MATLAB: Plotting complex zero causes plot one zero

MATLABplot complex zero default one

Hello

I have a set of complex data and I plot them in bulk. I use plot ( z , ' r * ' ) . All is fine and correct until there is only one number zero.

Function plot ( 0 , ' r * ' ) plots one point at (1,0). This is not exacty what I expected and this is critical. I dont see reson why. Separating the part real and imag does not work for me.

Can anybody please help me to clearify this. Maybe there is some rubish or default one which I can chage of clean. Thank you.

Best Answer

plot(real(z),imag(z)) % if only one argument is used matlab plots the variables against index in x axis read plot documentation

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MATLAB: Complex plots bug

"Am I doing something wrong or is this a bug in Matlab plotting?"

There is no bug in the plotting.

Exactly as documented, when z is complex, plot will plot the imaginary part against the real part.

What you have overlooked is that MATLAB silently removes the "complex" property from an operation's output array when the imaginary part of every value in that array is zero. And that is what happens with some of your values, the ones where you multiply i by zero... which not surprisingly have zero imaginary part, therefore z is not a complex array, and so the input to plot is not a complex array, so plot applies other rules (plotting the input value against the indices of that input array, which for a scalar are always 1).

You can check this yourself by only plotting the non-complex values inside your loop:

if isreal(z)
    plot(z,'b.')
end

I don't know if there is a neat way to force an array to be complex. To make z complex add this:

if isreal(z)
    z = complex(z);
end

or this

z = complex(real(z),imag(z));

Giving:

MATLAB: Do I get warning messages about ‘Imaginary Parts of Complex X and/or Y Arguments Ignored’

Explanation:

You are attempting to plot using two complex inputs to a plotting function, like PLOT or PLOT3. In this case, MATLAB will plot using the real part of the first input as the independent variable X and the real part of the second input as the dependent variable Y.

Common causes:

You have performed a square root or FFT operation on the vectors you are attempting to plot, and those operations resulted in a complex vector. Alternatively, you used the variables i and/or j in the computation of your input vectors but those variables did not exist when you performed your computation. In this case, MATLAB will treat i and/or j as the imaginary unit.

Solution:

Stop MATLAB on the line where the warning occurs. Verify that the two vectors or matrices you pass to the PLOT function, or the three you pass to PLOT3, are not complex. If you want to plot the real part of a vector versus the complex part, pass the vector as a single complex vector to the PLOT function. If you want to plot the magnitude of the elements, use the ABS function on the vector before passing it to the PLOT function.

http://www.mathworks.com/help/techdoc/ref/plot.html

http://www.mathworks.com/help/techdoc/ref/plot3.html

http://www.mathworks.com/help/techdoc/ref/abs.html

http://www.mathworks.com/help/techdoc/ref/fft.html

Example demonstrating this warning:

ImaginaryPartIgnored.m

Best Answer

Related Solutions

MATLAB: Complex plots bug

MATLAB: Do I get warning messages about ‘Imaginary Parts of Complex X and/or Y Arguments Ignored’

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