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

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Conor McCan el 22 de En. de 2016
Comentada: Conor McCan el 22 de En. de 2016
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)
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Kirby Fears el 22 de En. de 2016
Editada: Kirby Fears el 22 de En. de 2016
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.
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Conor McCan el 22 de En. de 2016
Thanks a million! Worked a charm!

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### Más respuestas (1)

jgg el 22 de En. de 2016
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.
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