for loop showing only last iteration and the graph shows dotted instead of lines

clc
close all
clear

syms x y E

v=[x y];
J=jacobian([y,E*(1-x^2)*y-x], v);
p=[0 0];
j=subs(J,v,p);


a=-8:0.1:8;

for E=a
    e = eig(j);
   
    c=subs(e,E);
    u=double(c);
    
   plot(E,real(u),'.'); hold on
end

i'm new to matlab and i scripted the above code from various helps online, in order to find the bifurcation but i can't get a line plot where all the dots are combined and i have tried changing the '.' to '-' but still doesn't work.

Also i need every iteration of u on the workspace, currently it's just showing only the last iteration value

Thank you.

Answer 1

[David Hill]

clc
close all
clear
syms x y E
v=[x y];
J=jacobian([y,E*(1-x^2)*y-x], v);
p=[0 0];
j=subs(J,v,p);
a=-8:0.1:8;
for E=1:length(a)
    e = eig(j);   
    c=subs(e,a(E));
    b=double(c);
    u(E)=b(1)+1i*b(2);%just guessing (did you expect a complex number)
end
plot(a,real(u),'Marker','*','MarkerSize',1);  

[akhil rous]

Sorry that is not the answer I'm expecting. I'll attach the fig file I've obtained for reference

[David Hill]

clc
close all
clear
syms x y E
v=[x y];
J=jacobian([y,E*(1-x^2)*y-x], v);
p=[0 0];
j=subs(J,v,p);
a=-8:0.1:8;
e = eig(j); 
for E=1:length(a)
    c(E,:)=double(subs(e,a(E))');    
end
plot(a,real(c(:,1)),'b',a,real(c(:,2)),'b');  

[akhil rous]

Thank you David Hill. that is the result i was looking for.

Can you explain to me the part,

for E=1:length(a)
    c(E,:)=double(subs(e,a(E))');    
end
plot(a,real(c(:,1)),'b',a,real(c(:,2)),'b'); 

and why my for loop and plot didn't work.

Thanks again for your help.

[David Hill]

Primarily, you just want to load all the data into matrix c in the loop before plotting it all at once outside the loop. If you plot single points, you cannot connect lines to them and you cannot see them without markers. Since matrix c has two columns of complex numbers that you want plotted, c(:,1) plots the first column and c(:,2) plots the second column with respect to a (I used the same color 'b'). It is better to use an index into vector (a) and to index matrix (c) than to have the for-loop go through all of (a) itself.

[akhil rous]

Thanks bro!!!

It really helped.