# Extracting coeffients and constants from a symbolic expression

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Mech Princess on 23 Jul 2012
Hi. im rather new to symbolic toolbox. Following is my code.
clear all; close all; clc;
N=3; alphavec = sym('alpha',[1 N]); %N is a variable input by user
derv_alpha=[-2*alphavec(2)-3*alphavec(3)+2-alphavec(1), 4*alphavec(2)-alphavec(3)]; %my expression (an example with 2 expressions); This is the output from another line of code and hence I have no control over it
derv_alpha=simplify(derv_alpha) %simplified expression
Now I want to get the coefficint matrix with the constant terms
xx = symvar(derv_alpha); %get variables in the expression
Coef_matrix = zeros(numel(derv_alpha),numel(xx)); %coefficient matrix: rows=no of expressions. columns=no of variables
for j = 1 %i.e. first expression
[a,b] = coeffs(derv_alpha(j),xx);
Coef_matrix(j,ismember(xx,b)) = a(1:numel(xx)); % I have got the coeff for alpha's
% Coef_matrix(j,ismember(xx,b)) = a % if I put this it gives me an
% error Subscripted assignment dimension mismatch.
end
for j = 2
[a,b] = coeffs(derv_alpha(j),xx);
Coef_matrix(j,ismember(xx,b)) = a; % I get coeff for alpha's
% Coef_matrix(j,ismember(xx,b)) = a(1:numel(xx)); % if I put this, I
% get only 2 coeff instead of 3. The '0' is missing in alpha1
end
Question1: Now I need to combine these j=1 and j=2 into one loop.How can I do this? i.e. j=1:numel(derv_alpha)
Question2: How can I also get the constant into this matrix? If the constant is zero, I want to capture that also. So that the final one looks like this:
Answer=[-1 -2 -3 2; 0 4 -1 0]
I searched and it says "you can also create symbolic constants with the sym" but in my case, the constant term varies every time
Thanks a million

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