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Error using imag: too many input arguments

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Sergio Manzetti
Sergio Manzetti 2021-6-22
评论： Sergio Manzetti 2021-6-22
Hi, I get the given error when trying to plot the contour of the given function
syms r x y k z
[ph,r] = meshgrid((0:5:360)*pi/180,0:.5:10);
[X,Y] = pol2cart(ph,r);
Z = X+i*Y;
J = besselj(k,l.*r);
J2 = besselj(k,m.*r);
Y = bessely(k,l.*r);
Y2 = bessely(k,m.*r);
H = besselh(k,r);
F1 = symsum((J).*exp(1i*k*ph),k,-5,5);
F2 = symsum((J2+Y2).*exp(1i.*k.*ph),k,-5,5);
F3 = symsum(H.*exp(1i.*k.*ph),k,-5,5);
pwu = nan(size(F1), 'like', F1);
mask = 0 <= r & r < 0.5;
pwu(mask) = F1(mask);
mask = 0.5 <= r & r < 1;
pwu(mask) = F2(mask);
mask = r >= 1;
pwu(mask) = F3(mask);
U = subs(pwu, {l, m}, {1.5, 3});hold on
contour(X,Y,imag(double(U),30))
axis equal
xlabel('r','FontSize',14);
ylabel('phi','FontSize',14);
The error lies in the 4th last line, however it appears to me as OK. What is wrong here?
thanks!

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采纳的回答

Walter Roberson
Walter Roberson 2021-6-22
编辑：Walter Roberson 2021-6-22
Count the bracket level!
contour(X,Y,imag(double(U),30))
1 2 3 2 10
Notice that the ,30 is at nesting level 2 -- inside the imag() call.
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Walter Roberson
Walter Roberson 2021-6-22
My code had
[x, y] = pol2cart(ph,r);
Z = x + 1i*y;
Your code has
[X,Y] = pol2cart(ph,r);
Z = X+i*Y;
Notice y compared to Y . This is important because you have
Y = bessely(k,l.*r);
which assigns a symbolic result to Y, and then later when you want to
contour(X,Y,imag(double(U)),30)
then you are using the symbolic Y, not the numeric Y.
You should have just taken my code and changed
surf(x, y, real(double(U)))
hold on
surf(x, y, zeros(size(x)))
hold off
to
contour(x, y, imag(double(U)), 30);

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