Sequence with varying step size defined by a vector

I need to create a sequence from 1 to let’s say 100 (it doesn’t matter), with alternating step size not following any particular distribution or rule. The exact step size can only be defined by a numerical vector:

vec <- c(1,1,2,1,3,1,1,1,1,4,1)

How do I end up with the following sequence out of the above vector?

seq <- c(1,2,3,5,6,9,10,11,12,13,17,18)

The parameter by = from the seq function does not accept a vector as input, so any ideas or suggestions on how to tackle this are greatly appreciated.

>Solution :

Use cumsum:

cumsum(c(1, vec))
#[1]  1  2  3  5  6  9 10 11 12 13 17 18

I asked a similar question a year ago, and one very flexible solution is:

seq2 <- function(from, to, by) {
  
  vals <- c(0, cumsum(rep(by, abs(ceiling((to - from) / sum(by))))))
  if(from > to) return((from - vals)[(from - vals) >= to])
  else (from + vals)[(from + vals) <= to]
}

seq2(from = 1, to = sum(vec), by = vec)
#[1]  1  2  3  5  6  9 10 11 12 13 17