External Interfaces

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: 1.8 $ */
  
  #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

© 1994-2005 The MathWorks, Inc.