How to use columns indexing

Hello everybody,

I have a small question and I was wondering if someone could help me out! I have a matrix (2617x2) with two columns Time and Velocity represented here in this graph. The red line is a threshold of the velocity. The departures and the stops are calculated using the velocity (if velocity < 36 then “stop”; if velocity >36 then “departure”).

Now, I would like to know calculate the stops time and the departures time. I suppose I could use columns indexing but i don’t know how.

thanks!!