[R] for loop

[aledanda]

Hi,
I need your help.
I have a vector of numbers reflecting the switch in the perception of a
figure. For a certain period I have positive numbers (which reflect the
perception A) then the perception changes and I have negative numbers
(perception B), and so on for 40000 iterations. I need to take the rate of
this switch for my analysis. Namely, I need a new vector with numbers which
reflect how many digit follows in sequence before the change in perception
(and then I have to take the reciprocal of these numbers in order to obtain
the rate but is not a problem). For example, suppose that the new vector
looks like this: new <- c(5,7,8,9) , 5 numbers positive then the perception
changes and 7 negative numbers follow, then it changes again and 8 positive
follows and so on.. 
In brief I need to write a little script that detects the change in sign of
the elements of the vector and count how many positive and how many negative
digits there are in sequence.

I would use the for loop, I started but then i don't know how to continue

rate <- vector()
for(i in (length(a)) rate <- (a[i]> 0 ????

..can you help me?

Alessandra

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