# How to extract coefficients of custom variables in symbolic expression in MATLAB?

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FAISAL ALQARNI le 25 Juil 2023
Commenté : Chunru le 26 Juil 2023
I want to extract the coefficient of i_B from the attached symbolic experession called omega_B and call it p for example, how to do that?
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Dyuman Joshi le 25 Juil 2023
Hello @FAISAL ALQARNI, can you post the whole code?
FAISAL ALQARNI le 25 Juil 2023
I found a way to achieve this using Symbolic substitution subs
Since the expression is already factored in i_B, j_B and k_B, the coefficient of any of these symbolic variables can be simply extracted by setting its value to 1 and setting the other variables to 0 using subs
Example:
Extracting the coefficient of i_B from a symbolic expression called omega_B and call it p
syms psi phi theta i_B j_B k_B psi_dot phi_dot theta_dot
omega_B = (phi_dot - psi_dot*sin(theta))*i_B + (theta_dot*cos(phi) + psi_dot*cos(theta)*sin(phi))*j_B + (psi_dot*cos(phi)*cos(theta) - theta_dot*sin(phi))*k_B
omega_B =
p = subs(omega_B,[i_B,j_B,k_B],[1,0,0])
p =
Please let me know if there is a better approach.

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### Réponse acceptée

Paul le 25 Juil 2023
Modifié(e) : Paul le 25 Juil 2023
syms psi phi theta i_B j_B k_B psi_dot phi_dot theta_dot
omega_B = (phi_dot - psi_dot*sin(theta))*i_B + (theta_dot*cos(phi) + psi_dot*cos(theta)*sin(phi))*j_B + (psi_dot*cos(phi)*cos(theta) - theta_dot*sin(phi))*k_B
omega_B =
[c,t] = coeffs(omega_B,i_B)
c =
t =
p = c(t == i_B)
p =
Get all the components simultaneously. Using both output arguments of coeffs forces the first output argument to be in the desired order (at least I think that's how it works)
clear c t
[omega,t] = coeffs(omega_B,[i_B j_B k_B])
omega =
t =
p = omega(1); q = omega(2); r = omega(3); % if individual components are needed as individual variables
[p;q;r]
ans =
[p,q,r]=struct('omega',num2cell(omega)).omega % one line approach
p =
q =
r =
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### Plus de réponses (1)

Chunru le 25 Juil 2023
Modifié(e) : Chunru le 25 Juil 2023
Let iB=1, jB=0 and kB=0. Then compute omegaB.
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Dyuman Joshi le 25 Juil 2023
Not a function let, but the english word 'let'.
He suggested the exact thing you found out on your own (what you mentioned in a comment above).
Using subs to substitute and obtain value is what Chunru meant by computing.
Chunru le 26 Juil 2023
Exactly.

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