quickstart_firstexample.md

Your first REBOUND simulation

If you have successfully installed REBOUND, then you are now ready to run your first simulation. On this page, we'll walk you through one simple example, line by line.

Python version

There are different ways to run python code:

• interactively, by using the python interpreter
• by executing a python script
• by using a Jupyter notebook

All of these methods work with REBOUND. Choose whichever you are most comfortable with.

We start by importing REBOUND:

import rebound

To run an N-body simulation, we need to create a simulation object first:

sim = rebound.Simulation()

Then, we add particles to the simulation:

sim.add(m=1.)                # Central object
sim.add(m=1e-3, a=1., e=0.1) # Jupiter mass planet 
sim.add(a=1.4, e=0.1)        # Massless test particle

Now we can integrate the particles forward in time using the default integrator (IAS15) for 100 time units:

sim.integrate(100.)

Finally, let us output the cartesian coordinates and the orbital parameters at the end of the simulation:

for p in sim.particles:
    print(p.x, p.y, p.z)
for o in sim.calculate_orbits(): 
    print(o)

As a next step, have a look at the examples and tutorials in the python_examples and ipython_examples directories.

C version

A very short example is provided in the examples/simplest/ directory. Go to this directory with

cd examples/simplest/

Then have a look at the source code in the problem.c file. First, we include the REBOUND header file which contains all the public function prototype and dataype definitions for REBOUND:

#include "rebound.h"

In the main function, we first create a REBOUND simulation with

struct reb_simulation* r = reb_create_simulation();

This function has now allocated memory for the simulation and initialized all the variables in the simulation to their default values. We can then add particles to the simulation:

reb_add_fmt(r, "m", 1.);                // Central object
reb_add_fmt(r, "m a e", 1e-3, 1., 0.1); // Jupiter mass planet
reb_add_fmt(r, "a e", 1.4, 0.1);        // Massless test particle

We then integrate the simulation for 100 time units with the default integrator (IAS15):

reb_integrate(r,100.);

After the integration is done, we can output the cartesian coordinates and the orbital parameters:

for (int i=0; i<r->N; i++){
    struct reb_particle p = r->particles[i];
    printf("%f %f %f\n", p.x, p.y, p.z);
}
struct reb_particle primary = r->particles[0];
for (int i=1; i<r->N; i++){
    struct reb_particle p = r->particles[i];
    struct reb_orbit o = reb_tools_particle_to_orbit(r->G, p, primary);
    printf("%f %f %f\n", o.a, o.e, o.f);
}

To compile the example, simple type

make

into a terminal window while you're in the examples/simplest/ directory. Then run the simulation with

./rebound