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How to apply a function to a list of variables
Variables in Kratos can have different types. This makes them difficult to be used in a vector and passed to a function. The main difficulty is that the standard vector cannot hold entities with different types. On the other side, the VariablesList class can store variables with different types but you may access them as a VariableData which is the base class and does not have the type information needed to manipulate data.
In fact, the Visitor Pattern is the standard way to overcome such situation. The main idea behind this is to let each variable accept a function class (A.K.A functional) and then call the visit function of it using its known type.
This pattern is implemented in Kratos and variables accepting a functional which should be derived from the VariablesListVisitorBase
and implementing the Visit
methods for each type of the variables which wants to work with them. For the list of the supported types in the base class, one could verify directly the definition of the VariablesListVisitorBase
class but the list includes bool
, int
, double
, array_1d<double,3>
, Vector
, and Matrix
. It is important to emphasize that if the visitor is applied to a variable that is not supported by the derived class, the base class will send a runtime error.
In order to see better the details, let's create a class that sets the nodal values in a model part to one. First of all I should create a class derived from the VariablesListVisitorBase
:
class SetModelPartVariableToOne : public VariablesListVisitorBase
{
ModelPart& mrModelPart;
public:
SetModelPartVariableToOne(ModelPart& TheModelPart) : mrModelPart(TheModelPart){}
};
As you can see, we take the modelpart we want to act over in the constructor. Now we should implemente the Visit
methods:
class SetModelPartVariableToOne : public VariablesListVisitorBase
{
ModelPart& mrModelPart;
public:
SetModelPartVariableToOne(ModelPart& TheModelPart) : mrModelPart(TheModelPart){}
virtual void Visit(Variable<bool> const& TheVariable) override {
SetScalarToOne(TheVariable);
}
virtual void Visit(Variable<int> const& TheVariable) override {
SetScalarToOne(TheVariable);
}
virtual void Visit(Variable<unsigned int> const& TheVariable) override {
SetScalarToOne(TheVariable);
}
virtual void Visit(Variable<double> const& TheVariable) override {
SetScalarToOne(TheVariable);
}
virtual void Visit(Variable<array_1d<double,3>> const& TheVariable) override {
SetVectorToOne(TheVariable);
}
virtual void Visit(Variable<Vector> const& TheVariable) override {
SetVectorToOne(TheVariable);
}
virtual void Visit(Variable<Matrix> const& TheVariable) override {
// Do nothing.
}
private:
template<typename TVariableType>
void SetScalarToOne()(TVariableType const& TheVariable) const {
for (auto& r_node : mrModelPart.Nodes()) {
auto& r_value = r_node.FastGetSolutionStepValue(TheVariable);
r_value = 1.0;
}
}
template<typename TVectorType>
void SetVectorToOne()(Variable<TVectorType> const& TheVariable) const {
for (auto& r_node : mrModelPart.Nodes()) {
auto& r_value = r_node.FastGetSolutionStepValue(TheVariable);
r_value = ScalarVector(3,-1.0);
}
}
};
You may observe that the real implemenation is in the two template private methods which take a variable and set its nodal value in the modelpart to one. It is important to mention that one can skip any type of the variable just by creating an empty Visit
method for that type. (like the one for matrix). This would avoid a runtime error from the base class if the variable list contain a Variable<Matrix>
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