`as.na` function

This seems to be consistently faster than your function:

as.na <- function(x) {
    rep(c(x[0], NA), length(x))
}

(Thanks to Joshua Ulrich for pointing out that my earlier version didn't preserve class attributes.)

---

Here, for the record, are some relative timings:

library(rbenchmark)

## The functions
flodel <- function(x) {x[] <- NA; x}
joshU <- function(x) {is.na(x) <- seq_along(x); x}
joshO <- function(x) rep(c(x[0], NA), length(x))

## Some vectors to  test them on
int  <- 1:1e6
char <- rep(letters[1:10], 1e5)
bool <- rep(c(TRUE, FALSE), 5e5)

benchmark(replications=100, order="relative",
    flodel_bool = flodel(bool),
    flodel_int  = flodel(int),
    flodel_char = flodel(char),
    joshU_bool = joshU(bool),
    joshU_int  = joshU(int),
    joshU_char = joshU(char),
    joshO_bool = joshO(bool),
    joshO_int  = joshO(int),
    joshO_char = joshO(char))[1:6]        
#          test replications elapsed relative user.self sys.self
# 7  joshO_bool          100    0.46    1.000      0.33     0.14
# 8   joshO_int          100    0.49    1.065      0.31     0.18
# 9  joshO_char          100    1.13    2.457      0.97     0.16
# 1 flodel_bool          100    2.31    5.022      2.01     0.30
# 2  flodel_int          100    2.31    5.022      2.00     0.31
# 3 flodel_char          100    2.64    5.739      2.36     0.28
# 4  joshU_bool          100    3.78    8.217      3.13     0.66
# 5   joshU_int          100    3.95    8.587      3.30     0.64
# 6  joshU_char          100    4.22    9.174      3.70     0.51

Why not use is.na<- as directed in ?is.na?

> l <- list(integer(10), numeric(10), character(10), logical(10), complex(10))
> str(lapply(l, function(x) {is.na(x) <- seq_along(x); x}))
List of 5
 $ : int [1:10] NA NA NA NA NA NA NA NA NA NA
 $ : num [1:10] NA NA NA NA NA NA NA NA NA NA
 $ : chr [1:10] NA NA NA NA ...
 $ : logi [1:10] NA NA NA NA NA NA ...
 $ : cplx [1:10] NA NA NA ...