N-body simulation with collisions - c++

Question

I wrote a brute-force 3-dimensional N-body simulation in C++ which works as expected.
I tried to implement inelastic particle collisions by including the line "myPhys.Collisions(particleList);" as part of my for-loop in the main.cpp where "Collisions" is defined in the "Physics" class (also shown below).
The code runs and produces output in data.csv. However, sometimes the run is interrupted by "Segmentation fault: 11" and the collisions lead to unexpected behavior.
I am wondering if I am somehow not handling vectors correctly in the "Collide" function, especially in the lines
// create particle with new parameters
Particle newParticle(newMass, newPosition, newVelocity);
// replace first particle in collision with new particle
particleList[idx1] = newParticle;
// erase second particle in collision
particleList.erase(particleList.begin()+idx2);
I should also mention that I'm new to C++ so I apologize if I'm doing something stupid.
The full code is available at https://www.dropbox.com/sh/b3oq7x546c9lktv/AAAdUE7Nrzaiov0TaGtgjbfNa?dl=0
main.cpp
#include <iostream>
#include <fstream>
#include <vector>
#include "vectorops.h"
#include "particle.h"
#include "physics.h"
using namespace std;
int main(){
ofstream myData;
myData.open("data.csv");
// initialize a vector of particles
int numParticles = 20;
vector<Particle> particleList;
for(int i = 0; i < numParticles; i++){
particleList.push_back(Particle(1));
}
// declare physics engine
Physics myPhys;
// time evolution
for(int t = 0; t < 10000; t++){
// calculate force on every particle
vector< vector<double > > currentForces = myPhys.forceList(particleList);
// iterate over particleList
for(int i = 0; i < numParticles; i++){
if(i < particleList.size())
{
// compute accelerations
particleList[i].updateAcceleration(currentForces[i]);
// increment velocities
particleList[i].updateVelocity();
// increment positions
particleList[i].updatePosition();
// print coordinates of each particle
for(int j = 0; j < 3; j++){
myData << particleList[i].getPosition().at(j) << ",";
}
}
else
{
for(int j = 0; j < 3; j++){
myData << 0 << ",";
}
}
}
// update particleList after collisions
myPhys.Collisions(particleList);
myData << "\n";
}
myData.close();
return 0;
}
extract from Physics.cpp
void Physics::Collide(vector<Particle> &particleList, int idx1, int idx2){
// compute parameters of new particle
double m1 = particleList[idx1].getMass();
double m2 = particleList[idx2].getMass();
double newMass = m1+m2;
vector<double> newVelocity;
vector<double> v1 = particleList[idx1].getVelocity();
vector<double> v2 = particleList[idx2].getVelocity();
for(int i = 0; i < 3; i++){
newVelocity.push_back((m1*v1[i]+m2*v2[i])/newMass);
}
vector<double> newPosition;
vector<double> x1 = particleList[idx1].getPosition();
vector<double> x2 = particleList[idx2].getPosition();
for(int i = 0; i < 3; i++){
newPosition.push_back((m1*x1[i]+m2*x2[i])/newMass);
}
// create particle with new parameters
Particle newParticle(newMass, newPosition, newVelocity);
// replace first particle in collision with new particle
particleList[idx1] = newParticle;
// erase second particle in collision
particleList.erase(particleList.begin()+idx2);
}
void Physics::Collisions(vector<Particle> &particleList){
for(int i = 0; i < particleList.size();i++){
for(int j=0; j < i; j++){
double dist = Vectors.distance(particleList[i].getPosition(),particleList[j].getPosition());
if(dist < 0.01){
Collide(particleList,i,j);
}
}
}
}

Answer 1

The problem with the code is that after "particleList" is resized as a result of a collision, the positions of the particles in "myData" are not correctly aligned with the previous time step.
A quick workaround is to simply move one of the colliding particles to a large distance from the others, rather than deleting it from the vector
// create particle with new parameters
Particle newParticle(newMass, newPosition, newVelocity);
// replace first particle in collision with new particle
particleList[idx1] = newParticle;
// move second particle to large distance
vector<double> farAway;
for(int i = 0; i < 3; i++){
farAway.push_back(1000000);
}
particleList[idx2].setPosition(farAway);
This is not particularly elegant, however. I am thinking that a more elegant approach might be to store the position histories in the particle objects themselves.