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An error when "syms" is used

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Mahsa Babaee
Mahsa Babaee 2021-5-31
关闭： Mahsa Babaee 2021-5-31
Hello
I would be appreciated if you could guide me why the following error has occured in phi(1), phi(2), and phi(3) as symbolic variables?!
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for variables.
A piece of my code is :
a1 = 5.79e-08; a2 = 1.23e-07; a3 = 6.67e-08; a21=a2/a1; a32=a3/a2;
o1= w1*(Tb-Tts)+q_met1/k1;
o2= w2*(Tb-Tts)+q_met2/k2;
o3= w3*(Tb-Tts)+q_met3/k3;
syms s
syms t
syms x y z
Tb=310; Tts=310; Tini=309; % Assumption: Tb=Tts;
phi= zeros(3,1);
TT= Tini-Tts;
phi(1)= TT+(a1*o1)/s;
phi(2)= TT+(a2*o2)/s;
phi(3)= TT+(a3*o3)/s;
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John D'Errico
John D'Errico 2021-5-31
What is w21? w2? w3? Will you keep that a secret? :) If you don't tell us, how can we help you?
Then you try to convert something (unknown, again, you give us no clue) into a double. But does that variable contain symbolic unknowns? (It surely does, given the error.)
How can MATLAB convert something into a number when it is an unknown value?
And how can we help you if you don't give us any useful information?
Would you go to a doctor and tell him that you have an unknown problem, and then ask him to tell you the color car you drive?
Mahsa Babaee
Mahsa Babaee 2021-5-31
Mr @John D'Errico
I forgot to paste that! o1, o2, o3 and q_met1, q_met2, q_met3 and k1, k2, k3 are all known values!
I just want to define phi as a symbolic array and I did it as Torson explained.
Thank you for your time.

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