[deal.II] Reading a list from parameter file

[Paras Kumar]

Dear All,

I am trying to read a list of integers from the parameter file using the 
dealii::Patterns::List class.
However, I get a runtime error which can be reproduced using the attached 
MWE.

I also tried the approach described (for tensor) in 
https://groups.google.com/g/dealii/c/TvtcOnxeVjw/m/9uaSAh-kAwAJ but it 
doesn't seem to work for the current case.

It would be great help if someone could help me understand where I go wrong?

Thanks in advance and best regards,
Paras



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#include <deal.II/base/patterns.h>
#include <deal.II/base/parameter_handler.h>
#include <string>

using UnsignedIntType = unsigned int;

 void Declare_blockNonlinearSolverParameters(dealii::ParameterHandler &parameterHandler)
{
    parameterHandler.declare_entry(
      "Num Max Newton Iters",
      "10|10",
      dealii::Patterns::List(dealii::Patterns::Integer(1), 2, 8, "|"),
      "Allowed maximum number of newton iterations for inner loops");
}

int main()
{
    // decalre the parameters to be read
    dealii::ParameterHandler paramHandler;
    paramHandler.enter_subsection("Block Nonlinear Solver");
    Declare_blockNonlinearSolverParameters(paramHandler);
    paramHandler.leave_subsection();

    // generate and read the param file
    const std::string paramsBNlSolverTest01 =
  "subsection Block Nonlinear Solver                                       \n"
  "    # Allowed maximum number of newton iterations for inner loops       \n"
  "    set Num Max Newton Iters = 27|30|45                                 \n"
  "end                                                                     \n";
    std::stringstream paramFileString;
    paramFileString << paramsBNlSolverTest01;
    paramHandler.parse_input(paramFileString);

    //extract the read params
    
    paramHandler.enter_subsection("Block Nonlinear Solver");
    const std::string nMaxItersString = paramHandler.get("Num Max Newton Iters");
    std::cout << "nMaxItersString: " << nMaxItersString << std::endl;    
    std::vector<UnsignedIntType> nIters;
    nIters = dealii::Patterns::Tools::Convert<decltype(nIters)>::to_value(nMaxItersString);
    paramHandler.leave_subsection();
}