MATLAB: Warning: Imaginary parts of complex X and/or Y arguments ignored

arguments ignoredimaginary parts of complex variable

function main
options=odeset('RelTol',1e-06)
Xo = [1;0];
tspan = [0,5];
[t,X] = ode45(@TestFunction,tspan,Xo,options);
figure
hold on
plot (t,X(:,1));plot (t,X(:,2),':')
legend ('x1','x2');ylabel('x');xlabel('t')
return
    function dx_dt=TestFunction(t,x)
        %function which returns a rate of change vector
        M=[i,i;i,i]
        dx_dt=M*x;
        return
full error message:
Warning: Imaginary parts of complex X and/or Y arguments ignored 
> In ode_plot_6 (line 17)
The program plots the graph, but generates the above error. Please help in eliminating the error message. Thank you.

Best Answer

You know that in
M=[i,i;i,i]
that the "i" there is going to refer to sqrt(-1) ? You are going to get complex output, and MATLAB cannot display complex output when you use that plotting syntax.
You could use
plot (t, real(X(:,1)), 'b-', t, imag(X(:,1)), 'r-');
plot (t, real(X(:,2)), 'b:', t, imag(X(:,2)), 'r:')
This would plot the real components in blue and the imaginary components in red.
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