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

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FAISAL ALQARNI
FAISAL ALQARNI 2023-7-25
评论： Chunru 2023-7-26
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?
  2 个评论
FAISAL ALQARNI
FAISAL ALQARNI 2023-7-25
Ran in:
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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采纳的回答

Paul
Paul 2023-7-25
编辑：Paul 2023-7-25
Ran in:
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 = 

更多回答（1 个）

Chunru
Chunru 2023-7-25
编辑：Chunru 2023-7-25
Let iB=1, jB=0 and kB=0. Then compute omegaB.
  3 个评论
显示 1更早的评论
Dyuman Joshi
Dyuman Joshi 2023-7-25
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.

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