I want to pass arguments to a function that aren't to be evaluated until they're used within the function. As a trivial example, how would I go about if I wanted to pass ... = foo, 'bar' to the following function, while foo is defined within the function:
myfunc <- function(X, ...) {
for (foo in seq_along(X)) {
cat(..., '\n')
}
}
myfunc(X = 1:5, foo, 'bar')
I tried using cat(substitute(...), '\n'), which seems to merely omit foo.
Thanks!
1) Use eval(substitute(...))
myfunc <- function(X, ...) {
for (foo in seq_along(X)) {
eval(substitute(cat(..., "\n")))
}
}
myfunc(X = 1:5, foo, 'bar')
giving:
1 bar
2 bar
3 bar
4 bar
5 bar
2) defmacro Another approach is to create a macro using defmacro in gtools:
library(gtools)
myfunc2 <- defmacro(X, DOTS, expr = {
for (foo in seq_along(X)) {
cat(..., "\n")
}
})
myfunc2(X = 1:5, foo, 'bar')
Here's one way
myfunc <- function(X, ...) {
dots <- eval(substitute(alist(...)))
for (foo in seq_along(X)) {
do.call("cat", c(dots, '\n'))
}
}
myfunc(X = 1:5, foo, 'bar')
# 1 bar
# 2 bar
# 3 bar
# 4 bar
# 5 bar
we capture all the parameters passed to the ... in an unevaluated pair list. We then use "do.call" to inject those values into a call to cat() and evaluate in the local environment.
This would work as long as you don't need to do something like
test <- function() {
bar <- "bar"
myfunc(X = 1:5, foo, bar)
}
test()
# Error in cat(foo, bar, "\n") : object 'bar' not found