[R] Return a Vector of Positions

allan miller amiller at a2software.com
Tue Jan 10 18:56:18 CET 2006


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))

nv <- c(2,4,NA,NA)


 > 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 

Thank You.

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