How can a plot from an "if" inside of a while loop?

So in this piece of code I'm trying to work out how long it takes for gas particles to go from side of a box to the other, assuming only one can go at a time for each interval and also has an equal chance of going left-to-right as well as right-to-left. I have the code working in such a way that I can get the values in the command window, but it won't graph each value of m and n on the same plot against t. Here's the code just trying to plot (t,m)

N=input('The total number of particles in the system is:');         %The total number of particls in the system
n=N;          %The number of particles on left side at start.
m=0;          %The number of particls remaining on the right side of the system at start.
t=0;
hold on
while m<N/2

      t=t+1;          %Time interval
      r=rand;         %Determines if a particle moves from left to right

      if r<n/N        %Moves particle left to right
          n=n-1;
          m=m+1;
      elseif r>n/N    %Moves particle right to left
          n=n+1;
          m=m-1;

      end

      display(t);
      display(n);
      display(m);

end
hold off
 plot(t,m)

Answer 1

[Kirby Fears]

Pre-allocate a large array to load values into during the loop. At the end of the loop, you can plot them all at once. I added tMax to limit the while loop and give an initial size for m and n arrays.

N=input('The total number of particles in the system is:');
n=N;          %The number of particles on left side at start.
m=0;          %The number of particls remaining on the right side of the system at start.
tMax = 10000; % Maximum number of periods before exiting loop
mArray = NaN(tMax,1); % Initialize array to store m values
nArray = NaN(tMax,1); % Initialize array to store n values
t=0;

while m<N/2 && t<tMax

        t=t+1;          %Time interval
        r=rand;         %Determines if a particle moves from left to right

        if r<n/N        %Moves particle left to right
            n=n-1;
            m=m+1;
        elseif r>n/N    %Moves particle right to left
            n=n+1;
            m=m-1;

        end

        nArray(t) = n;
        mArray(t) = m;

end
mArray = mArray(1:t);
nArray = nArray(1:t);
plot([mArray,nArray]);
legend({'m','n'});

Hope this helps.

[Conor McCan]

Thanks a million! Worked a charm!

Answer 2

[jgg]

Try this:

 N=input('The total number of particles in the system is:');         %The total number of particls in the system
n=N;          %The number of particles on left side at start.
m=0;          %The number of particls remaining on the right side of the system at start.
t=0;
big = 10000;
lim = big;
t_vals = zeros(big,1);
m_vals = zeros(big,1);
 hold on
 while m<N/2
      t=t+1;          %Time interval
      r=rand;         %Determines if a particle moves from left to right
      if r<n/N        %Moves particle left to right
          n=n-1;
          m=m+1;
      elseif r>n/N    %Moves particle right to left
          n=n+1;
          m=m-1;
      end
      display(t);
      display(n);
      display(m);

      if t+1 > lim
          t_vals = [tvals; zeros(big,1)];
          m_vals = [tvals; zeros(big,1)];
          lim = length(m_vals);
      end

      t_vals(t+1) = t;
      m_vals(t+1) = m;

 end
 hold off
 ind = find(t_vals == 0,2);
 plot(t_vals(1:ind(2)-1),m_vals(1:ind(2)-1))

You can also just change plot to scatter and move it inside your while loop.