# Equation vs. Rule vs. Function

“What is the difference between an Equation, a Rule and a Function ?”

All three are algebraic expressions, however:

• An Equation is an equality relation the simulator is expected to keep throughout simulation time. Thus whenever dependent symbols change the referred symbol (symbol-ref) is recalculated.
• A Rule is a recurrance relation rule available in System, i.e. the expression may depend on the output symbol. All rules in a system are executed synchronously. The time-step of the system controls the frequency of the execution of the rule.
• A Function is a parametric function definition that is calclulated whenever used (using symbol() notation). For convenience, parameter-free functions are also accessible in plain symbol notation.
Attribute Evaluation
Equation symbol-ref Evaluated whenever input symbols change and assigned to the referred symbol (e.g. Property).
Rule symbol-ref Evaluated with the time-step of the enclosing System and assigned to referred symbol.
Function symbol Evaluated when symbol / function is used.
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