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

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FAISAL ALQARNI 2023-7-25

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2000293-how-to-extract-coefficients-of-custom-variables-in-symbolic-expression-in-matlab

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?

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Dyuman Joshi 2023-7-25

Hello @FAISAL ALQARNI, can you post the whole code?

FAISAL ALQARNI 2023-7-25

在 MATLAB Online 中打开

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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Answer 1

Paul 2023-7-25

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2000293-how-to-extract-coefficients-of-custom-variables-in-symbolic-expression-in-matlab#answer_1278458

编辑：Paul 2023-7-25

在 MATLAB Online 中打开

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 =