R fill vector efficiently

Question

I have a fairly big vector (>500,000 in length). It contains a bunch of NA interspersed with 1 and it is always guaranteed that it begins with 1<

Answer 1

A vectorised solution would look like:

v1[-1] <- ifelse(diff(v2), 0, v1[-length(v1)])

But the above won't work, and I don't think you can avoid an explicit loop since, if I understand correctly, you want to propagate new values. So, how about:

cmp <- diff(v2)
for (i in 2:length(v1)){
    v1[i] <- if(cmp[i-1]) 0 else v1[i-1]
}