Simple Equality Equations

Simple equality equations is the traditional kind of equation known from mathematics that expresses an equality relation between two expressions. There are two syntactic forms of such equations in Modelica. The first form is equality equations between two expressions, whereas the second form is used when calling a function with several results.

1 Function f

function f input Real a; input Real b; output Real c; output Real d; output Real e; algorithm c := a + b; d := a - b; e := a * b; end f;

2 Example Call of f

Try calling f:

f(2, 5)

3 EqualityEquations

The first equation in the class EqualityEquations involving the function with two inputs and three results is correct, whereas the second equation is illegal since there are expressions in the left hand side.

class EqualityEquations Real x; Real y; Real z; Real u; Real v = 2; equation u = v; // Equality equations between two expressions (x, y, z) = f(1.0, 2.0); // Correct! (x+1, 3.0, z/y) = f(1.0, 2.0); // Illegal! Not a list of variables on the left hand side end EqualityEquations;

4 Simulation of EqualityEquations

Errors after simulation of EqualityEquations are shown below. Since it is illegal to write the code (x+1, 3.0, z/y), the simulation cannot be completed.

simulate( EqualityEquations ) // Should be illegal!

$Failed

5 EqualityEquationsCorrect

class EqualityEquationsCorrect Real x; Real y; Real z; Real u; Real v = 2; equation u = v; // Equality equations between two expressions (x, y, z) = f(1.0, 2.0); // Correct! end EqualityEquationsCorrect;

6 Simulation of EqualityEquationsCorrect

simulate( EqualityEquationsCorrect )
plot({x, y, v})