Accessing elements in an mxArray

Question

Caroline 2013-7-24

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https://ww2.mathworks.cn/matlabcentral/answers/83058-accessing-elements-in-an-mxarray

I am using the MATLAB engine to read a excel file using C++. I am trying to access elements in an mxArray that was created using engEvalString and xlsread, they were used in the following way:

mxArray *num;

mxArray *text;

mxArray *raw;

engEvalString(ep, "[num, text, raw] = xlsread('C:\\rest_of_file_path')")

However, now I am trying to access the elements of raw by using mxGetCell(*mxArray, int ind). I know that this returns a pointer to the cell with the index ind.I used mxGetCell in the following way (dereference the pointer to return the actual value in the cell with the specified index) :

cout << *mxGetCell(num, 40) << endl;

but I got the following error:

binary '<<': no operator found which takes right-hand operand of type 'mxArray' (or there is no acceptable conversion).

The right hand operator shouldn't be an mxArray, shouldn't it be a number? What is wrong with they way I am dereferencing the pointer? How could I get the values in the mxArray?

Any help that anyone could provide would be much appreciated.

Thanks!

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Answer 1

James Tursa 2013-7-24

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mxGetCell(num, 40) returns an (mxArray *), i.e. a pointer to an mxArray. Thus, by extension, *mxGetCell(num, 40) deferences that pointer into an mxArray. Hence the error message you are seeing. Each "element" of a cell array is an (mxArray *). You need to get at the data of the underlying mxArray. E.g.,

cout << *mxGetPr(mxGetCell(num, 40)) << endl;

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James Tursa 2013-7-25

编辑：James Tursa 2013-7-25

在 MATLAB Online 中打开

Since you have both that you need to deal with and you won't know until runtime what you have, you will need to wrap some logic around your output statement. E.g., something like this

char *cp;
mxArray *cell;
    :
cell = mxGetCell(num,1);
if( cell == NULL ) {
    ; // Do nothing this branch since there is nothing in the cell
} else if( mxIsChar(cell) ) {
    cp = mxArrayToString(cell); // MATLAB char array to C-style string
    cout << cp << endl;
    mxFree(cp); // Free the dynamically allocated C-style string
} else {
    cout << *mxGetPr(cell) << endl; // Assume mxArray is double
}