Mex Function for Mahalanobis Function

armadillo,

mahalanobis,

mahalanobis distance,

matlab,

mex,

numerical computing,

programming,

software

In my last post I showed how to write mex functions with the armadillo library to get both fast and clean mex functions. Armadillo is a library for C++ that has a syntax similar to matlab.

Here I give an example of a mex function using armadillo. It computes the Mahalanobis distance and is based on the template from the last post.

Code snippet - Mex Function for Mahalanobis Function on Snipplr

#include "mex.h"
#include "math.h"
#include<armadillo>

using namespace arma;

void importMatlab(mat& A, const mxArray *mxdata){
  access::rw(A.mem)=mxGetPr(mxdata);
  access::rw(A.n_rows)=mxGetM(mxdata);
  access::rw(A.n_cols)=mxGetN(mxdata);
  access::rw(A.n_elem)=A.n_rows*A.n_cols;
};

void freeVar(mat& A, const double *ptr){
  access::rw(A.mem)=ptr;
  access::rw(A.n_rows)=1;
  access::rw(A.n_cols)=1;
  access::rw(A.n_elem)=1;
};

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    if (nrhs != 3)
        mexErrMsgTxt("Incorrect number of input arguments");
    if (nlhs != 1)
        mexErrMsgTxt("Incorrect number of D arguments");

  mat X(1,1);
  const double* Xmem=access::rw(X.mem);
  importMatlab(X,prhs[0]);

  mat Y(1,1);
  const double* Ymem=access::rw(Y.mem);
  importMatlab(Y,prhs[1]);

  mat W(1,1);
  const double* Wmem=access::rw(W.mem);
  importMatlab(W,prhs[2]);

//    check if the input corresponds to what you are expecting
  if(( X.n_cols !=  Y.n_cols ) || (Y.n_cols != W.n_cols ) )
    mexErrMsgTxt("Columns of X, Y, and W must be of equal length!");
   if( W.n_rows !=  W.n_cols )
    mexErrMsgTxt("W must be a square matrix!");

    plhs[0] = mxCreateDoubleMatrix(X.n_rows, Y.n_rows, mxREAL);

    double *out = mxGetPr(plhs[0]);
    mat diff;
    mat M;
    int k=0;
    for(int y=0;y<Y.n_rows;y++)
      for(int x=0;x<X.n_rows;x++){
// (X(x,:)-Y(y,:))*W*((X(x,:)-Y(y,:))')
            diff=X.row(x)-Y.row(y);
            M=diff*W*trans(diff);
            (*out++)=M(0);
        }


    freeVar(X,Xmem);
    freeVar(Y,Ymem);
    freeVar(W,Wmem);
    return;
}

Enjoy. Please leave a comment below for suggestions and questions.

No Response to "Mex Function for Mahalanobis Function"

Subscribe to replies to this post

This conversation is missing your voice. Your feedback is appreciated.
Post a Comment

You can use some HTML tags, such as <b>, <i>, <a>

If you see a message that says "your request could not be processed" press preview first and then post.

You can follow the discussion of this post by subscribing.

You are free to include information from this article on your own site if you provide a backlink. You can use the following markup:
<a href="http://www.myoutsourcedbrain.com/2009/08/mex-function-for-mahalanobis-function.html">Mex Function for Mahalanobis Function</a>