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

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Robert 2013-6-13

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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.

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回答（2 个）

James Tursa 2013-6-13

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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.

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Jan 2013-6-19

在 MATLAB Online 中打开

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 2013-6-19

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 2013-6-13

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https://ww2.mathworks.cn/matlabcentral/answers/78908-is-there-an-easy-way-to-copy-part-of-an-array-using-mex-functions#answer_88650

编辑：Jan 2013-6-19

在 MATLAB Online 中打开

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.

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James Tursa 2013-6-20

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 2013-6-20

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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