Well, for loop structure might be the most common control structure we used in R programming. The code normally looks like this:

for (i in 1:length(x)) {}

It works well for most of the case, how ever when the x vector is empty, 1:length(x) will be (1,0) , so the program will have an error. A better way to handle this is using seq() function.

for (i in seq(x)) {}

And let’s see how the seq() function handle the empty vector.

> x <- c(4, 10)
> seq(x)
[1] 1 2
> x <- NULL
> seq(x)
integer(0)

The seq() function gives the same result as the length() function, but correctly evaluates to NULL, if x is empty, resulting in zero iteration in the loop.

*Reference: The Art of R Programming by Norman Matloff*

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