Create index for contiguous runs of values

Question

I have a vector:

test <-c(1,1,0,2,2,3,4,1,1,0)  
test  
# [1] 1 1 0 2 2 3 4 1 1 0

I want to construct an grouping variable which indicates when values change:

# [1] 1 1 2 3 3 4 5 6 6 7

What is the best way to do this?

Answer 1

Use run length encoding (rle), seq_along and rep

r <- rle(test)

changes <- rep(seq_along(r$lengths), r$lengths)
changes
## [1] 1 1 2 3 3 4 5 6 6 7

Answer 2

Alternative option, which will admittedly only work for numeric data.

test <-c(1,1,0,2,2,3,4,1,1,0)  
cumsum(c(1L, diff(test) != 0))
# [1] 1 1 2 3 3 4 5 6 6 7

And a convoluted variation that will work for any data types:

head(cumsum(c(TRUE, c(tail(test, -1), NA) != test)), -1)
# [1] 1 1 2 3 3 4 5 6 6 7