MEX: iterator and mxArray

Hey!

I've got a problem using iterators with mxArrays ! for now, i'm trying with something more linear but it doesn't work so good...

I use this function to read the whole buffer:

mxArray * bin = mxCreateNumericArray(ind_dim_order,dimsData,mxDOUBLE_CLASS, mxREAL);
   double * outputbin = mxGetPr(bin);
   for(size_t k =0;k<nbrSamplesTotal;k++,++tmp)
    {
      outputbin[k]=*(tmp);
    }
plhs[2] = bin;

For example i get this (10*3 matrix) but i can also have more dimensions.

1 - 101 - 201

2 - 102 - 202

3 - 103 - 203

4 - 104 - 204

5 - 105 - 205

6 - 106 - 206

7 - 107 - 207

8 - 108 - 208

9 - 109 - 209

10 - 110 - 210

My problem is : I have a cellArray in prhs[1] which says what i have to extract in this buffer. For example i've got : prhs[1] = {2, [5:12]}. so I've got to extract only the line two for the first dimension and lines 5 until 12 for the second dimension.

Second example : prhs[1] = {{1,6},[ ],{5:10}} I've got to extract lines 1 and 6 for the first dimension, nothing for the second, and 5 until 10 for the third dimension.

I did : mxArray * dim1 = mxGetCell(prhs[1],ee) to extract dim1, dim2,dim3 and dim4 which correspond to each cell of the cellArray.

I'm thinking about iterators to extract the good data, but i don't know how to do this.

Sorry for my English, it's hard to explain about mathematics and logic...

Thanks a lot !

[James Tursa]

What do you mean, exactly, by "extract"? Do you mean you need to index into the array to read these values in a loop? Do you mean you need to build a new array that contains these values? Or something else?

[Lea L]

initially, in the mxArray bin, all the values are there, as I wrote above.

In the case where there is a second argument, prhs [1], I must choose the correct values to put in the mxArray bin.

For Example 1: I have to take the second column, lines 5 to 12. and put them in the mwArray bin.

For Example 2 : I have to take the values of the first and the the sixth column, nothing for the second dimension, et lines from 5 to 12 for the third dimension, and put them in the mxArray bin.

sorry for not making myself clear..!

[Jan]

I still do not get it. Does "prhs[1] = {2, [5:12]}" mean that you want to get the subarray "A(2, 5:12)" as output, if A is the input?

Use mxDuplicateArray or memcpy to duplicate an complete array.

[Lea L]

It's almost that! this prhs[1] means i want get the subarray "A({5:12},2)"

here the input is : float * tmp.

and the output is bin.

I don't know how to do this in mex...

and how do you do wen prhs[1] is like {2, [ ], [5:12]} with an empty dimension?

[Lea L]

oops : A(5:12,2)

[Lea L]

do you have any idea, how to do that?

Answer 1

[Jan]

Extracting "A(index1, index2, ...)" in a MEX function is not trivial. But this is implemented in Matlab already. You can use mexCallMATLAB to call the SUBSREF command, but this needs a struct as input and it is not trivial to create it. I assume this is the easiest method:

function Y = Cut(X, varargin)
Y = X(varargin{:});

This can be called through mexCallMATLAB, if you really need it in a MEX. Implementing this in C will need more than 100 lines of code, because a lot of exceptions have to be considered: less indices than dimensions, more indices with trailing ones, ':' to specify the complete slice, indices as SINGLE or (U)INT8/16/32/64, logical indexing, rejection of complex and negative and non-integer values, linear indexing, ... Although a good C-implementation might be faster than calling the one-liner shown above through mexCallMATLAB, I bet that the programming and debug time will exceed the saved time by factor 1000.

[Lea L]

thanks for your answer! I will use mxCallMATLAB then, but I don't really understand how to do that (I'm in intership and I've just begun in Mex and MATLAB...)

I don't understand :

function Y = Cut(X, varargin)

Y = X(varargin{:});

what is X ? do I have to create a function in MATLAB?

[Lea L]

I tried :

mxArray * bini = mxCreateNumericArray(ind_dim_order,dimsData,mxDOUBLE_CLASS, mxREAL);

double * outputbin = mxGetPr(bini);

for(size_t k =0;k<nbrSamplesTotal;k++,++tmp){outputbin[k]=*(tmp);}

mxArray *rhs[8];

mxArray * dimStruct;

rhs[0] = mxCreateString("."); rhs[1] = dim4;

rhs[2] = mxCreateString("."); rhs[3] = dim3;

rhs[4] = mxCreateString("."); rhs[5] = dim2;

rhs[6] = mxCreateString("."); rhs[7] = dim1;

cout<<"ok1"<<endl;

mexCallMATLAB(1, &dimStruct, 8, rhs, "substruct");

for(int t=0; t<8; t++) {mxDestroyArray(rhs[t]);}

plhs[2]= dimStruct;

cout<<"ok2"<<endl;

rhs[0] = (mxArray *)bini;

rhs[1] = dimStruct;

cout<<"ok3"<<endl;

mexCallMATLAB(1,&bin,2, rhs, "subsref");

cout<<"ok4"<<endl;

it's ok for compilation,the struct is created, but MATLAB is not working when i call subsref :

??? Error using ==> subsref

The "subs" field for the subscript argument to SUBSREF and SUBSASGN must be a cell or character array.

for remember : mxArray * dim1 = mxGetCell(prhs[1],ee);

and prhs[1] = {2, {5:9}};

and mxArray * bin;