roll apply with window of indices

wapply.R

wapply <- function(x, window=1:10, fun=mean) {
    res <- rep(NA, length(x))
    names(res) <- names(x)
    
    i <- max(window) + 1
    
    while(i <= length(x)) { 
    
        res[i] <- fun(x[i - window])
        i <- i + 1
    }
    res
}