Extracting variables from symbolic expression in a matrix form

I have a long equation in symbolic math toolbox which has the following form:

A*u1+B*u2+C*u3

A, B and C are long relationships consisting of symbolic variables. I would like to extract the A B C in from u1, u2 and u3 and write them in a matrix form. In other words I would like to shape a relationship such us

[A B C]*{u}

where u is

u=[u1;u2;u3]

How can I do that?

Answer 1

[Walter Roberson]

You could use coeffs but the output is a bit of a nuisance for multivariates.

You could call into MuPAD and use coeff.

But really, the easiest way is likely to use

A = subs(Expression, {u2,u3}, {0,0});
B = subs(Expression, {u1,u3}, {0,0});
C = subs(Expression, {u1,u2}, {0,0});

[Giorgos Papakonstantinou]

Thank you very much Walter.

I would like to ask a relevant question with the symbolic math toolbox.

Suppose I have:

h=a+b;

k=c1*h +c2*h+c3*h;

then matlab would give me a result like this:

k=(a+b)*c1+(a+b)*c2+(a+b)*c3

Is there anyway to force matlab to display k at the following more compact form:

k=c1*h +c2*h+c3*h;

[Walter Roberson]

If you define k before you define h, then I think it might display in the shorter form.

syms h a b
k = c1*h +c2*h+c3*h;
h = a + b;
disp(k)