Comparing between 2 instances of same object

How do I compare 2 instances of same object when I need an action triggered performed on one of the instances based on one of the instance variables when they collide.

If you know for sure what the value of the instance variable is on the object you want to affect.. you could just do:

+on object collision with object

+object.variable = [number]

     ->your actions

If however, you don't know what the value is.. but need to check for the one with a lower value for example.. (like health points) you could do:

+on object collision with object

+for each object ordered by object.health ascending

     -> stop loop

     -> your actions etc

So the for each after the on collision condition would only affect those involved in the collision?

What I wanted to do is to propagate certain values on collision, something like a viral effect.