I have a working C++ function:

/// Structure for surface nodes
// Each node has a current x- and y-position and a reference x- and y-position.
struct nstruct {
/// Constructor
node_struct() {
    x = 0;
    y = 0;
    x_ref = 0;
    y_ref = 0;
    Ax = 0;
    Ay = 0;
/// Elements
double x; // current x-position
double y; // current y-position
double x_ref; // reference x-position
double y_ref; // reference y-position
double Ax; // node  (x-component)
double Ay; // node  (y-component)
/// Structure for object
struct pstruct {
/// Constructor
pstruct() {
    num_nodes = pnodes;
    node = new node_struct[num_nodes];
/// Elements
int num_nodes; // number of surface nodes
node_struct center; // center node
node_struct *node; // list of nodes

void Function(pstruct particle) {

for (int X = 0; X < Nx; ++X) {
   for (int Y = 0; Y < Ny; ++Y) {
       Ax[X][Y] = 0;
       Ay[X][Y] = 0;

for (int n = 0; n < particle.num_nodes; ++n) {
   int xStart = static_cast <int> (particle.node[n].x - 3.0);
   int xEnd = static_cast <int> (particle.node[n].x + 3.0);
   int yStart = static_cast <int> (particle.node[n].y - 3.0);
   int yEnd = static_cast <int> (particle.node[n].y + 3.0);
   for (int X = xStart; X < xEnd; ++X) {
       for (int Y = yStart; Y <= yEnd; ++Y) {
          **// here I want to use Lagrangian Interpolation**
            const double xDistance = X - particle.node[n].x;
            const double yDistance = Y - particle.node[n].y;
            const double delta = dirac_4(xDistance, yDistance);
          // dirac_4 : is Dirac Delta function
            Ax[X][Y] += (particle.node[n].Ax * delta);
            Ay[X][Y] += (particle.node[n].Ay * delta);

Its is an immersed boundary problem. I have Lagrangian points over an object (circular, or any shape), corresponding acceleration, etc. What I want to do, is to interpolate flow variables over Euler mesh using information of Lagrangian points (defined in pstruct and nstruct). I would like to use Lagrangian Interpolation instead of dirac_4 function to compute Ax and Ay. How I will implement Lagrangian/Polynomial interpolation in such a situation?


  • 4
    $\begingroup$ I am pretty sure a lot of contributors can help you with this. However, probably no one will, unless you give some context. Posting a code and asking for a solution is not a question. $\endgroup$ Commented Mar 28, 2016 at 12:28
  • 3
    $\begingroup$ Also, the code has no documentation, does not include a declaration or definition of the pstruct or dirac_4 functions, not basically anything else. Not much anyone can help you here without more context. $\endgroup$ Commented Mar 28, 2016 at 14:33
  • $\begingroup$ @WolfgangBangerth, I edited my original post with some more explanation. Hope, now everyone can understand my problem better. $\endgroup$
    – TheCoder
    Commented Mar 29, 2016 at 1:36
  • $\begingroup$ I still have no idea what the dirac_4 function is. Mathematically speaking, the Dirac function is zero everywhere except for one point where it is infinite. That's clearly not a useful definition for a computer code. $\endgroup$ Commented Mar 29, 2016 at 11:32
  • $\begingroup$ Refer to the attached link for the dirac delta function Eq6.28. Dirac Delta function is not problem here, it is doing the same work here as an interpolant. I want to replace this dirac_4 function with Lagrangian Interpolation. $\endgroup$
    – TheCoder
    Commented Mar 30, 2016 at 12:55

1 Answer 1


So a general Lagrange Interpolation in 2D can be written as the following:

$$ f(x,y) = \sum_i^N f_i L_i(x,y) $$


$$ L_i(x,y) = \prod_{j,j\neq i}^N \frac{(x-x_j)(y-y_j)}{(x_i-x_j)(y_i-y_j)}$$

Based on the above formulas, you should be able to program this up. The thing to note is that when $N$ is large, you end up with high order polynomial interpolants, which can result in high errors between data points. I would consider another approach unless you are interpolating using a few data points or you are certain this sort of fit would work well in your situation.


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