How can I find all expressions added to a large formula?

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Hamza Yusuf
Hamza Yusuf 2020-11-10
评论： Walter Roberson 2020-11-10
In maple there is coeff(p, x^n) where you find all x coefficients to the n-th power in the polynomial p. in Matlab there is a coeffs(p,x)); where you find all the x on polynomial p. How do find a x^n on a polynomial n just like the one on maple. I have tried to make like this
(coeffs(p, x^2))
And for that I get the error.
Error using symengine
Invalid indeterminate.
Error in sym/coeffs (line 60)
cSym = mupadmex('symobj::coeffs',p.s, args{:});
Error in randomfile (line 36)
g = ((coeffs(p, x^2)));
it works when n needs to be one.

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Walter Roberson
Walter Roberson 2020-11-10
Use coeff with two outputs and ismember the desired power in the second output to locate the corresponding coefficients. Or use the 'all' option, in which case you can use indexing to get the coefficients.
Doing the work in a single call without using any true functions is a bit messy.
Possibly there might be a way using feval(symengine)
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Walter Roberson
Walter Roberson 2020-11-10
n = randi([0,4]);
syms x
[c,t] = coeffs(16*x^2 + 19*x + 11)
[found, idx] = ismember(x^n, t) ;
if found
c = c(idx) ;
else
c = sym(0);
end
Walter Roberson
Walter Roberson 2020-11-10
Ran in:
n = randi([0,2]);
syms x
y = 16*x^2 + 19*x + 11;
feval(symengine, 'coeff', y, x, n)
ans = 
16

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