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Iīm glad you mentioned guard clauses, dws. They are particularly appropriate in light of jeffa mentioning Karnough maps.
Much of the point of guard clauses (and more elaborate if-then-else structures) is to reduce to a known state at any given moment, i.e., normalize your input states.
Interestingly, in EE, where Karnough maps are drilled into your head during beginning digital design courses, this takes on added importance when you implement your logic using hardware circuits. The logical reductions provided by Karnough maps do not come for free in the hardware world: they result is so called "donīt care" states, areas on the K map that represent theoretically irrelevant input states that under normal circumstances should never be reached.
In hardware, however, there is no absolute guarantee as to what state the circuit will land in once you flip on the power. Or, at least, thereīs no guarantee in a poorly designed circuit. You have to guard against these unknown initial conditions with some additional circuitry up front that will guarantee a known starting state.
None of this, naturally, does diddly squat for your TTL project if you dump a coke on it.
Beware the Santa Clauses!.