Record Definitions

Record Definitions

Earlier, we introduced the idea of a model definition. Although we haven’t seen any yet, Modelica also includes a record type. A record can have variables, just like a model, but it is not allowed to include equations. Records are primarily used to group data together. But as we will see shortly, they are also very useful in describing the data associated with Annotations.

Syntax

The record definition looks essentially like a model definition, but without any equations:

record RecordName "Description of the record"
  // Declarations for record variables
end RecordName;

As with a model, the definition starts and ends with the name of the record being defined. An explanation of the record can be included as a string after the name. All the variables associated with the record are declared within the record definition.

The following are all examples of record definitions:

record Vector "A vector in 3D space"
  Real x;
  Real y;
  Real z;
end Vector;

record Complex "Representation of a complex number"
  Real re "Real component";
  Real im "Imaginary component";
end Complex;

Record Constructors

Now that we know how to define a record, we need to know how to create one. If we are declaring a variable that happens to be a record, the declaration itself will create an instance of the record and we can specify the values of variables inside the record using modifications, e.g.,

parameter Vector v(x=1.0, y=2.0, z=0.0);

But there are some cases where we might want to create an instance of a record that isn’t a variable (e.g., to use in an expression, pass as an argument to a function or use in a modification). For each record definition, a function is automatically generated with the same name as the record. This function is called the “record constructor”. The record constructor has input arguments that match the variables inside the record definition and returns an instance of that record. So in the case of the Vector definition above, we could also initialize a parameter using the record constructor as follows:

parameter Vector v = Vector(x=1.0, y=2.0, z=0.0);

In this case, the value for v comes from the expression Vector(x=1.0, y=2.0, z=0.0) which is a call to the record constructor.