Clear Filters
Clear Filters

Matlab recommends me to apply 'subs' command but I have already used it to convert a symbolic matrix into a numerical one. What fails?

10 views (last 30 days)
Dear all,
I am trying to obtain the eigenvalues of a matrix which was previously defined with symbolic variables. Here is a bit of the code:
P_simb=[Cf_ss Lo_ss Ro_ss omega_b_ss Tgt_ss K_q_ss K_d_ss Ta_ss K_w_ss omega_g0_ss omega_f_ss Kp_ex_ss Ki_ex_ss Tex_ss vkd_ss vkq1_ss Rs_ss L1_ss Lmd_ss Lmq_ss Rfd_ss Lfd_ss Rkd_ss Lkd_ss Rkq1_ss Lkq1_ss Lf1d_ss Rdamp_ss ism_d_ss ism_q_ss ifd_ss ikd_ss ikq1_ss vo_d_ss vo_q_ss omega_sm_ss deltathetasm_ss pm_ss vfd_ss];
P_num=[1e-5 0.2 0.01 314.15926 0.5 0 0 2 20 1 1000 0.02353 0.005028 0.1 0 0 0.004044 0.0545 1.117 0.4772 8.476e-4 0.256 0.041 0.288 0.02859 1.1948 1.117 6.6666666e2 1 0 1 1 0 1 0 1 0 0.5 1];
A_num=subs(A_simb, P_simb, P_num);
However, despite of having used 'subs' command, I obtain the following errors:
"Error using symengine
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute
values for variables.
Error in sym/double (line 800)
Xstr = mupadmex('symobj::double', S.s, 0);
Error in VSM_baseline (line 178)
*First of all, I feel sorry that I cannot share the symbolic matrix A, since it comes from a very long set of equations that would extend excessively the content of this question.
Anyway, could someone give me an advice on how to fix this issue? I have read many many previous similar threads without sucess. Besides, the error message seems very inconsistent as I already applied 'subs' command in previous lines....
And the following is also very weird. Although A_num is categorised by Matlab as a symbolic matrix, A_num actually has numeric values and it is even possible to perform matrix operations with it....

Accepted Answer

Paul on 22 Jan 2022
It looks like there is still one or sym objects left in A_num after the command
A_num = subs(A_simb, P_simb, P_num);
What is the output of this command
if you execute it just after that call to A_num = subs(..... ?
  1 Comment
Manyu Hati
Manyu Hati on 23 Jan 2022
Thanks a lot!! Indeed, there was still a symbolic variable with incorrect name. Thank to this command 'symvar()' I could find the bad labeled variable.

Sign in to comment.

More Answers (0)

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!