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.

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