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Is there an easy way to copy part of an array using mex functions?

3 Ansichten (letzte 30 Tage)
Robert
Robert am 13 Jun. 2013
I'm aware of mxDuplicateArray to copy a whole mxArray, but if I just one to copy just one "column" or "row" of an mxArray, is there a helper mex function for this?
In other words, is there a mex function equivalent to:
B = A(:, i);
C = A(i, :);
etc.

Antworten (2)

James Tursa
James Tursa am 13 Jun. 2013
There is no mex API function for this directly, but it would be easy to accomplish a simply copy (e.g., a single column) with a few lines of code. The challenge would be how robust you want the code to be in more general cases. E.g., do you want it to handle all class of variables (not just double)? All possible dimensions? Complex? Sparse? Etc.
If you need help with code for a specific copy (e.g., single row or column) let me know.
  5 Kommentare
Jan
Jan am 19 Jun. 2013
A Matlab array is an array, which can be used inside Matlab. It can have different types and has a header, which is used internally in Matlab. A C-array is a pure bolck of reserved memory.
mxArray *MatlabArray;
double *CArray;
MatlabArray = mxCreateDoubleMatrix(2,4, mxREAL);
CArray = (double *) mxMalloc(2 * 4 * sizeof(double));
Robert
Robert am 19 Jun. 2013
Ok, now I understand what you were asking (I think there is a typo: "Do you want the copy as a Matlab or C array"). I would prefer the copy as a C array, but still your answer for creating a Matlab array (mxArray) was helpful. I'm still working on my specific implementation though...

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Jan
Jan am 13 Jun. 2013
Bearbeitet: Jan am 19 Jun. 2013
To copy a subvector to a new Matlab array:
mxArray *A, *B, *C;
mwSize s1, s2, i;
double *Ap, *Cp;
A = (mxArray *) prhs[0]; // The 1st input, ignore const qualifier
Ap = mxGetPr(A);
s1 = mxGetM(A);
s2 = mxGetN(A);
// B = A(:, i)
B = mxCreateDoubleMatrix(s1, 1, mxREAL);
memcpy(mxGetPr(B), Ap+(i-1)*s1, s1 * sizeof(double)); % EDITED
// C = A(i, :)
C = mxCreateDoubleMatrix(1, s2, mxREAL);
Cp = mxGetPr(C);
for (i = 0; i < s2; i++) {
Cp[i] = Ap[i * s1];
}
This is not tested! Please check this for typos carefully.
  8 Kommentare
James Tursa
James Tursa am 20 Jun. 2013
mxGetM would crash MATLAB if you passed an invalid pointer to it, or if something upstream corrupted memory and mxGetM just happened to be the point at which the crash finally happened. You will probably need to post your code (not just a code snippet) for us to look at.
Jan
Jan am 20 Jun. 2013
A = (mxArray *) prhs[0]; n=mxGetN(A); could fail, when you call the MEX without input arguments, because A is a null pointer then. I cannot test this currently, but as far as I remember mxGetM/N does not test this before.

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