Plot two variable sum function

Hi can you tell me please how can I plot with 2 variables in sum function like this?

I need graph plot(omega,theta) and plot(omega,amplitude)

Answer 1

[Alan Stevens]

Looks like you are translating from SMath Studio to MATLAB. The following is one way to do what you want:

% Basic data
beta(1) = 0.000125;   lambda(1) = 0.0124;
beta(2) = 0.001424;   lambda(2) = 0.0305;
beta(3) = 0.001274;   lambda(3) = 0.111;
beta(4) = 0.002568;   lambda(4) = 0.301;
beta(5) = 0.000748;   lambda(5) = 1.13;
beta(6) = 0.000273;   lambda(6) = 3;

% Define range of w
w = 0:0.001:1;

% Call function
[Amplitude, Theta] = fn(w,beta,lambda);

% Plot results
subplot(2,1,1)
plot(w,Amplitude),grid
xlabel('Omega'), ylabel('Amplitude')
subplot(2,1,2)
plot(w,Theta),grid
xlabel('Omega'), ylabel('Theta')

function [Amp, Theta] = fn(w,beta,lambda)
        
         n = numel(w);
         Amp = zeros(n,1);
         Theta = zeros(n,1);
         for i = 1:n
             Re = sum(beta./(lambda.^2 + w(i)^2));
             Im = w(i)*10^-3 + w(i)*sum(beta.*lambda./(lambda.^2 + w(i)^2));
             Amp(i) = sqrt(Re^2 + Im^2);
             Theta(i) = atan(Im/Re);
         end
 
end

[EuroLion]

Thank you for your answer. but called this error "Unrecognized function or variable 'fn'."

[Alan Stevens]

That's strange, because it works for me, giving

Did you copy and paste or rewrite it yourself?

[EuroLion]

Copied

[Alan Stevens]

Then it should work for you as well! Have you checked line-by-line (including the correct number of 'end')?

Try changing the name of the function (in both places where it is used).

[EuroLion]

Tried. I have two PC. same feedbacks :(

[Alan Stevens]

What version of MATLAB are you using (not that that should matter much!)?

[EuroLion]

R2020a. I think we ve problem with call function

[Alan Stevens]

The words Call function should be commented out! Copy and paste your version of the code here.

[Alan Stevens]

Better to post the actual code, not a picture. Use the > button in the code section of the Comment menu.

[EuroLion]

>> % Basic data
beta(1) = 0.000125;   lambda(1) = 0.0124;
beta(2) = 0.001424;   lambda(2) = 0.0305;
beta(3) = 0.001274;   lambda(3) = 0.111;
beta(4) = 0.002568;   lambda(4) = 0.301;
beta(5) = 0.000748;   lambda(5) = 1.13;
beta(6) = 0.000273;   lambda(6) = 3;
% Define range of w
w = 0:0.001:1;
% Call function
[Amplitude, Theta] = fn(w,beta,lambda);
% Plot results
subplot(2,1,1)
plot(w,Amplitude),grid
xlabel('Omega'), ylabel('Amplitude')
subplot(2,1,2)
plot(w,Theta),grid
xlabel('Omega'), ylabel('Theta')
function [Amp, Theta] = fn(w,beta,lambda)
        
         n = numel(w);
         Amp = zeros(n,1);
         Theta = zeros(n,1);
         for i = 1:n
             Re = sum(beta./(lambda.^2 + w(i)^2));
             Im = w(i)*10^-3 + w(i)*sum(beta.*lambda./(lambda.^2 + w(i)^2));
             Amp(i) = sqrt(Re^2 + Im^2);
             Theta(i) = atan(Im/Re);
         end
 
end
Unrecognized function or variable 'fn'.

[Alan Stevens]

I see the problem. You are running it directly in the workspace. You need to create a script (New script - top left hand corner of the Home section of the workspace menu), save it and run that.

[EuroLion]

Thank you. running! now I have another problem. How can I scale log x and y axis?

For example

[Alan Stevens]

In the workspace type help loglog, or help semilogx, or help semilogy as appropriate.

[EuroLion]

Thank u so much!

[Alan Stevens]

Also lookup logspace. As follows:

% Basic data
beta(1) = 0.000125;   lambda(1) = 0.0124;
beta(2) = 0.001424;   lambda(2) = 0.0305;
beta(3) = 0.001274;   lambda(3) = 0.111;
beta(4) = 0.002568;   lambda(4) = 0.301;
beta(5) = 0.000748;   lambda(5) = 1.13;
beta(6) = 0.000273;   lambda(6) = 3;

% Define range of w
w = logspace(-3,3,100);

% Call function
[Amplitude, Theta] = fn(w,beta,lambda);

% Plot results
subplot(2,1,1)
loglog(w,Amplitude),grid
xlabel('Omega'), ylabel('Amplitude')
subplot(2,1,2)
semilogx(w,Theta),grid
xlabel('Omega'), ylabel('Theta')

function [Amp, Theta] = fn(w,beta,lambda)
        
         n = numel(w);
         Amp = zeros(n,1);
         Theta = zeros(n,1);
         for i = 1:n
             Re = sum(beta./(lambda.^2 + w(i)^2));
             Im = w(i)*10^-3 + w(i)*sum(beta.*lambda./(lambda.^2 + w(i)^2));
             Amp(i) = sqrt(Re^2 + Im^2);
             Theta(i) = atan(Im/Re);
         end
 
end

This produces

[EuroLion]

Now I need combine my plot with different coefficients. We have a constant for Im value. I need define an interval for this constant from 10^-8 to 10^-3. Like this

Im = w(i)*a + w(i)*sum(beta.*lambda./(lambda.^2 + w(i)^2));

a = 10^-8:10:10^-3;

And plot 6 curves in same graph.

[Alan Stevens]

As follows

% Basic data
beta(1) = 0.000125;   lambda(1) = 0.0124;
beta(2) = 0.001424;   lambda(2) = 0.0305;
beta(3) = 0.001274;   lambda(3) = 0.111;
beta(4) = 0.002568;   lambda(4) = 0.301;
beta(5) = 0.000748;   lambda(5) = 1.13;
beta(6) = 0.000273;   lambda(6) = 3;

% Define range of w
w = logspace(-3,3,100);
a = logspace(-8,-3,6);
na = numel(a); nw = numel(w);
Amptude = zeros(nw,na);
Thta = zeros(nw,na);
for j = 1:na
% Call function
[Amplitude, Theta] = fn(w,beta,lambda,a(j));
Amptude(:,j)=Amplitude;
Thta(:,j) = Theta;
end

% Plot results
subplot(2,1,1) 
loglog(w,Amptude),grid
xlabel('Omega'), ylabel('Amplitude')
subplot(2,1,2) 
semilogx(w,Thta),grid
xlabel('Omega'), ylabel('Theta')


function [Amp, Theta] = fn(w,beta,lambda,a)
        
         n = numel(w);
         Amp = zeros(n,1);
         Theta = zeros(n,1);
         for i = 1:n
             Re = sum(beta./(lambda.^2 + w(i)^2));
             Im = w(i)*a + w(i)*sum(beta.*lambda./(lambda.^2 + w(i)^2));
             Amp(i) = sqrt(Re^2 + Im^2);
             Theta(i) = atan(Im/Re);
         end
 
end

Results in: