ans =
problem with coeffs command
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I have a symbollic expression
5*w - 5*z - u*((603367941593515*x)/4503599627370496 - y/2) - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8
which I want to extract the coefficient of u from. But the coeff command gives an erroneous result:
ans =
5*w - 5*z - u*((603367941593515*x)/4503599627370496 - y/2) - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8
coeffs(ans,u)
ans =
[5*w - 5*z - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8, y/2 - (603367941593515*x)/4503599627370496]
The first 'ans' is a multilinear polynomial in x y z u v and w, so shouldn't this work as coeffs is for polynomials.
采纳的回答
syms w z u y v x
5*w - 5*z - u*((603367941593515*x)/4503599627370496 - y/2) - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8
coeffs(ans,u).'
The second term in the result is the coefficient of u and the first term is everything else.
Is that not the expected result?
4 个评论
Note that the convention used by coeffs (returning in ascending order of power when called with 1 output) is different than the convention used by some of the other polynomial functions in MATLAB like polyfit, which returns the coefficients in descending order of power.
p = [1 2 3]; % x^2+2*x+3
x = 1:10;
y = polyval(p, x);
polyfitCoefficients = polyfit(x, y, 2) % Quadratic term first
syms z
polynomialSym = p(1)*z^2+p(2)*z+p(3)
coeffsCoefficients1Output = coeffs(polynomialSym, z) % Constant term first
But if you call coeffs with two outputs, it will return it in descending order of power like polyfit does.
[coeffsCoefficients2Output, powersOfZ] = coeffs(polynomialSym, z)
Hi @Ali Kiral
The coeffs(ans, u) function essentially extracts the coefficients for the 
and 
terms. If you specify the output arguments clearly using [coefficients, terms], you will observe the results.
and terms. If you specify the output arguments clearly using [coefficients, terms], you will observe the results. syms w z u y v x
expr = 5*w - 5*z - u*((603367941593515*x)/4503599627370496 - y/2) - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8
[coefficients, terms] = coeffs(expr, u)
If you only want to retrieve the coefficient for the u term, then enter this:
coefficients(1)
The coeffs(ans, u) function essentially extracts the coefficients for the 
and 
terms.
and terms. That's true if called with two outputs. If not it returns them in the opposite order, 
and 
.
and .syms w z u y v x
expr = 5*w - 5*z - u*((603367941593515*x)/4503599627370496 - y/2) - v*(x/2 + (603367941593515*y)/4503599627370496) - 1/8;
oneOutput = coeffs(expr, u);
[twoOutputs, terms] = coeffs(expr, u);
isequal(oneOutput, twoOutputs) % false
isequal(flip(oneOutput), twoOutputs) % true
Ali Kiral
2026-2-6
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