Hi, I’ve been going through the SDK tutorials again, in the Multiple Actors tut, there is a hard coded result in the Rock/Paper/Scissors canister when calling…
The battle_round function calls the alice and bob functions, which determine what Alice and Bob will play respectively. Currently these two functions return hard-coded results, with Alice always playing paper, Bob always playing rock. It’s the Hollywood principle - “You don’t call us, we’ll call you” 
To turn this into an actual game, you’d have to edit these two methods, such that the choice for each round would be based on eg what a real-life player picks, or be based on the previous moves of the other player if you want to implement a computer opponent.
Chances are that, for your first draft, the battle_rounds function can remain untouched. All it does is call alice and bob, and grant points according to who won the current round.
Is there a way to generate a random number so that their decisions are more interesting?
Kind of, but it won’t be all too straight-forward.
Motoko doesn’t provide access to randomness as of now. Meaning if you want randomness you’ll need to generate it client-side (ie in JS), or implement a PRNG of your own. Both are suitable for some basic PoC game, but take some work.
Will that be added in future? Or is it an inherent constraint due to an architectural decision that the methods must be deterministic?
I found https://github.com/stanleygjones/hero-app-generator/blob/master/src/hackathon/Utils/Math.mo but it starts from a fixed seed and so always generates the same numbers.
In the future it should be possible to derive randomness from dfinitys random beacon natively.