plot a function consisting of real and imaginary numbers

Question

J K 2019-7-27

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473735-plot-a-function-consisting-of-real-and-imaginary-numbers

评论： Rena Berman 2019-9-19

Hi guys,

I have this code and upon execution Matlab says that Imaginary parts of complex X and/or Y arguments were ignored. How can I plot the function without ignoring imaginary parts? I tried "plot(t, real(squeeze(shifted(:,61,61))),imag(squeeze(shifted(:,61,61))), '.-');" but Matlab said "Data must be a single matrix Y or a list of pairs X,Y" Tried a few other things but they did not work either.

Here is the full code before any modifications and attempts.

c = 0.3;                     % um/fs 
z = 5;
w_o = 2*pi*c;
w = -6*w_o:0.1:6*w_o;
dw = w(2)-w(1);
kx = -6:0.1:6;
dkx=kx(2)-kx(1);
ky = kx;
dky = ky(2)-ky(1);
delta =pi;
deltaW = 3*pi;
deltaK = 1;
nw = length(w);
dt = pi/w(end);
t =[(-nw/2+1:nw/2)]*dt;
nkx = length(kx);
dx = pi/kx(end);
x =[(-nkx/2+1:nkx/2)]*dx;
nky = length(ky);
dy = pi/ky(end);
y = [(-nky/2+1:nky/2)]*dy;
[Kx, W, Ky] = meshgrid(kx, w, ky);
[X, T, Y] = meshgrid(x, t, y);
input = exp(-((W-w_o).^2)/deltaW.^2).*exp(-(Kx.^2+Ky.^2)/(deltaK.^2)).*exp(1*i.*sqrt((W/c).^2-(Kx.^2+Ky.^2)).*z(j));
fourier = fftn(fftshift(input));
shifted = fftshift(fourier);
figure(6);
plot(t, squeeze(shifted(:,61,61)), '.-');

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Walter Roberson 2019-7-28

在 MATLAB Online 中打开

Note that you have

input = exp(-((W-w_o).^2)/deltaW.^2).*exp(-(Kx.^2+Ky.^2)/(deltaK.^2)).*exp(1*i.*sqrt((W/c).^2-(Kx.^2+Ky.^2)).*z(j));

which involves z(j) but that j has not been assigned in your code. When you do not assign to j then j is interpreted as sqrt(-1) which is not a valid subscript.

Rena Berman 2019-9-19

(Answers Dev) Restored edit

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Answer 1

Walter Roberson 2019-7-27

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/473735-plot-a-function-consisting-of-real-and-imaginary-numbers#answer_385100

在 MATLAB Online 中打开

plot(t, real(shifted(:, 61, 61)), '.-b', t, imag(shifted(:, 61, 61)), '.-r')

Where does your code define shifted?

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Walter Roberson 2019-7-27

For fft of real values it is common for the real and imaginary components to effectively end up with different scales. In particular, the first output slot is sum() of the input and that will be real valued for real input. If the input has a nonzero mean, that first output slot can end up fairly large compared to the other outputs.

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