Creating and tabulating an element

This demo shows how Basix can be used to create an element and tabulate the values of its basis functions at a set of points

First, we import Basix and Numpy.

import numpy as np

import basix
from basix import CellType, ElementFamily, LagrangeVariant

Next, we create a degree 4 Lagrange element on a quadrilateral using the function create_element. The first input is the element family: for Lagrange elements, we use ElementFamily.P. The second input is the cell type. The third input is the degree of the element. For Lagrange elements, we must provide a fourth input: the Lagrange variant. In this example, we use the equispaced variant: this will place the degrees of freedom (DOFs) of the element in an equally spaced lattice.

lagrange = basix.create_element(
    ElementFamily.P, CellType.quadrilateral, 4, LagrangeVariant.equispaced
)

We now print the number of DOFs that this element has.

print(lagrange.dim)

We see that the element has 25 DOFs: for this element, there will be 1 DOFs at each vertex, 3 DOFs on each edge, and 9 DOFs on the interior of the quadrilateral.

Next, we create a set of points as a numpy array, and tabulate the basis functions of the Lagrange space at these points. The first input of tabulate is the number of derivative to tabulate: we set this to 0 so will to compute the values of the functions (and no derivatives). We pass in the points as the second input.

points = np.array([[0.0, 0.0], [0.1, 0.1], [0.2, 0.3], [0.3, 0.6], [0.4, 1.0]])
tab = lagrange.tabulate(0, points)
print(tab)
print(tab.shape)

The result of tabulating is a 1 by 5 by 25 by 1 Numpy array. The first dimension is 1 as we are only tabulating the function values; it would be higher if we had asked for derivatives too. The second dimension (5) is the number of points. The third dimension (25) is the number of DOFs. The fourth dimension (1) is the value size of the element: this will be greater than 1 for vector-values elements.

C++ demo

The following C++ code runs the same demo using Basix’s C++ interface:

// ==================================
// Creating and tabulating an element
// ==================================
//
// This demo shows how Basix can be used to create an element
// and tabulate the values of its basis functions at a set of
// points.

#include <basix/finite-element.h>
#include <basix/mdspan.hpp>
#include <iostream>

namespace stdex
    = MDSPAN_IMPL_STANDARD_NAMESPACE::MDSPAN_IMPL_PROPOSED_NAMESPACE;
template <typename T, std::size_t d>
using mdspan_t = MDSPAN_IMPL_STANDARD_NAMESPACE::mdspan<
    T, MDSPAN_IMPL_STANDARD_NAMESPACE::dextents<std::size_t, d>>;

using T = double;

int main(int argc, char* argv[])
{
  // Create a degree 4 Lagrange element on a quadrilateral
  // For Lagrange elements, we use `basix::element::family::P`.
  auto family = basix::element::family::P;
  auto cell_type = basix::cell::type::quadrilateral;
  int k = 3;

  // For Lagrange elements, we must provide and extra argument: the Lagrange
  // variant. In this example, we use the equispaced variant: this will place
  // the degrees of freedom (DOFs) of the element in an equally spaced lattice.
  auto variant = basix::element::lagrange_variant::equispaced;

  // Create the lagrange element
  basix::FiniteElement lagrange = basix::create_element<T>(
      family, cell_type, k, variant, basix::element::dpc_variant::unset, false);

  // Get the number of degrees of freedom for the element
  int dofs = lagrange.dim();
  assert(dofs == (k + 1) * (k + 1));

  // Create a set of points, and tabulate the basis functions
  // of the Lagrange element at these points.
  std::vector<T> points = {0.0, 0.0, 0.1, 0.1, 0.2, 0.3, 0.3, 0.6, 0.4, 1.0};

  auto [tab_data, shape] = lagrange.tabulate(0, points, {points.size() / 2, 2});

  std::cout << "Tabulate data shape: [ ";
  for (auto s : shape)
    std::cout << s << " ";
  std::cout << "]" << std::endl;

  mdspan_t<const T, 4> tab(tab_data.data(), shape);
  std::cout << "Tabulate data (0, 0, :, 0): [ ";
  for (std::size_t i = 0; i < tab.extent(2); ++i)
    std::cout << tab(0, 0, i, 0) << " ";
  std::cout << "]" << std::endl;

  return 0;
}