collatz <- function(raw_x, max_iter) {
    stopifnot("Length of x must be 1" = length(raw_x) == 1)
    x <- as.integer(raw_x)
    stopifnot("Cannot interprete x as integer" = !is.na(x))
    sequ <- c(x)
    len <- NA
    for (n in (1:(max_iter-1))) {
        if (x == 1) {
            len <- n
            break
        } else if (x %% 2 == 0) {
            x <- x / 2
        } else {
            x <- 3*x+1
        }
        sequ <- c(sequ, x)
    }
    return(list(seq=sequ, len=len))
}

str(collatz(1, 1e4))
## List of 2
##  $ seq: int 1
##  $ len: int 1
str(collatz(2, 1e4))
## List of 2
##  $ seq: int [1:2] 2 1
##  $ len: int 2
str(collatz(3, 1e4))
## List of 2
##  $ seq: int [1:8] 3 10 5 16 8 4 2 1
##  $ len: int 8
str(collatz(3, 5))
## List of 2
##  $ seq: int [1:5] 3 10 5 16 8
str(collatz("4", 1e4))
## List of 2
##  $ seq: int [1:3] 4 2 1
##  $ len: int 3
str(collatz("four", 1e4))
## Warning in collatz("four", 10000): NAs introduced by coercion
## Error in collatz("four", 10000): Cannot interprete x as integer
str(collatz(1:5, 1e4))
## Error in collatz(1:5, 10000): Length of x must be 1
str(collatz(5.0, 1e4))
## List of 2
##  $ seq: int [1:6] 5 16 8 4 2 1
##  $ len: int 6
str(collatz(5.1, 1e4))
## List of 2
##  $ seq: int [1:6] 5 16 8 4 2 1
##  $ len: int 6
str(collatz(5.9, 1e4))
## List of 2
##  $ seq: int [1:6] 5 16 8 4 2 1
##  $ len: int 6