determine continuous states of an FMU?

[GarronFish]

It can be useful to know the names of the continuous states in a ME FMU. One way to do this using FMPy is described here: https://stackoverflow.com/questions/75730464/how-to-using-fmpy-extract-the-list-of-continuous-time-states. Is there an equivalent method for fmi4c?

What methods would you suggest for parsing the model_description file to extract that information?

[robbr48]

That is a good suggestion! It is not possible now but should be fairly easy to add. Would it be sufficient with a function for fetching the value references? Something like this:

fmiX_getNumberOfContinuousStates(fmu, nStates);
fmiX_getContinuousStateValueReferences(fmu, valueReferences, nStates);
for(int i=0; i<nStates; ++i) {
    var = fmiX_getVariableByValueReference(valueReferences[i]);
    names[i] = fmiX_getVariableName(var)
}

[GarronFish]

That would be good, thanks

[robbr48]

After a second thought I came up with a different solution. The information is already available in the ModelStructure element of the model description XML. By fetching all derivative variables, and checking which variables they are the derivative of, a list of continuous states (and of course state derivataives) can easily be obtained:

numDerivatives = fmi2_getNumberOfModelStructureDerivatives();
for(int i=0; i<numDerivatives; ++i) {
    fmi2ModelStructureHandle unknown = fmi2_getModelStructureDerivative(i);
    int derIndex = fmi2_getModelStructureIndex(unknown);
    fmi2VariableHandle der = fmi2_getVariableByIndex(derIndex);
    int varIndex = fmi2_getVariableDerivativeIndex(der);
    fmi2VariableHandle var = fmi2_getVariableByIndex(varIndex);
    printf("Continuous State: %s\n", fmi2_getVariableName(var));
}

Would this work for you?

Notice that according to the FMI 2.0.4 specification,

The “continuous states” of a model are all variables that appear differentiated in the model and are independent from each other.

If you want to be strict you also need to check the second requirement. This information is also available in the XML and exposed in the API.

[GarronFish]

That would work, thanks!