Coeff & constants of a symbolic equation

syms a b c
exp=[-a-2*b-3*c+2, 4*b-c, -5*a+3]

I need the coefficient matrix in the form

% Coef_matrix=[ -1, -2, -3, 2; 0, 4, -1, 0; -5, 0, 0, 3];

How do I get this?

I can get the first line from the following syntax (from a previous blog), but there is an error when I try to do the second & third lines (i.e. j=1:3) as there is a 'zero' which it wont recognize

xx = symvar(exp) %find the variables in expression 'exp'
Coef_matrix = zeros(numel(exp),numel(xx));
for j = 1
    [v1,v2] = coeffs(exp(j),xx);
    Coef_matrix(j,ismember(xx,v2)) = v1(1:numel(xx));
end

And when I get the coefficients, I cant get the constant term at the end.

The solution should be able to scale up for more variables, as this is a simplified version of my problem. Thanks

Answer 1

[Oleg Komarov]

syms a b c
exp = [-2*b-a-3*c+2; -c+4*b; -5*a+3];

xx = symvar(exp);
Coef_matrix = zeros(numel(exp),numel(xx)+1);
for j = 1:3
    [v1,v2]   = coeffs(exp(j),xx);
    [idx,loc] = ismember(v2,xx);
    idx(end)  = true;
    loc(loc == 0) = 4;
    Coef_matrix(j,loc) = v1(idx);
end

[Mech Princess]

Thanks, it works for the symbols.

      -1    -2    -3
       0     4    -1
      -5     0     0

How do I get the constant term at the end?

      -1    -2    -3     2
       0     4    -1     0
      -5     0     0     3

Thanks in advance

[Azzi Abdelmalek]

why don't you change your expression

 syms a b c d
 exp = [-2*b-a-3*c+2*d; -c+4*b; -5*a+3*d]

then just ignore d

[Mech Princess]

Thanks. This seems great if it was a straight fwd Qn. I dont have control of the immediate expression. My actual problem is way more complicated.

"exp" is the derivative of another expression "LD_new".
LD_new is arrived at by substituting expression "st" in "LD".
LD is arrived at by summing all the expressions of "LD0".
LD0 is my input vector multiplied by the variables.

Is there an easier way to do this? Thanks again

[Oleg Komarov]

@Mech: I edited the code to generate the coeff matrix with the constant term as well.

[Mech Princess]

Thanks a million. I've been working on this for weeks!

[Oleg Komarov]

You weren't far from the solution.