Isolate Coefficients of Multivariate Linear Polynomial

Hi, I have a symbolic expression

eqn = r - 2*x +7/4

I would like to isolate the coefficients of the variables (x, r and y (y's coefficient is 0)). How do I do that? I've tried using coeffs but that only lets me specify one variable. It would also be ok if I got two vectors back:

[ -2 , 1 , 7/4]
[x , r , 1]

even if they don't include y. Thanks!

Answer 1

[Alexander]

If you know that you have only linear terms and only x, r, and y, you could use diff to get the coefficients:

 syms x r y
 eqn = r - 2*x +7/4;
 [diff(eqn, x), diff(eqn, r), diff(eqn, y)]

This gives:

 ans =
 [ -2, 1, 0]

[Reuvenk]

That doesn't give the independent term, but I can easily find that by subbing 0 for all of the variables. Thanks for the answer!

Answer 2

[Christopher Creutzig]

syms x r
eqn = r - 2*x +7/4
coeffs(eqn, [x, r])

ans =

[ 7/4, 1, -2]

Note that coeffs returns the coefficients in increasing order and leaves out zeroes:

>> coeffs(7*x^2*r^2 + 6*x^2*r + 5*x^2 + 4*x*r^2 + 3*x*r + 2*r^2, [x, r])

ans =

[ 2, 3, 4, 5, 6, 7]