CODEWITHSUNDEEP
matlabgraph
user2579288
user2579288

Reputation: 61

Plotting a graph with logarithic axis - 3 y-values for every x-value

I am trying to plot a graph of value value of a variable x on the x-axis; however, there are three values for each point, that is to say, natural frequency 1, natural frequency 2 and natural frequency 3. This is the code and it is not working:

M = [3 0 0; 0 2 0; 0 0 0.5]
% disp('Mass matrix [M]');
% disp(M);

for i=1:1000:60e06
    K = [i+7e06 i -6e06; i i+3e06 -3e06; -6e06 -3e06 10e06];
    % disp(K)
    [V,L]=eig(K,M);     % eig is a standard Matlab function
    values=diag(L);     % diag gets the values from the leading diagonal
    [values,I]=sort(values);    % sort into ascending order of frequency
    V(:,I)=V;                   % now sort the mode shape vectors
    freq=sqrt(values)/(2*pi);   % this divides all elements by 2 pi

    % disp('Eigenvalues [s^(-2)]');
    % disp(values');        % the quote mark after values prints the column vector as a row

    % disp('Frequencies [Hz]');
    % disp(freq');

    % disp('Mode shape vectors in the columns of matrix [V]');
    % disp(V);
      loglog(i, freq(1:1), i, freq(2:1), i, freq(3:1)
end

Apologies for the mistakes, I am a beginner.

Upvotes: 0

Views: 92

Answers (2)

Prashant Kumar
Prashant Kumar

Reputation: 22529

Your problem is that you have the loglog inside the loop.

Before the loop, I put:

i_vals = 1:1000:60e06;
freq = zeros(3, length(i_vals));

I change the top of the loop to:

for n=1:length(i_vals)
    i = i_vals(n);
    K = [i+7e06 i -6e06; i i+3e06 -3e06; -6e06 -3e06 10e06];
    ...

Also change your freq assignment:

freq(:, n)=sqrt(values)/(2*pi);

Delete your loglog and put this after/outside your loop. 

loglog(i_vals, freq')

Doing all of that gives me this:

enter image description here

Upvotes: 2

fuesika
fuesika

Reputation: 3330

You are missing a closing bracket in the end of your line

loglog(i, freq(1:1), i, freq(2:1), i, freq(3:1)

Also, I don't see the reason for your 2:1, 3:1, ...?!

Replace it by

loglog(i, freq(1), i, freq(2), i, freq(3))

and I don't see anything wrong with your plotting.

In general I would suggest to store the values of freq in some array and plot it separately. This will speed the script up a lot, i.e.

%preallocate freq, the number of iterations in the loop is important here
freq = zeros(3,length(1:1e3:60e6))

for i = ...
    %your loop goes here
    %remove freq = ...
    %remove loglog( ...
   freq(:,i) = sqrt(values)/(2*pi);   % this divides all elements by 2 pi
end

loglog(1:1e3:60e6, freq(1,:)) %plot first curve
loglog(1:1e3:60e6, freq(2,:)) %plot second curve
loglog(1:1e3:60e6, freq(3,:)) %plot third curve

Upvotes: 2

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