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Isolate Coefficients of Multivariate Linear Polynomial

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Reuvenk
Reuvenk on 17 Jun 2012
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!

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Accepted Answer

Alexander
Alexander on 18 Jun 2012
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]

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Reuvenk
Reuvenk on 18 Jun 2012
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!

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More Answers (1)

Christopher Creutzig
Christopher Creutzig on 27 Jun 2012
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]

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