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how to fix Warning: Imaginary parts of complex X and/or Y arguments ignored

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Faria Siddiqui
Faria Siddiqui on 1 Apr 2021
Edited: Paul Hoffrichter on 2 Apr 2021
plot(V,y(:,1),V,y(:,2),V,y(:,3),V,y(:,4),V,y(:,5),V,y(:,6));
This is the line of code the warning pops up for.
my initial conditions are
% yo = [Fa,Fb,Fc,Fd,Fe,Ff,T,x,P]
yo = [9.614 19.435 0 0 35.03391 0 870 0 101.325]
and my V is calculated by
V = ((FA+FB+FC+FD+FE+FF)*R*T)/P
hope someone can help with this!
  1 Comment
Walter Roberson
Walter Roberson on 1 Apr 2021
My prediction is that P is not a scalar, so the / operator is "matrix right divide", which is similar to
V = ((FA+FB+FC+FD+FE+FF)*R*T) * pinv(P)
Is that the intention?
Anyhow, look at
whos FA FB FC FD FE FF P R T V
and you will see that at least one of them is marked as complex; you will need to trace your code to figure out which one.

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Answers (1)

Paul Hoffrichter
Paul Hoffrichter on 2 Apr 2021
Edited: Paul Hoffrichter on 2 Apr 2021
If you have a negative number raised to a non-integer power, you can sometimes get a complex number. Take a look at:
In this case, there was a very small percentage of complex numbers. Maybe the plot routine knew what was best for the writer by converting automatically the complex numbers to real. The other numbers were real. Matlab is a smart cookie.

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