External Interfaces/API

Handling Complex Data

Complex data from MATLAB is separated into real and imaginary parts. The MATLAB API provides two functions, mxGetPr and mxGetPi, that return pointers (of type double *) to the real and imaginary parts of your data.

This example takes two complex row vectors and convolves them.

• /*
   * =============================================================
   * convec.c
   * Example for illustrating how to pass complex data 
   * from MATLAB to C and back again
   *
   * Convolves two complex input vectors.
   *
   * This is a MEX-file for MATLAB.
   * Copyright (c) 1984-2000 The MathWorks, Inc.
   * =============================================================
   */
  
  /* $Revision: 1.9 $ */
  
  #include "mex.h"
  
  /* Computational subroutine */
  void convec(double *xr, double *xi, int nx, double *yr, 
              double *yi, int ny, double *zr, double *zi)
  {
    int i,j;
    
    zr[0] = 0.0;
    zi[0] = 0.0;
    /* Perform the convolution of the complex vectors. */
    for (i = 0; i < nx; i++) {
      for (j = 0; j < ny; j++) {
        *(zr+i+j) = *(zr+i+j) + *(xr+i) * *(yr+j) - *(xi+i) 
            * *(yi+j);
        *(zi+i+j) = *(zi+i+j) + *(xr+i) * *(yi+j) + *(xi+i) 
            * *(yr+j);
      }
    }
  }
  

Below is the gateway routine that calls this complex convolution.

• /* The gateway routine. */
  void mexFunction(int nlhs, mxArray *plhs[],
                   int nrhs, const mxArray *prhs[])
  {
    double  *xr, *xi, *yr, *yi, *zr, *zi;
    int     rows, cols, nx, ny;
      
    /* Check for the proper number of arguments. */
    if (nrhs != 2)
      mexErrMsgTxt("Two inputs required.");
    if (nlhs > 1)
      mexErrMsgTxt("Too many output arguments.");
  
    /* Check that both inputs are row vectors. */
    if (mxGetM(prhs[0]) != 1 || mxGetM(prhs[1]) != 1)
      mexErrMsgTxt("Both inputs must be row vectors.");
    rows = 1; 
  
    /* Check that both inputs are complex. */
    if (!mxIsComplex(prhs[0]) || !mxIsComplex(prhs[1]))
      mexErrMsgTxt("Inputs must be complex.\n");
    
    /* Get the length of each input vector. */
    nx = mxGetN(prhs[0]);
    ny = mxGetN(prhs[1]);
  
    /* Get pointers to real and imaginary parts of the inputs. */
    xr = mxGetPr(prhs[0]);
    xi = mxGetPi(prhs[0]);
    yr = mxGetPr(prhs[1]);
    yi = mxGetPi(prhs[1]);
    
    /* Create a new array and set the output pointer to it. */
    cols = nx + ny - 1;
    plhs[0] = mxCreateDoubleMatrix(rows, cols, mxCOMPLEX);
    zr = mxGetPr(plhs[0]);
    zi = mxGetPi(plhs[0]);
  
    /* Call the C subroutine. */
    convec(xr, xi, nx, yr, yi, ny, zr, zi);
  
    return;
  }
  

Entering these numbers at the MATLAB prompt

• x = [3.000 - 1.000i, 4.000 + 2.000i, 7.000 - 3.000i];
  y = [8.000 - 6.000i, 12.000 + 16.000i, 40.000 - 42.000i];
  

and invoking the new MEX-file

• z = convec(x,y)
  

results in

• z =
     1.0e+02 *
  
  Columns 1 through 4 
  
  0.1800 - 0.2600i 0.9600 + 0.2800i 1.3200 - 1.4400i 3.7600 - 0.1200i
  
  Column 5 
  
  1.5400 - 4.1400i
  

which agrees with the results that the built-in MATLAB function conv.m produces.

Passing Structures and Cell Arrays

Handling 8-,16-, and 32-Bit Data