findnz.c

/*=================================================================
 * findnz.c 
 * Example for illustrating how to handle N-dimensional arrays in a 
 * MEX-file.  NOTE: MATLAB uses 1 based indexing, C uses 0 based indexing.
 *
 * Takes a N-dimensional array of doubles and returns the indices for
 * the non-zero elements in the array. Findnz works differently than
 * the FIND command in MATLAB in that it returns all the indices in
 * one output variable, where the column element contains the index
 * for that dimension.
 *
 * This is a MEX-file for MATLAB.  
 * Copyright 1984-2006 The MathWorks, Inc.
 *============================================================*/

/* $Revision: 1.1 $ */

#include "mex.h"

/* If you are using a compiler that equates NaN to zero, you must
 * compile this example using the flag -DNAN_EQUALS_ZERO. For example:
 *
 *     mex -DNAN_EQUALS_ZERO findnz.c
 *
 * This will correctly define the IsNonZero macro for your Compiler */

#if NAN_EQUALS_ZERO
#define IsNonZero(d) ((d)!=0.0 || mxIsNaN(d))
#else
#define IsNonZero(d) ((d)!=0.0)
#endif

void mexFunction(int nlhs,       mxArray *plhs[],
         int nrhs, const mxArray *prhs[])
{
    /* Declare variables */ 
    mwSize elements,j,cmplx;
    mwSize number_of_dims;
    mwSize nnz=0, count=0; 
    double *pr, *pi, *pind;
    const mwSize *dim_array;         
    
    /* Check for proper number of input and output arguments */    
    if (nrhs != 1) {
    mexErrMsgTxt("One input argument required.");
    } 
    if (nlhs > 1){
    mexErrMsgTxt("Too many output arguments.");
    }
    
    /* Check data type of input argument */
    if (!(mxIsDouble(prhs[0]))) {
      mexErrMsgTxt("Input array must be of type double.");
    }
    
    /* Get the number of elements in the input argument */
    elements=mxGetNumberOfElements(prhs[0]);
   
    /* Get the data */
    pr=(double *)mxGetPr(prhs[0]);
    pi=(double *)mxGetPi(prhs[0]);
    cmplx = ((pi==NULL) ? 0 : 1);

    /* Count the number of non-zero elements to be able to allocate
       the correct size for output variable */
    for(j=0;j<elements;j++){
    if(IsNonZero(pr[j]) || (cmplx && IsNonZero(pi[j]))) {
        nnz++;
    }
    }
    
    /* Get the number of dimensions in the input argument. Allocate the
       space for the return argument */
    number_of_dims=mxGetNumberOfDimensions(prhs[0]);
    plhs[0]=mxCreateDoubleMatrix(nnz,number_of_dims,mxREAL);
    pind=mxGetPr(plhs[0]);
    
    /* Get the number of dimensions in the input argument. */
    dim_array=mxGetDimensions(prhs[0]);

    /* Fill in the indices to return to MATLAB. This loops through the
     * elements and checks for non-zero values. If it finds a non-zero
     * value, it then calculates the corresponding MATLAB indice and
     * assigns them into the output array.  The 1 is added to the
     * calculated indice because MATLAB is 1 based and C is zero
     * based. */
    for(j=0;j<elements;j++) {
    if(IsNonZero(pr[j]) || (cmplx && IsNonZero(pi[j]))) {
        mwSize temp=j;
        mwSize k;
        for (k=0;k<number_of_dims;k++){
        pind[nnz*k+count]=(double)((temp % (dim_array[k])) +1);
        temp/=dim_array[k];
        }
        count++;
    }
    }
}