MATLAB: Dynamically allocated c++ array covert-convert to mxArray

c++ dynamically allocatedconvert mxarray

Hello every one, i am new to matlab. So what i am trying to do is solve a problem like: Ax=B

I get a nXn matrix from my c++ code and i want to pass its values to mxArray. Specifically i do something like:

double **Ac = NULL;

Ac = new double*[size];

for(i = 0; i < size; i++){

Ac[i] = new double[size];

}

mxArray *Am = NULL;

Am = mxCreateDoubleMatrix (size, size, mxREAL);

memcpy( ( void* )mxGetPr(Am), ( void* )Ac, size*size*sizeof(double));

engPutVariable(ep, "Am",Am);

engEvalString(ep, "plot(Am);");

It shows a plot but not the correct one. Any directions?

Thank you all

Best Answer

Ac is not an array which contains the double values, but an array of pointers to vectors, which contain the values. If you prefer this nested kind of data storage for any good reasons, you need to copy the data vector by vector also:

double *Am_p;
Am_p = mxGetPr(Am);
for (i = 0; i < size; i++) {
  memcpy(Amp, Ac[i], size * sizeof(double));
  Amp += size;
}

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Use regular output functions. E.g., ( after you assign the value of cresult but before you destroy the mxArray mresult ):

printf( "The result is %g\n", *cresult );

But note that you will need to pause your program at the end in order to see this output on the screen (otherwise the window will close immediately after printing and you will not see it).

SIDE NOTE:

All of this code:

  int const SIZE=1;
  int const SIZE2=1;
  double x[SIZE][SIZE2];
  x[1][1] = 1.0;
  mxArray *A=NULL;
  A=mxCreateDoubleMatrix(SIZE2, SIZE, mxREAL);
  memcpy((void *)mxGetPr(A), (void *)x, sizeof(double)*SIZE*SIZE2);

can be simplified to this:

  mxArray *A=NULL;
  A = mxCreateDoubleScalar(1.0);

Also, you should be getting in the habit of checking your pointers before using them. E.g., A, mresult, and cresult should all be checked to see that they are not NULL before you try to dereference them.

MATLAB: Matlab engine giving wrong result when called in a function in Cpp file

These lines:

int call_matlab_processing(double double_array[], int size_double_array,double * sorted, double * indices_cpp)
{
    :
  memcpy((void *)mxGetPr(T), (void *)double_array, sizeof(double_array));

The first argument of the function call_matlab_processing is the variable double_array. It turns out that this variable is of type "pointer to double" (i.e., double * ). I know it looks like you have declared it as an array, but that syntax is misleading. When used in a function argument, the notation

double variable_name[]

is equivalent to the notation

double *variable_name

In fact, even if you had use an explicit size it would have made no difference to the compiler. I.e., this notation in a function argument

double variable_name[10]

is also equivalent to the notation

double *variable_name

That is, the compiler sees that argument as a pointer, not as an array (you can't pass whole arrays in C/C++ function arguments this way). So downstream in your code when you use sizeof(double_array), it is equivalent to doing sizeof(double * ). The result will be either 4 (on 32-bit) or 8 (on 64-bit). So you are definitely not copying all of the elements in that memcpy call. You need to do this instead:

  memcpy((void *)mxGetPr(T), (void *)double_array, size_double_array*sizeof(double));

Also, FYI you don't really need the (void *) casts since converting pointers to/from void * is something the C/C++ compiler will automatically do for you.

Best Answer

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MATLAB: Matlab engine giving wrong result when called in a function in Cpp file

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