referring to lists within a coll object?

Hi,
I am trying to develop a system that allows a user to generate random melodies from a specified set of notes. The direction (ascending/descending) in pitch that the next note in a sequence will take will be controlled by the motion of a stylus on the y axis of a wacom tablet.
I am packing a coll object with the necessary lists (e.g. index number 1, midi number 48 = note C, and so on) to create a particular scale from which notes are randomly selected.
My problem lies with ensuring that a subsequent note will be higher or lower in pitch than the previous. I wondered if there may be a way to refer to the lists in the coll object and discriminate accordingly i.e randomly select any number from the coll as long it is higher/lower than the current.
Any help with this would be much appreciated.

Thanks