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Considering the following state machine diagram, it is CORRECT to state that:

enter image description here

Select one or more:

a. the Restart event will always be executed in the Halted state.

b. After a Halt and Restart event and assuming no previous history is available the re-enter state will be State1

c. Assuming the current state is State2 and Halt occurs and that there is no preferred entry state, the next state transition (after re-entry) will result in the current state to be State3.

d. "Endop" is the final state.

e. Only the Complete event can lead to the Completed state.

The correct options are B and C. I don't understand why. I know how state machine diagrams work but why is A not correct, there is a Restart arrow coming out of Halted so surely restart event will always be executed in halted state. For B, after a Halt and Restart it goes to the H, which is not State1. For C, I don't even understand what its asking. Could someone explain why these options are correct?

computerscienceisapain
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2 Answers2

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A: "Restart" can happen at any time (state machine diagrams don't show when a specific event occurs, but instead when it's handled. I.e. "Restart" can happen anytime, but is only handled in the "Halted" state - in any other state it can occur, but is ignored. (I find it a bit misleading that the verb "execute" is used for an event, however.)

C: "H" is the "History connector": It stores the last state we were in before leaving "operating" and restores that state on re-entry. Thus, the next state after re-entry will be "State 3" ("State 2" will be restored and the machine will immediately transition to "State 3").

B: "Assuming no previous history is available" means we've never been in "Operating" before (otherwise, we had one after a "Halt" and "Restart"). With no history, we'll obviously end up in State 1 after re-entry.

tofro
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  • Regarding C, wouldn't we expect a visible transition/arrow from H to State2? Why is only the "no history" arrow explicitly shown? – Erik Eidt Apr 03 '22 at 00:02
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    @ErikEidt, H is not an actual state. It is a marker indicating that the composite state it is drawn in maintains a history and a transition to H should be interpreted as a transition to the sub-state that was active at the time we left the composite state. The explicit arrow is only to have a defined behavior if there is no last active sub-state. – Bart van Ingen Schenau Apr 04 '22 at 07:27
  • @ErikEidt Exactly - In code, you simply retain the inner state of the FSM and go from there on re-entry without re-initializing the FSM. – tofro Apr 04 '22 at 09:45
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  • not A: the Complete event can be executed in the Halted state.
  • not D: "Completed" is the last final state, the proposed final state is merely that of the component.
  • B is correct as you said: assuming no previous history.
  • C is correct, (H) = history will preset with State 2 and come to State 3.

So the piece of knowledge one needs is that (H) is the history connector. But B indeed gives a hint.

Joop Eggen
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