Error - I am new to Matlab and I just need help to graph this data I am getting a strange error.

clear all;

close all;

clc;

x = 0; % [m]

t = [0:300]; % [s]

h = 100; % [W/m^2*K]

% k = 2; % [W/m*K]

alpha = 5.6*10^-6; % [m^2/s]

% Temperatures

T_i = 325; % [Degrees Celcius]

T_infinity = 15; % [Degrees Celcius]

for k = [2;20;200]

T_xt = ((erfc((x)/(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2)))*erfc((x/(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;

%fprintf('Temperature in Degrees Celcius: %0.3f\n\n', T_xt);

plot(t,T_xt, '-','DisplayName', ['k' num2str(k)]);

hold on;

end

hold off;

fprintf('Temperature in Degrees Celcius: %0.3f\n\n', T_xt);

Error is in line 17 of code for the T_xt formula it says:

Error using /

Matrix dimensions must agree.

[Pooja Kumari]

Can you share the formula you are using for T_xt ?

[David]

Certainly. My objective here is to graph time (t) and temperature (T_xt) at serveral different values of k.

The formula:

I hope this helps.

[Pooja Kumari]

This code will run for three values only.

clear all;
close all;
clc;
x = 0;                          % [m]
t = [0:300];                      % [s]
h =  100;                       % [W/m^2*K]
% k = 2;                          % [W/m*K]
alpha = 5.6*10^-6;              % [m^2/s]
% Temperatures
T_i = 325;                      % [Degrees Celcius]
T_infinity = 15;                % [Degrees Celcius]

for k = [2,20,200]    
    T_xt = ((erfc((x)./(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2))).*erfc((x./(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;
%      T_xt = ((erfc((x)/(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2)))*erfc((x/(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;
    %fprintf('Temperature in Degrees Celcius: %0.3f\n\n', T_xt);
    plot(t,T_xt, '-','DisplayName', ['k' num2str(k)]);
    hold on;
end
hold off;

Answer 1

[Walter Roberson]

for k = [2;20;200]

An obscure fact is that "for" iterates along columns of the expression. Your right hand side is a column vector so all of it is copied to k. And then you have /k but k is a vector...

You should change your right hand side of the for to have a row vector.

[David]

So I changed it to k = [2,20,200] and I'm still getting the same answer. :(

Thank you for the help!

[Walter Roberson]

t = [0:300];

That is a vector

T_xt = ((erfc((x)/(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2)))*erfc((x/(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;

Notice the /(2*sqrt(alpha*t)) . But t is a vector, so you have / vector

When you have / and a vector on the right hand side, the vector must be a column vector and the left side must be something that has the same number of rows. The result will be a vector with the same number of columns as the left side.

It is a common mistake to think that / is the division operator. It is not. A/B is a fitting operator similar to A*pinv(B) where * is algebraic matrix multiplication (inner product). The division operator is ./ not /

Answer 2

[Voss]

t is a vector, so you need to use element-wise operations with it, e.g., .* ./ .^

Also, the way you defined your for loop, k is a vector in each iteration:

iteration = 0;
for k = [2;20;200] % for loop with column vector
    iteration = iteration+1
    disp(k);       % k is a column vector of all values, loop iterates once
end

iteration = 1

     2
    20
   200

iteration = 0;
for k = [2 20 200] % for loop with row vector
    iteration = iteration+1
    disp(k);       % k takes one element at a time, loop iterates 3 times
end

iteration = 1

     2

iteration = 2

    20

iteration = 3

   200

iteration = 0;
for k = 2:20:200 % this row vector may be what was intended ( [] not necessary )
    iteration = iteration+1
    disp(k);
end

iteration = 1

     2

iteration = 2

    22

iteration = 3

    42

iteration = 4

    62

iteration = 5

    82

iteration = 6

   102

iteration = 7

   122

iteration = 8

   142

iteration = 9

   162

iteration = 10

   182

clear all;
close all;
clc;
x = 0;                          % [m]
t = [0:300];                      % [s]
h =  100;                       % [W/m^2*K]
% k = 2;                          % [W/m*K]
alpha = 5.6*10^-6;              % [m^2/s]
% Temperatures
T_i = 325;                      % [Degrees Celcius]
T_infinity = 15;                % [Degrees Celcius]

% for k = [2;20;200]
for k = 2:20:200
    
%     T_xt = ((erfc((x)/(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2)))*erfc((x/(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;
    T_xt = ((erfc((x)./(2*sqrt(alpha*t)))) - (exp(((h*x/k))+(h^2*alpha*t/k^2))).*erfc((x./(2*sqrt(alpha*t)))+((h*sqrt(alpha*t))/k))) * (T_infinity - T_i) + T_i;
    
    %fprintf('Temperature in Degrees Celcius: %0.3f\n\n', T_xt);
    plot(t,T_xt, '-','DisplayName', ['k' num2str(k)]);
    hold on;
end
hold off;

% fprintf('Temperature in Degrees Celcius: %0.3f\n\n', T_xt);

[David]

Thank you Voss, this looks very close. I'm only wanting the output only those three k values 2, 20, and 200 however.

It should look something like this...

[David]

Ahh, I think I have it. Your first two comments seem to have solve the problem. Thank you for the tutelage @Voss and @Walter Roberson. :)

This was very helpful.

Best,

D