RE: [deal.II] How to get the shape_value at the quadrature point in the reference cell?

Michael Li Sun, 12 Sep 2021 17:15:47 -0700

Dr. Bangerth,

Thank you for your hints. I tried using shape_value, but it seems finite_element_output is a protected member. The error shows as follows:

error: ‘dealii::internal::FEValuesImplementation::FiniteElementRelatedData<2, 2> dealii::FEValuesBase<2, 2>::finite_element_output’ is protected within this context

59 | double shape_value = fe_values.finite_element_output.shape_value(i, q_index);

^~~~~~~~~~~~~~~~~~~~~

error: ‘class dealii::internal::FEValuesImplementation::FiniteElementRelatedData<2, 2>’ has no member named ‘shape_value’; did you mean ‘shape_values’?

By the way, I also want to get the shape_grad of the reference cell to compare with some other code implementation regarding shape function.

Based on Thong’s method, the following codes gives me the desired results for a Q1 element:

const FE_Q<2> Q_2d_element(1);

const Point<2> center(1, 1);

for(unsigned int i=0; i<Q_2d_element.n_dofs_per_cell(); i++)

{

std::cout << " shape value = " << Q_2d_element.shape_value(i, center);

std::cout << " shape grad = " << Q_2d_element.shape_grad(i, center) << std::endl;

}

I wonder if there are other ways to access the shape_grad on the ref cell directly. Do I need to use fe_values.inverse_jacobian(q_index) to convert from shape_grad on the real cell to the reference cell?

Best,

Michael

From: Wolfgang Bangerth
Sent: Sunday, September 12, 2021 5:13 PM
To: [email protected]
Subject: Re: [deal.II] How to get the shape_value at the quadrature point in the reference cell?

On 9/10/21 4:48 PM, Michael Lee wrote:

> I want to require the value of a shape function at some quadrature point. But

> the following code throws an error saying finite_element_output is protected.

> double shape_value = fe_values.finite_element_output.shape_values(i, q_index);

You already solved the problem, but just for completeness: You get the error

because you try to access a 'protected' member variable called 'shape_values'

(plural). What you want is to use the 'public' member function 'shape_value'

(singular).

Best

------------------------------------------------------------------------

Wolfgang Bangerth email: [email protected]

www: http://www.math.colostate.edu/~bangerth/

The deal.II project is located at http://www.dealii.org/

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RE: [deal.II] How to get the shape_value at the quadrature point in the reference cell?

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