Detecting first iteration (cycle) in R loop (without counter) - r

Question

I'd like to detect the first iteration in a loop within a function from inside the body of the loop (i.e., without using some counter variable defined outside the loop), and in the most flexible possible manner.
Here would be one basic solution, just to demonstrate the idea:
vect = c('x', 'y', 'z')
for (elem in vect) {
print(elem)
isfirst(elem, vect)
}
isfirst = function(ele, vec) {
if (ele == vec[1]) {
print('this is the first cycle!')
} else {
print('this is NOT the first cycle!')
}
}
The "problem" with this is that I want this function to be easily reusable in any loop: that means that it should not need loop-specific arguments such as elem and vect. That is: another loop might use e.g. for (my_item in my_list) etc., and so then the isfirst arguments would need to be modified correspondingly, e.g. isfirst(my_item, my_list). The ideal way would be to just have an isfirst() without any arguments needed.
I'm not sure whether this is even possible, but I welcome any ideas.
(About why I need this: I would simply want to provide users with a function that behaves differently based on whether or not the iteration is the first, and that they can flexibly use in any loop and don't need to make even this small adjustment of changing the arguments.)

Answer 1

Well, here is the closest I could get:
vect = c('x', 'y', 'z')
for (elem in enum(vect)) {
print(elem)
isfirst()
}
enum = function(vec) {
assign("first_iteration", TRUE, envir = .GlobalEnv)
vec = mapply(c, 1:length(vec), vec, SIMPLIFY = FALSE) # this is just a small extra, not related to the question
return(vec)
}
isfirst = function() {
if (first_iteration == TRUE) {
print('this is the first cycle!')
assign("first_iteration", FALSE, envir = .GlobalEnv)
} else {
print('this is NOT the first cycle!')
}
}
But I'm still hoping for a better solution.