How to perform simple numeric integration with odeint in C++ - c++

Question

Can you provide me with a simple example of performing a numeric integration with odeint in C++?
I would like to use the convenient integrate function, documented as:
integrate( system , x0 , t0 , t1 , dt )
Also I'm not sure, how to pass it instead of a function or a functor, a class method, if that's possible.

Answer 1

In C++11 you can use a simple lambda function wrapping the call to your member method
Class c;
auto f = [&c]( const state_type & x , state_type &dxdt , double t ) {
c.system_func( x , dxdt , t ); };
integrate( f , x0 , t0 , t1 , dt );
std::bind might also work, but then you have to take care if values are passed by reference of by value.
In C++03 you need to write a simple wrapper around your class method
struct wrapper
{
Class &c;
wrapper( Class &c_ ) : c( c_ ) { }
template< typename State , typename Time >
void operator()( State const &x , State &dxdt , Time t ) const
{
c.system_func( x , dxdt , t );
}
};
// ...
integrate( wrapper( c ) , x0 , t0 , t1 , dt );
(Boost.Bind will not work correctly with more then two arguments).

Answer 2

You mean examples in addition to the ones provided online?
#include <iostream>
#include <boost/array.hpp>
#include <boost/numeric/odeint.hpp>
using namespace std;
using namespace boost::numeric::odeint;
const double sigma = 10.0;
const double R = 28.0;
const double b = 8.0 / 3.0;
typedef boost::array< double , 3 > state_type;
void lorenz( const state_type &x , state_type &dxdt , double t )
{
dxdt[0] = sigma * ( x[1] - x[0] );
dxdt[1] = R * x[0] - x[1] - x[0] * x[2];
dxdt[2] = -b * x[2] + x[0] * x[1];
}
void write_lorenz( const state_type &x , const double t )
{
cout << t << '\t' << x[0] << '\t' << x[1] << '\t' << x[2] << endl;
}
int main(int argc, char **argv)
{
state_type x = { 10.0 , 1.0 , 1.0 }; // initial conditions
integrate( lorenz , x , 0.0 , 25.0 , 0.1 , write_lorenz );
}
And regarding the system, you can provide anything where the following expression is valid:
sys( x , dxdt , t ) // returning void
Check the user's guide (and more examples) online.