How can I label a solution as NaN in a MEX code?

I am using solvers of LAPACK libraries in a MATLAB MEX file for solving linear system of equations. For some of the cases, the system that I solve is singular. For example, the system is as follows for one of my cases:

    A =
     0.00000000  0.00000000  0.00000000
     0.00000000  0.00000000  0.00000000
     0.00000000  0.00000000  77.31867171

    b:
    -0.00000000  -0.00000000  -0.00000000

What would be the best approach to label this solution as NaN similar to MATLAB?

Answer 1

[James Tursa]

Use the API function mxGetNaN:

http://www.mathworks.com/help/matlab/apiref/mxgetnan.html?searchHighlight=mxgetnan

E.g., to put a NaN value into a double for returning to the MATLAB workspace:

double d = mxGetNaN( );

Answer 2

[José-Luis]

 if (some_condition)
     some_value = 0.0/0.0; %This will produce a NaN
  end

[AP]

I understand this. I wanted to label some_value as NaN.

[José-Luis]

I am afraid I don't understand the problem. To test for singularity you could use:

 rank(some_matrix)

And if you find out some_matrix is singular then set some_value as NaN.

[AP]

Since I am writing a c++ code in MATLAB, I am trying to find a way to label the solution of a singular matrix to NaN in the c++ code. I am using LAPACK solvers to solve a linear system of equation. At some point the solution to this system fail and I want to be able to label this as NaN in my c++ code. Hope this clarifies my question.

[José-Luis]

 double myNaN = 0.0/0.0;

will produce a NaN in C++. Alternatively, you could set it to some other value

 double flag = numeric_limits<double>::max();

that you then change into NaN in Matlab.