Re: Using expm in C++

[Ben Abbott]

On Feb 6, 2012, at 5:58 PM, loisp wrote:

> Hello,
> 
> I'm writing a C++ program and want to use the Octave function expm()
> (exponential of a matrix). Here is the relevant snippet from my code. 
> 
>       hamiltonian = Matrix (N+1, N+1);                                        
>         
>       for(int i = 0; i < N+1; i++) 
>       {
>               for(int j = 0; j < N+1; j++) 
>               { 
>                       if((i+1 == j) || (i == j+1))
>                               hamiltonian (i, j) = -1;
>               } 
>       }
> 
>       hamiltonian = hamiltonian.expm();
> 
> 
> When I try to compile my program using mkoctfile I get the following error 
> 
>       dmatrix.cpp: In function ‘int main()’:
>       dmatrix.cpp:54: error: ‘class Matrix’ has no member named ‘expm’
> 
> I was wondering if the classes Matrix and ComplexMatrix provide that
> function and if they don't, do you have any work around to suggest? I am
> using MacOSX and have Octave installed though fink.
> 
> Thanks in advance. 
> 
> Lois

I may be wrong, but I don't think there is an expm is an on the c++ side. 
However, there is an m-file version. You can call it via feval (although I've 
never tried).

http://www.gnu.org/software/octave/doc/interpreter/Calling-Octave-Functions-from-Oct_002dFiles.html

Ben