Finding coefficients of variables of specific powers

Hello,

I am trying to find coefficients of variables of specific powers, or of 2+ variables multiplied together, however I am running into issues.

Problem 1:

syms x
y = x^2 + 3;
disp(coeffs(y,x));

The equation y does not include a x^1 term, however the returned matrix from coeffs is (3 1) which could be the same for the equation y = x + 3. I need something like (3 0 1) to show zero coefficients of intermediate powers of the variable.

Problem 2:

Trying to find coefficients of multiple variables. For instance z = x^2*y + 3y + 5x^2 +4y^2 + 3xyr + 5

If I try to find the coefficients of xy I want it to return 3r and ignore the x^2y term. Is there an easy way to do this or is it a nested (or 2-line) coeffs? If so this also runs into problem #1 should the equation not include the powers of that variable.

Thanks!

Answer 1

[Voss]

Problem 1: Try using the 'All' flag in your call to coeffs.

Answer 2

[Steven Lord]

syms x y r
z = x^2*y + 3*y + 5*x^2 +4*y^2 + 3*x*y*r + 5

z = 

[coeffsOfX, powersOfX] = coeffs(z, x, 'all')

coeffsOfX = 

powersOfX = 

[coeffsOfX2y, powersOfY] = coeffs(coeffsOfX(powersOfX == x^2), y, 'all')

coeffsOfX2y = 

powersOfY = 

coefficientOfX2NoY = coeffsOfX2y(powersOfY == 1)

coefficientOfX2NoY = 

5

[Connor LeClaire]

This is great! I would accept the answer if I hadn't posted 2 different problems in the question... sorry

[Connor LeClaire]

Should there exist no 2nd order power of x for example, using the index of powersOfX == x^2 throw an error?