for(i=0;i<num_balls;i++){
        for(j=i+1;j<num_balls;j++)
           if(are_colliding(positions[i],positions[j],velocities[i],velocities[j],
              ball_radius)==true)
                collide(positions[i],positions[j],velocities[i],velocities[j]);
        enforce_walls(positions[i],velocities[i],ball_radius,width,height);
   }
   /*second loop, updates everything according to time_step*/
   for(i=0;i<num_balls;i++)
       for(j=0;j<2;j++)
           positions[i][j]=positions[i][j]+velocities[i][j]*time_step;