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how to correctly use the function "subs"

29 vues (au cours des 30 derniers jours)
Tony Cheng
Tony Cheng le 24 Juil 2024 à 3:08
Commenté : Umar le 24 Juil 2024 à 7:39
Hi there,
I am using subs to replace some symbols in a symbolic expression Jacobian_mtx_1. The symbols that I want to be replaced are A, B and C, which are a 6*1 symbolic vector, a symbolic scalar, and a 2*1 symbolic vector, respectively. I write the code:
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , { A , B , C } , { A_val ,B_val , C_val } ) ;
where A_val is a 6*1 vector containing 6 function calls, B_val is a numeric scalar, and C_val is a 2*1 numeric vector.
However, the software reminds me that
Errors using sym/subs
Entries in second argument must be scalar.
Could anyone tell me how to resolve this issue?
Many thanks!
  3 commentaires
Tony Cheng
Tony Cheng le 24 Juil 2024 à 6:47
Hi Umar,
Many thanks to your reply! I think your way is feasible, but the following manner is a little bit easier
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , A , A_all ) ;
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , B , B_val ) ;
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , C , C_val) ;
cause on
it says If old and new are both vectors or cell arrays of the same size, subs replaces each element of old with the corresponding element of new.
Then we can use vector replacements directly.
Cheers
Umar
Umar le 24 Juil 2024 à 7:39
No problem, Tony, glad to help you out. Please let us know if you have any further questions.

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Piyush Kumar
Piyush Kumar le 24 Juil 2024 à 4:07
Hi Tony,
I tried reproducing the issue you are facing -
syms B
A = sym('A', [6 1]);
C = sym('C', [2 1]);
A_val = [1, 2, 3, 4, 5, 6];
B_val = 7;
C_val = [8, 9];
Jacobian_mtx_1 = [A(1)*B + C(1), A(2), A(3);
A(4), A(5)*B + C(2), A(6);
B, C(1), C(2)];
disp(Jacobian_mtx_1);
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , { A , B , C } , { A_val ,B_val , C_val } ) ;
Error using sym/subs (line 156)
Entries in second argument must be scalar.
Instead of directly replacing A and C with vectors, I tried replacing the elements one by one and it wokred for me -
syms B
A = sym('A', [6 1]);
C = sym('C', [2 1]);
A_val = [1, 2, 3, 4, 5, 6];
B_val = 7;
C_val = [8, 9];
Jacobian_mtx_1 = [A(1)*B + C(1), A(2), A(3);
A(4), A(5)*B + C(2), A(6);
B, C(1), C(2)];
disp(Jacobian_mtx_1);
for i = 1:length(A)
Jacobian_mtx_1 = subs(Jacobian_mtx_1, A(i), A_val(i));
end
for i = 1:length(C)
Jacobian_mtx_1 = subs(Jacobian_mtx_1, C(i), C_val(i));
end
% Now replace B
Jacobian_mtx_1 = subs(Jacobian_mtx_1, B, B_val);
disp(Jacobian_mtx_1);
  1 commentaire
Tony Cheng
Tony Cheng le 24 Juil 2024 à 6:47
Hi Kumar,
Many thanks to your reply! I think your way is feasible, but the following manner is a little bit easier
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , A , A_all ) ;
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , B , B_val ) ;
Jacobian_mtx_1 = subs( Jacobian_mtx_1 , C , C_val) ;
cause on
it says If old and new are both vectors or cell arrays of the same size, subs replaces each element of old with the corresponding element of new.
Then we can use vector replacements directly.
Cheers

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