cosv_matrix.cpp

#include <Rcpp.h>
using namespace Rcpp;


// cosine distance between two vectors
double cosv(NumericVector a, NumericVector b)
{
    double theta;
    theta = sum(a * b) / sqrt(sum(pow(a, 2)) * sum(pow(b, 2)));
    return theta;
}


// initialise an Rcpp matrix
NumericMatrix M(int row, int col)
{
    NumericMatrix mat(row, col);
    return mat;
}


// fill matrix with values calculated from cosine similarities
// [[Rcpp::export]]
NumericMatrix cosv_matrix(int nrow, int ncol, NumericVector a, NumericVector b)
{
    // create matrix to store values
    NumericMatrix M(nrow, ncol);

    for (int i = 0; i < nrow; i++)
    {
        for (int j = 0; j < ncol; j++)
        {
            M[i, j] = cosv(a, b); // need to get the vectors from the dataframe
                                  // ideally without writing a separate loop
        }
    }

    return M;
    
}