Once statement in R i.e. evaluate if statement only once - r

Question

I am looking for a method in R to run the block inside the if statement only the first time the if statement is evaluated as TRUE, but the block would not be run again even if the if condition is TRUE again. Specifically, the method would be useful in a loop.
This would be the "once" statement (it is called so in some exotic languages).
Example:
for (id in id_list){ # runs over a list of several id's which are random
if (id == "snake"){ # I want to run this block only the first time and NOT each time id == "snake"
# now, do some calculations
# ...
}
# do some other calculations by default for all other runs inside the loop
# ...
}
I would be also curious to know how would this work in Python.

Answer 1

1) duplicated Using the test input shown in the first line iterate over an index and add a condition using duplicated. This avoids using a flag making it less error prone.
id_list <- c("a", "snake", "b", "snake") # test input
dup <- duplicated(id_list)
for(i in seq_along(id_list)) {
if (id_list[i] == "snake" && (!dup)[i]) print("snake")
print(i)
}
giving:
[1] 1
[1] "snake"
[1] 2
[1] 3
[1] 4
2) match Another approach to determine which iteration represents the first instance of snake and using that in the condition.
ix <- match("snake", id_list, nomatch = 0)
for(i in seq_along(id_list)) {
if (i == ix) print("snake")
print(i)
}
giving:
[1] 1
[1] "snake"
[1] 2
[1] 3
[1] 4
3) once
Another approach is to create a once function which returns TRUE the first time it is run and FALSE otherwise. This does use a mutable variable, x, (similar to a flag) but at least it is encapsulated. The genOnce function outputs a fresh once function.
It is important to use && in the condition to ensure that the right hand side of && is only run if the left hand side is TRUE. & does not have that short circuiting property.
genOnce <- function(x = 0) function() (x <<- x + 1) == 1
once <- genOnce()
for(id in id_list) {
if (id == "snake" && once()) print("***")
print(id)
}
giving:
[1] "a"
[1] "***"
[1] "snake"
[1] "b"
[1] "snake"

Answer 2

Suggested solution using a 'global' variable (i.e. 'flag') to denote first pass into if clause:
first <- TRUE
for (i in 1:5) {
if (first & i > 0) {
print("run this block only the first time")
first <- FALSE
}
print("do some other calculations")
}
Output:
[1] "run this block only the first time"
[1] "do some other calculations"
[1] "do some other calculations"
[1] "do some other calculations"
[1] "do some other calculations"
[1] "do some other calculations"