how to view c source code that has been loaded into R package

recently, I want to know more details about GWmodel package.I typed gwr.binomial.wt at console in Rstudio：

function (y, x, bw, W.mat, verbose = T) 
{
  tol = 1e-05
  maxiter = 20
  dp.n <- nrow(x)
  var.n <- ncol(x)
  betas <- matrix(nrow = dp.n, ncol = var.n)
  betas1 <- betas
  S <- matrix(nrow = dp.n, ncol = dp.n)
  n = rep(1, length(y))
  it.count <- 0
  llik <- 0
  mu <- 0.5
  nu <- 0
  if (verbose) 
    cat(" Iteration    Log-Likelihood:(With bandwidth: ", 
        bw, ")\n=========================\n")
  wt2 <- rep(1, dp.n)
  repeat {
    y.adj <- nu + (y - n * mu)/(n * mu * (1 - mu))
    for (i in 1:dp.n) {
      W.i <- W.mat[, i]
      gwsi <- gw_reg(x, y.adj, W.i * wt2, FALSE, i)
      betas1[i, ] <- gwsi[[1]]
    }
    nu <- gw_fitted(x, betas1)
    mu <- exp(nu)/(1 + exp(nu))
    old.llik <- llik
    llik <- sum(lchoose(n, y) + (n - y) * log(1 - mu/n) + 
                  y * log(mu/n))
    if (is.na(llik)) 
      llik <- old.llik
    if (verbose) 
      cat(paste("   ", formatC(it.count, digits = 4, width = 4), 
                "    ", formatC(llik, digits = 4, width = 7), 
                "\n"))
    if (abs((old.llik - llik)/llik) < tol) 
      break
    wt2 <- n * mu * (1 - mu)
    it.count <- it.count + 1
    if (it.count == maxiter) 
      break
  }
  res <- list(wt2, llik, y.adj)
  res
}

Then, I don't quite understand the "gw_reg" function. I continued to type gw_reg in the console of Rstudio:

function (x, y, w, hatmatrix, focus) 
{
  .Call("GWmodel_gw_reg", PACKAGE = "GWmodel", x, y, w, hatmatrix, 
        focus)
}

I find that this function doesn't tell me much about how it works. As I understand it, .call() means that the function is implemented in C/C ++ code. So, how do I look at this function in more detail? Or how do I look at the C code that implements this function?

Solution

The C++ code of the function can be found in src/GWmodel.cpp in the package's source files. The actual C++ function is also called gw_reg but gets renamed at the point of export by Rcpp.

You can look at the function here on Github to see how it works in context, but the function itself is

List gw_reg(mat x, vec y, vec w, bool hatmatrix, int focus)
{
    mat wspan(1, x.n_cols, fill::ones);
    mat xtw = trans(x % (w * wspan));
    mat xtwx = xtw * x;
    mat xtwy = trans(x) * (w % y);
    mat xtwx_inv = inv(xtwx);
    vec beta = xtwx_inv * xtwy;
    if (hatmatrix)
    {
        mat ci = xtwx_inv * xtw;
        mat s_ri = x.row(focus - 1) * ci;
        return List::create(
                Named("beta") = beta,
                Named("S_ri") = s_ri,
                Named("Ci") = ci);
    }
    else
    {
        return List::create(
                Named("beta") = beta);
    }
}

I find the easiest way to track down such source code is to find the package on Github and use the search tool to look up the name of the C++ function.