Warning: Imaginary parts of complex X and/or Y arguments ignored

I got this problem while applying FFT,

>> f=1803*(0:(2^(n-1)-1))/2^n;

the corresponding matrix 'f' is coming out to be a row vector instead of column. due to this MATLAB warns me 'Warning: Imaginary parts of complex X and/or Y arguments ignored' when I try to plot(f, Pyy(1:2^(n-1))) Pyy is a column vector.

How can I solve this problem and get the plot?

Answer 1

[Greg Heath]

That error message warns that you are trying to plot a complex function. Try

whos

to see which complex function you are trying to plot.

Hope this helps.

Greg

Answer 2

[Walter Roberson]

f = (1803 * (0:(2^(n-1)-1)) / 2^n) .';

And then f will be a column vector.

You will find, though, that this absolutely will not solve your complex problem.

Hint: you would get exactly the same problem if you were to attempt

plot(1 + 2i)

Answer 3

[Wayne King]

I'm guessing from your variable, Pyy, that you are trying to plot a spectrum and f is your frequency vector. Then assuming that f and Pyy have the same length.

   isequal(length(f),length(Pyy))

should return a 1.

Just do

     plot(f,abs(Pyy))
     % or usually better if you have a power spectral density estimate
     % use dB
     plot(f,10*log10(abs(Pyy)))

[Walter Roberson]

Should that be

plot(f,10*log10(abs(Pyy)))

[Wayne King]

Yes, you are absolutely right Walter, thank you! I've corrected the typo.