicitly document that it does not work. Much of the discussion in that help file relates to the ... argument of the other functions, not necessarily to replicate().
If you look at the code for replicate:
> replicate
function (n, expr, simplify = TRUE)
sapply(integer(n), eval.parent(substitute(function(...) expr)),
simplify = simplify)
<environment: namespace:base>
You see that the function is evaluated in the parent frame, where the ... from your calling function no longer exists.
There actually is a way to do this:
# Simple function:
ff <- function(a,b) print(a+b)
# This will NOT work:
testf <- function(...) {
replicate(expr = ff(...), n = 5)
}
testf(45,56) # argument "b" is missing, with no default
# This will:
testf <- function(...) {
args <- as.list(substitute(list(...)))[-1L]
replicate(expr = do.call(ff, args), n = 5)
}
testf(45,56) # 101
An alternative way to do that:
g <- function(x, y) x + y
f <- function(a = 1, ...) {
arg_list <- list(...)
replicate(n = 3, expr = do.call(g, args = arg_list))
}
f(x = 1, y = 2)
