R help - Jan 2006 - Return a Vector of Positions

Hello,

I wrote a version of which.na (similar to S's) in R that returns a 
vector of positions at which NA appears in the target vector v:

which.na<-function(v) {
  retv <- c()
  for (i in 1:length(v)) {
     if (is.na(v[i])) {
       retv<-append(i, retv, after = length(retv))
     } 
     
  }
  return(retv)
}

nv <- c(2,4,NA,NA)
nas<-which.r(nv)
print(nas)

[run]


 > source("which.r")
[1] 4 3
 >

Two questions about this:

(1) It seems that append actually "prepends"

(2) Is there a more simpler way to build the vector of positions (versus 
using append in a loop, as above), using functions such as match() and 
apply()?

Thank You.

> (2) Is there a more simpler way to build the vector of positions (versus
> using append in a loop, as above), using functions such as match() and
> apply()?
How about
which.na <- function(x) which(is.na(x))
?

Hadley