Plotting a 2D "Time Snapshot"

Hello. I am having trouble plotting a 2D "time snapshot" of a solution to a parabolic heat equation. The mesh plot seen near the bottom of my code works like a charm. However, I am supposed to try and plot a 2D "time snapshot" at time t = 20, so essentially just taking a slice of my 3D mesh. I am unsure how to do this. I have tried numerous things and nothing seems to be working. One of my classmates was able to use plot(:,401); to get his to work, where 401 is the final n iterate. This does not seem to work for me though. I get an error about an undefined function or variable with plot. From what I can tell, it does not like the semi-colon. I am not sure why this is a problem for me and not him, as our code is essentially the same. If anyone is able to show me how to make this 2D plot I would greatly appreciate it. A photo of my code is below:

[KSSV]

Copy your code here....not a image.

[Matthew Osler]

% This code solves the Parabolic Heat Equation

L = 2*pi;       % length of metal rod
T = 20;         % length of time interval

c = 1;          % diffusion coefficient

m = 20;         % number of steps in spatial grid
n = 400;         % number of steps in temporal grid

h = L/m;        % spatial step size
k = T/n;        % temporal step size

x = 0:h:L;      % initialize spatial vector
t = 0:k:T;      % initialize temporal vector

f = -sin(x/4);  % temperature distribution at time t=0

g1 = sin(t);    % left endpoint boundary condition
g2 = 0;         % right endpoint boundary condition

% initialize solution matrix u(x,t)
u = zeros(m+1,n+1);
u(:,1) = f;
u(1,:) = g1;
u(m+1,:) = g2;

r = (c*k)/(h^2);            % constant coefficient

[X,T] = meshgrid(x,t);      % define function q(x,t)

q = 10*X.^(-2)+T.^(-2);     % function q(x,t)
Q = q.';                    % orient matrix Q same as matrix u

% solve the parabolic heat equation
for jj = 1:n
    for ii = 2:m
        u(ii,jj+1)=r*u(ii-1,jj)+(1-2*r)*u(ii,jj)+r*u(ii+1,jj)+k*Q(ii,jj+1);
    end
end

figure(1)
mesh(x,t,u');
title('Metal Rod with Applied Heat Source')
xlabel('Length of Metal Rod')
ylabel('Time')

[Matthew Osler]

Sorry about that... getting used to using this forum.

Answer 1

[KSSV]

plot(t,u(20,:))

The one you are trying:

plot(:,401);  % this is wrong 

YOu have not specified the variable there. It should be

plot(u(:,401))

[Matthew Osler]

Thank you so much KSSV!

plot(u(:,401))

This was exactly what I needed. So Matlab essentially couldn't tell what variable to plot there with the way I was doing it, in a sense?

Also, I believe you answered another question I had on here a couple of months ago. I really appreciate your help! I just started learning Matlab this semester.