MATLAB: Help ploting different particles in a box

script

I want to create a code that simulates multiple particles coliding with the walls of a box but I dont want the particles to colide with each other.
I alredy made a script for one particle, but Im having trouble making one for multiple ones.
Can someone help me please?
So far I got this:
Here's the main script:
a=20;
b=10;
R=0.5;
dt=0.1;
np=3; %Number of particles
rx=a*rand(1,np);
ry=b*rand(1,np);
r=[rx;ry];
v=5*(2*rand(2,np)-1);
tmax=100;
t=0; 
while (t<tmax)
    for i=1:np
        deltat(i,1)=(R-r(1,i))/v(1,i); deltat(i,2)=(a-R-r(1,i))/v(1,i);
        deltat(i,3)=(R-r(2,i))/v(2,i); deltat(i,4)=(b-R-r(2,i))/v(2,i);
        
        if deltat(i,1)<=4*eps
            deltat(i,1)=1e6;
        end
        if deltat(i,2)<=4*eps
            deltat(i,2)=1e6;
        end
        if deltat(i,3)<=4*eps
            deltat(i,3)=1e6;
        end
        if deltat(i,4)<=4*eps
            deltat(i,4)=1e6;
        end
    end
    deltat_wall=[deltat(:,1),deltat(:,2),deltat(:3),deltat(:,4)]; %must be a matrix of np particles and 4 columns 
    [deltat, column]=min(deltat_wall,[],2);
    [deltat,particle]=min(deltat); 
    wall=column(particle); 
    animacion(r,v,a,b,R,dt,deltat) % the function is just to plot the box and the posicion of the particles
    r=r+v*deltat;
    ep=[[1;0],[-1;0],[0;1],[0;-1]];
    v=v-2*dot(v,ep(:,wall))*ep(:,wall);
    t=t+deltat;
end
Here´s the function:
function  []= animacion(r,v,a,b,R,dt,deltat)
    x1=0;
    x2=a;
    y1=0;
    y2=b;
    side_a=[x1,x2,x2,x1,x1];
    side_b=[y1,y1,y2,y2,y1];
       
    for t=0:dt:deltat
        plot(side_a,side_b,'b-','Linewidth',2)
        hold on
        plot(r(1,:),r(2,:),'ko','MarkerFaceColor','k', 'MarkerSize',28*R)
        xlim([-10,30])
        ylim([-10,20])
        drawnow
        hold off
        r=r+v*dt;
    end
end

Best Answer

If the last step is to just calculate a new speed after the shock then this would apply to a single particle only. So we would modify the velocity of just that one particle instead of trying to modify the velocities for each. Try changing 
v=v-2*dot(v,ep(:,wall))*ep(:,wall);
to
v(:, particle) = v(:, particle) - 2*dot(v(:, particle),ep(:,wall))*ep(:,wall);
where the only difference is how we index into v.
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