# Mealy and moore automata in the scope of system

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Mealy and Moore automata In the scope of system modeling, extensions of finite-state automata are frequently used that are based on the concepts of so-called Mealy [Mealy 1955] and Moore automata [Moore 1956] , respectively. In Mealy automata, the output of an automaton depends on the current state of the automaton as well as on the input. In contrast, in Moore automata, the output merely depends on the current state. Start navigation Determine GPS coordinates Calculate route Query traffic info Output route [avoid congestions] Input destination [do not avoid congestions] Ask for desire to calculate the route dynamically

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6.7 Requirements Modeling in the Behavioral Perspective 83 6.7.1 Statecharts Statecharts = state machines + hierarchization + conditions + concurrency Due to challenges that arise when using finite state automata in practice (such as missing support for abstraction), the automata concept has been developed into a technique of modeling the reactive behavior of a system. A widely applied technique to model the behavior of a system is the use of statecharts [Harel 1987] . Statecharts are a type of automata that is based on finite-state automata but are extended to support hierarchization of states to document conditions of state transitions and to model concurrent behavior. Figure 6-15 shows the modeling elements of statecharts in the notation suggested by Harel [Harel 1987] . Figure 6-15 Modeling elements of statecharts State A state defines a period of time in which the system shows a specific behav- ior and waits for a particular event to occur in order to perform a defined transition. Transition with condition and activity A transition is triggered by a particular event once it occurs in a spe- cific state. A transition describes the change from one state to the next. The change of states can additionally be dependent on some condition. The system can perform particular activities if it is in a particular state (typical for Moore automata) or if it performs a transition to another state (typical for Mealy automata). These activities can be directed toward the system itself or the environment of the system. Hierarchization and abstraction Statecharts allow for the hierarchical refinement of states that in turn represent automata. The initial state is referred to as super state and is defined by a number of refining states. Hierarchization allows abstracting from the irrelevant details of a state by—depending on the purpose of the model—only regarding and/or modeling the super state rather than the entire sub automaton that defines the super state. The detailed behavior of the system can, if necessary, be refined by defining the respective partial automata. Concurrency Along with hierarchical decomposition of a state into refining autom- ata, a state can be decomposed into several concurrent automata. The con- current automata can be synchronized by means of transition conditions Transition State Initial state [Name] Final state Event [condition] / activity Hierarchization [Name] Concurrent states Automaton 1 Automaton 2 Sub state Super state Special state

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