Trouble modifying Motion-Based Multiple Object Tracking to calculate velocities of each object

I am trying to modify displaytrackingresults method from the Motion-Based Multiple Object Tracking sample - I am trying to iterate through the ids of reliabletracks array to get each id and bboxes array to get the x, y of the centroid of each track, but I am only able to get 1 element, instead of all one by one for the frame.

Also, I encounter a matrix index out of bounds error when I try to access data for the previous frame from a structure that I have used to try and store data for each id in every frame.

How do I check the struct to see if a particular id exists in the previous frame before trying to access it so I don't get index errors? Any help will be greatly appreciated.