Split a vector in R based on sign (+/-)

Question

I would like to split a vector based on its sign. I have a vector that looks like:

v <- c(1, 2,-4,-8 ,-9, 4)

There are 3 groups in the vector. A positive group (index 1 and 2), a negative group (index 3,4,5) and another positive group (index 6). I want to get a vector of FIRST index of each group....

So the result I want is vector containing the indicies 1,3,6

I would like this to work if the vector has an arbitrary number of groups of arbitrary size.

Any help?

Thank you!

Answer 1

You can use sign to pick out the signs, and then diff to see where this changes.

c(1,which(diff(sign(v))!=0)+1)
[1] 1 3 6

Answer 2

An alternative to @James's solution is to use sequence and rle:

which(sequence(rle(sign(v))$lengths) == 1)
# [1] 1 3 6