Adding NA's to a vector

Question

Let's say I have a vector of prices:

foo <- c(102.25,102.87,102.25,100.87,103.44,103.87,103.00)

I want to get the percent change from x periods ago and, say, store it into another vector that I'll call log_returns. I can't bind vectors foo and log_returns into a data.frame because the vectors are not the same length. So I want to be able to append NA's to log_returns so I can put them in a data.frame. I figured out one way to append an NA at the end of the vector:

log_returns <- append((diff(log(foo), lag = 1)),NA,after=length(foo)) 

But that only helps if I'm looking at percent change 1 period before. I'm looking for a way to fill in NA's no matter how many lags I throw in so that the percent change vector is equal in length to the foo vector

Any help would be much appreciated!

Answer 1

You could use your own modification of diff:

mydiff <- function(data, diff){
  c(diff(data, lag = diff), rep(NA, diff))
}

mydiff(foo, 1)
[1]  0.62 -0.62 -1.38  2.57  0.43 -0.87    NA

data.frame(foo = foo,  diff = mydiff(foo, 3))

     foo  diff
1 102.25 -1.38
2 102.87  0.57
3 102.25  1.62
4 100.87  2.13
5 103.44    NA
6 103.87    NA
7 103.00    NA