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A state machine is a way to describe how a system behaves: it has a current situation, receives an input or event, and follows a rule that determines what happens next. Think of a login flow: the system responds differently to a logout event when you are logged in than it does when you are already logged out.
How a state machine works
A state machine keeps track of the state that matters to what the system should do next. When an input or event arrives, the machine uses its current state and a transition rule to decide whether to move to a different state. A state machine is a model of behavior; software does not literally need to contain circles and arrows.
State
A state describes the system’s current mode or situation. “Logged out” and “Logged in” are useful login states because they affect what the system should allow or do.
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An input or event is something the system responds to, such as a successful login, a failed login, or a logout request.
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Transition
A transition is the rule that maps a current state and an input to the next state. The formal finite-state-machine definition includes a set of states, a start state, an input alphabet, and a transition function that selects the next state. See NIST’s finite state machine definition.
A login example
Suppose a simplified login flow has two states. Its response depends on both the incoming event and the state it is already in:
| Current state | Event | Next state |
|---|---|---|
| Logged out | Login succeeds | Logged in |
| Logged out | Login fails | Logged out |
| Logged in | Logout | Logged out |
The failed-login case is a transition back to the same state: the event occurs, but the system remains logged out. In a state diagram, states are circles and transitions are arrows labeled with events. A self-loop arrow can show an event that leaves the state unchanged. MDN explains this basic model and its diagrams in its state machine glossary entry.
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Why make the behavior explicit?
When a system has distinct modes, writing down its states and allowed transitions makes its behavior easier to inspect. A diagram or transition table can clarify which paths are valid, which events change the system, and what happens when an event does not apply. MathWorks notes that state diagrams make relationships among states and transition conditions more apparent.
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This is an organizing technique, not a rule that every conditional statement should become a state machine. It is most useful when behavior changes according to a small number of meaningful modes and incoming events.
Where state machines are used
- Games: Apple’s GameplayKit documents state objects and transitions for character behaviors such as Chase, Flee, Dead, and Respawn, as well as turret behavior such as Ready, Firing, and Cooldown. See Apple’s GKStateMachine documentation.
- Workflows: A workflow can move through states in response to triggers, subject to conditions. Microsoft’s .NET Framework documentation describes this event-driven approach in State Machine Workflows.
- Reactive systems: State-machine models can describe behavior in areas such as robotics, telecommunications, and software. MathWorks illustrates the idea with a car transmission that changes gears; see Model a Finite State Machine.
How common variations differ
Deterministic and nondeterministic machines
In a deterministic finite-state machine, a given state and input select one next state. A nondeterministic machine can allow more than one possible next state for that same state and input. The distinction is about how many next-state possibilities the model permits.
Mealy and Moore machines
These variants differ in where outputs are associated: a Mealy machine associates outputs with transitions, while a Moore machine associates outputs with states. NIST’s entry covers these variants alongside finite-state-machine terminology.
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Hierarchical state machines
When a system grows, related states can be nested beneath a broader parent state. Shared behavior can be defined once at the parent level, while substates describe their differences. This avoids repeating the same behavior across many states, but adds structure that a tiny example does not need. The QP/C++ User Manual’s state-machine section describes hierarchical state machines, also called UML statecharts.
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