Octave / Matlab: How to plot the roots of a polynomial

Question

Im trying to plot the roots of a polynomial, and i just cant get it.

First i create my polynomial

p5 = [1 0 0 0 0 -1] %x^5 - 1
r5 = roots(p5)
stem (p5)

Im using the stem function, but I would like to remove the stems, and just get the circle around the roots.

Is this possible, is stem the right command?

Thanks in advance,

PS: This is not homework, but very close, will tag it if requested.

Answer 1

If you have complex roots that you want to plot with the real part on the x-axis and the imaginary part on the y-axis, you can just use the PLOT function:

plot(r5,'o');

If you are wanting to plot the function and the roots together, you will have to ignore the complex roots (as yuk mentions in the comment below):

p5 = [1 0 0 0 0 -1];
r5 = roots(p5);
realRoots = r5(isreal(r5)); %# Gets just the real roots
x = -2:0.01:2; %# x values for the plot
plot(x,polyval(p5,x)); %# Evaluate the polynomial and plot it
hold on; %# Add to the existing plot
plot(realRoots,zeros(size(realRoots)),'o'); %# Plot circles for the roots

• yuk: Notice that r5 contains complex numbers and plot will ignore imaginary part. To plot real root only you can do: plot(r5(imag(r5)==0),zeros(sum(imag(r5)==0)),'o')
• gnovice: @yuk: Good catch. I updated the answer using the ISREAL function. ;)
• Tom: @yuk, gnovice: Thank you both for your help, but how would i plot all roots (both img and real ) ?
• gnovice: @Tom: The first part of my new answer shows how to plot both the real and imaginary parts of all the roots together.
• Tom: @gnovice, thanks, ended up using plot (real(r5),imag(r5),'o')