Extract first continuous sequence in vector

Question

I have a vector:

as <- c(1,2,3,4,5,9)

I need to extract the first continunous sequence in the vector, starting at index 1, such that the output is the following:

1 2 3 4 5

Is there a smart function for doing this, or do I have to do something not so elegant like this:

a <- c(1,2,3,4,5,9)
is_continunous <- c()
for (i in 1:length(a)) {
  if(a[i+1] - a[i] == 1) {
    is_continunous <- c(is_continunous, i)
  } else {
    break
  }
}

continunous_numbers <- c()
if(is_continunous[1] == 1) {
  is_continunous <- c(is_continunous, length(is_continunous)+1)
  continunous_numbers <- a[is_continunous]
}

It does the trick, but I would expect that there is a function that can already do this.

Answer 1

It isn't clear what you need if the index of the continuous sequence only if it starts at index one or the first sequence, whatever the beginning index is.

In both case, you need to start by checking the difference between adjacent elements:

d_as <- diff(as)

If you need the first sequence only if it starts at index 1:

if(d_as[1]==1) 1:(rle(d_as)$lengths[1]+1) else NULL
# [1] 1 2 3 4 5

rle permits to know lengths and values for each consecutive sequence of same value.

If you need the first continuous sequence, whatever the starting index is:

rle_d_as <- rle(d_as)
which(d_as==1)[1]+(0:(rle_d_as$lengths[rle_d_as$values==1][1]))

---

Examples (for the second option):

as <- c(1,2,3,4,5,9) 
d_as <- diff(as)
rle_d_as <- rle(d_as)
which(d_as==1)[1]+(0:(rle_d_as$lengths[rle_d_as$values==1][1]))
#[1] 1 2 3 4 5

as <- c(4,3,1,2,3,4,5,9)
d_as <- diff(as)
rle_d_as <- rle(d_as)
which(d_as==1)[1]+(0:(rle_d_as$lengths[rle_d_as$values==1][1]))
# [1] 3 4 5 6 7

as <- c(1, 2, 3, 6, 7, 8)
d_as <- diff(as)
rle_d_as <- rle(d_as)
which(d_as==1)[1]+(0:(rle_d_as$lengths[rle_d_as$values==1][1]))
# [1] 1 2 3