[joe_the_user]

Even with the clue that he wants to "predict how cows move around in any landscape", his description seems like it wouldn't be enough any definite design advice.

What do the states of the cows and the landscape look like? What is changed by the if-then-else results. Does he already have his rules? Is this for a biology project or a video game, etc.

You could use a finite automaton system, an array of cow-states, a cow DSL or something else depending on the answers to these and other questions.

[insert joke about "till the cows come home"]

[ShabbyDoo]

Yes. The piece missing for me in most answers I've seen is eventing. Does one pump a series of events/state changes into the CowModel and see what actions pop-out? Does the model involve interactions with other cows where the actions of one cow cause actions of others? If so, one must have an event queue to handle chain reactions. How discrete are the events? Is there a "temp is 89F" event where, for every N minutes the cow is in that temperature in direct exposure, it becomes more likely to seek shade? If so, the "it is 89F" event must be triggered multiple times. And, such a rule requires that the past states of the cow be retained. I think the if/then/else logic is only part of the solution to a simulation problem of this sort.