How to - "sort" using mexCallMATLAB ?

Dear all,

I am writing a mex-function and need to sort some data using the mexCallMATLAB API so that I get the data in the required form. i.e I need the data in descending order along with the index values.

In matlab - [y,i] = sort(x,1,'descend'); does the job for me. Can I simulate the same through the mexCallMATLAB API? If yes, how does it fit in the syntax for the API?

or

Should I write a C-code for the same?

Thanks in advance

Answer 1

[Jan]

Using a quicksort in C directly might be more efficient. Especially if your data set is small, the overhead of calling Matlab will be important. For large data sets it matters, that Matlab's SORT is highly efficient and mexCallMATLAB could be the best choice:

mxArray *In[3];
mxArray *Out[2];
In[0] = <your data>;
In[1] = mxCreateDoubleScalar(1.0);
In[2] = mxCreateString("descend");  
mexCallMATLAB(2, Out, 3, In, "sort");

[Rajaram B S R]

Hi Jan,

I am not able to populate the arguments properly, especially the 24x1 array data for 'In[0]' that needs to be sorted. Here is my code.

double *Data;

mxArray *In[3];

Data = mxCalloc(24,sizeof(double));

ii = 0;

for (i = 0; i < 576; i+=25)

Data[ii++] = eV[i]; //Diagonal extraction from Eigen Value matrix (eV)

In[0] = mxCalloc(24,sizeof(double));

mxSetPr(In[0], Data);

/* In[1] & In[2] are populated as suggested by you */

mexCallMATLAB(0, NULL, 1, In[0], "disp"); // CRASHES HERE

mexCallMATLAB(2, Out, 3, In, "sort");

mexCallMATLAB(0, NULL, 1, In[1], "disp");

mexCallMATLAB(0, NULL, 1, In[2], "disp");

Could you point out if I am doing anything wrong here? I am using MSVC 2005 for debugging purposes.

Thanks in advance.

[Kaustubha Govind]

The right way to do this would be something like:

double *Data;

mxArray *In[3];

In[0] = mxCreateDoubleMatrix(24, 1, mxREAL);

Data = mxGetPr(In[0]);

ii = 0;

for (i = 0; i < 576; i+=25)

Data[ii++] = eV[i];

[Rajaram B S R]

Hi Kaustaubha,

Thank you very much!

Can you tell me why mxCalloc does not work here?

[Jan]

mxCalloc allocates bare bytes in the memory. mxCreateDoubleMatrix creates a Matlab variable of type DOUBLE. This has additional fields for the dimensions, a pointer to the real and imaginary data, and a lot of further information.

Answer 2

[Kaustubha Govind]

Yes, you should be able to call into the MATLAB 'sort' implementation using mexCallMATLAB. Whether or not to implement your own algorithm in C is subjective. There is potential for performance improvement in some cases because MATLAB functions are interpreted and there is overhead for data marshaling, but it appears that the MATLAB sort is multithreaded, so you might not be able to do any faster than MATLAB can, unless you plan to call into some highly-optimized libraries. If performance is very important to you, perhaps you should perform some benchmarking to see if the MATLAB implementation is good enough for your application.