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The basic function to create a structure in a mex-file is
mxCreateStructMatrix
which creates a structure array with a two
dimensional matrix, or mxCreateStructArray
.
mxArray *mxCreateStructArray (int ndims, int *dims, int num_keys, const char **keys); mxArray *mxCreateStructMatrix (int rows, int cols, int num_keys, const char **keys);
Accessing the fields of the structure can then be performed with
mxGetField
and mxSetField
or alternatively with the
mxGetFieldByNumber
and mxSetFieldByNumber
functions.
mxArray *mxGetField (const mxArray *ptr, mwIndex index, const char *key); mxArray *mxGetFieldByNumber (const mxArray *ptr, mwIndex index, int key_num); void mxSetField (mxArray *ptr, mwIndex index, const char *key, mxArray *val); void mxSetFieldByNumber (mxArray *ptr, mwIndex index, int key_num, mxArray *val);
A difference between the oct-file interface to structures and the
mex-file version is that the functions to operate on structures in
mex-files directly include an index
over the elements of the
arrays of elements per field
; Whereas, the oct-file structure
includes a Cell Array per field of the structure.
An example that demonstrates the use of structures in a mex-file can be found in the file mystruct.c shown below.
#include "mex.h" void mexFunction (int nlhs, mxArray* plhs[], int nrhs, const mxArray* prhs[]) { int i; mwIndex j; mxArray *v; const char *keys[] = { "this", "that" }; if (nrhs != 1 || ! mxIsStruct (prhs[0])) mexErrMsgTxt ("expects struct"); for (i = 0; i < mxGetNumberOfFields (prhs[0]); i++) for (j = 0; j < mxGetNumberOfElements (prhs[0]); j++) { mexPrintf ("field %s(%d) = ", mxGetFieldNameByNumber (prhs[0], i), j); v = mxGetFieldByNumber (prhs[0], j, i); mexCallMATLAB (0, NULL, 1, &v, "disp"); } v = mxCreateStructMatrix (2, 2, 2, keys); mxSetFieldByNumber (v, 0, 0, mxCreateString ("this1")); mxSetFieldByNumber (v, 0, 1, mxCreateString ("that1")); mxSetFieldByNumber (v, 1, 0, mxCreateString ("this2")); mxSetFieldByNumber (v, 1, 1, mxCreateString ("that2")); mxSetFieldByNumber (v, 2, 0, mxCreateString ("this3")); mxSetFieldByNumber (v, 2, 1, mxCreateString ("that3")); mxSetFieldByNumber (v, 3, 0, mxCreateString ("this4")); mxSetFieldByNumber (v, 3, 1, mxCreateString ("that4")); if (nlhs) plhs[0] = v; }
An example of the behavior of this function within Octave is then
a(1).f1 = "f11"; a(1).f2 = "f12"; a(2).f1 = "f21"; a(2).f2 = "f22"; b = mystruct (a); ⇒ field f1(0) = f11 field f1(1) = f21 field f2(0) = f12 field f2(1) = f22 b ⇒ 2x2 struct array containing the fields: this that b(3) ⇒ scalar structure containing the fields: this = this3 that = that3
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