How can i plot ALL values produced from while loop from the following code??

I have written the following code and wish to plot all values produced but at the moment it only gives one point

clear
q=-1;
m=3;
r0=[1 2 0];
v0=[2 2 0];
B=[0 0 1];
E=[0 0 0];
t=0;
dt=0.2;
h=dt/2;
tfinal=0.1;
while t<tfinal
    vpre=v0+(((q*E)/m)*h);
    b=norm(B,2);
    t=(((q*b)/m)*h);
    s=(2*t)/(1+(t^2));
    vxpre=vpre(1);
    vypre=vpre(2);

      vdash=vxpre + (vypre*t);
      vypost=vypre - (vdash*s);
      vxpost=vdash + (vypost*t);

      vpost=[vxpost vypost 0];

      v1=vpost+(((q*E)/m)*h);
      r1=(dt*v1)+r0;
      x=r1(1);
      y=r1(2);

     plot(x,y,'mo')
     hold on;

     t=t+dt; 
     r1=r0;
      v1=v0;

end

Answer 1

[the cyclist]

Simple way to modify your code:

• Define a counter n (initialized to zero) before your while loop.
• Update n=n+1 in every iteration of your while loop.
• Set x(n) = r1(1) and y(n) = r1(2).
• Pull the plotting routine out of the while loop.

If your while loop runs for many iterations, you will need to preallocate memory for x and y to make this run most efficiently.

[Byron Uzoka]

It still shows an empty graph

Answer 2

[Phoebe]

I did what you said so my amended code reads;

clear q=-1; m=1; r0=[1 2 0]; v0=[2 2 0]; B=[0 0 1]; E=[0 0 0]; t=0; dt=0.2; h=dt/2; tfinal=0.1; n=0; while t<tfinal vpre=v0+(((q*E)/m)*h); b=norm(B,2); t=(((q*b)/m)*h); s=(2*t)/(1+(t^2)); vxpre=vpre(1); vypre=vpre(2);

    vdash=vxpre + (vypre*t);
    vypost=vypre - (vdash*s);
    vxpost=vdash + (vypost*t);

    vpost=[vxpost vypost 0];
    n=n+1;
    v1=vpost+(((q*E)/m)*h);
    r1=(dt*v1)+r0;
    x(n)=r1(1);
    y(n)=r1(2);
    t=t+dt;

   r1=r0;
    v1=v0;

end plot(x,y,'mo') hold on;

same problem of a single point??

[the cyclist]

I ran your while loop. It only has one iteration, so it only generates one point.

You seem to be using the variable "t" in two different ways inside your loop. Once to just count up

 t = t + dt;

but also in a more complex way:

t=(((q*b)/m)*h);

That seems wrong to me, and may be why your loop expectedly exits after just one iteration.

Answer 3

[Luis Pinto]

just add the command "drawnow" at the end of the loop.