How to break up a vector into contiguous groups in R

Question

I am curious if there is a functional way to break up a vector of contiguous groups in R.

For example if I have the following

# start with this
vec <- c(1,2,3,4,6,7,8,30,31,32)

# end with this
vec_vec <- list(c(1, 2, 3, 4),c(6,7,8),c(30,31,32))
print(vec_vec[1])

I can only imaging this is possible with a for loop where I calculate differences of previous values, but maybe there are better ways to solve this.

score 7 · Accepted Answer · answered Aug 10 '22 at 14:50

7

split(vec, vec - seq_along(vec)) |> unname()
# [[1]]
# [1] 1 2 3 4
# 
# [[2]]
# [1] 6 7 8
# 
# [[3]]
# [1] 30 31 32

Subtratcing seq_along(vec) turns contiguous sequences into single values, which we can split by.

answered Aug 10 '22 at 14:50

Gregor Thomas

1

Clever, I was going to try diff but this is more elegant and efficient – Ottie Aug 10 '22 at 15:35
Thanks @Ottie, I've given more-or-less this answer before, but I couldn't find it to mark it as a duplicate. While I do like this version and find it elegant, G. Grothendieck's suggestions are probably both more efficient :) – Gregor Thomas Aug 10 '22 at 15:37

G. Grothendieck · Answer 2 · 2022-08-10T18:10:14.693

5

1) The collapse package has seqid for this:

library(collapse)
split(vec, seqid(vec))

giving:

$`1`
[1] 1 2 3 4

$`2`
[1] 6 7 8

$`3`
[1] 30 31 32

2) A base approach giving the same answer is

split(vec, cumsum(c(TRUE, diff(vec) != 1)))

3) In the comments seqle in cgwtools is suggested.

library(cgwtools)
lens <- seqle(vec)$lengths
split(vec, rep(seq_along(lens), lens))

edited Aug 10 '22 at 18:10

answered Aug 10 '22 at 14:53

G. Grothendieck

`cgwtools` has `seqle` function that is rle for sequences. – Chris Aug 10 '22 at 18:01

2 Answers2