r - Getting a vector of differences of a vector -

suppose have vector v of length n; wish create new vector of length, contain na in first k cells, , in cell number m contain difference v[m]-v[m-k].

i can create for-loop accomplishes task:

diffs <- rep(na, length(v)) (i in k+1:length(diffs)) {     diffs[i] <- v[i] - v[i-k] } 

but i've heard loops in r slow, , looks bit cumbersome that.

actually, goal create list of diff vectors , not single 1 - 1 each k in range. loops solution?

example

on input v <- 1:5 , k=2 i'd expect output [1] na na 2 2 2 (however, turns out, output of snippet above [1] na na 2 2 2 na na...)

?diff

vec <- data.frame(v=c(1,3,5,15,21)) vec$dif <- c(na, diff(vec$v, lag = 1, differences = 1))  > vec    v dif 1  1  na 2  3   2 3  5   2 4 15  10 5 21   6 

for other lag interval or magnitude of differences, change lag , differences arguments accordingly