Question

I want to loop over a range of numbers from 1:n, when n is the length of vector v in R. Usually, I would use the for (i in 1:length(v)) syntax, but this fails when n == 0.

What is the idiomatic way to do this loop? At the moment I do the followin, but it seems a little ugly:

# This is in my standard library
rng <- function(n)seq(from=1, to=n, length.out=n)

# Now when I come to the for loop:
for(i in rng(length(v))){
   print(paste("I ate", i, "kg of brocolli today"))
}

And yes, I know its better to vectorise, but there are situations when vectorisation isn’t possible or would require so much extra work that it is much harder to read the code.

Answer 1

It would be better to use seq_along:

> v <- letters[1:3]
> for (i in seq_along(v)) print(c(i, v[i]))
[1] "1" "a"
[1] "2" "b"
[1] "3" "c"
> 
> v <- numeric(0)
> for (i in seq_along(v)) print(c(i, v[i]))