sorting a complex vector by imaginary part in R

Question

roots <- polyroot(c(5, 4, 3, 2, 1))

I want to sort the roots by ascending order of the imaginary part. By default the sort function sor

Answer 1

Use Im function to extract the imaginary part and sort it.

roots <- polyroot(c(5, 4, 3, 2, 1))
#[1]  0.287815+1.416093i -1.287815+0.857897i -1.287815-0.857897i
#[4]  0.287815-1.416093i

roots[order(Im(roots))]
#[1]  0.287815-1.416093i -1.287815-0.857897i -1.287815+0.857897i
#[4]  0.287815+1.416093i

Answer 2

This also works:

> roots<-polyroot(c(5,4,3,2,1))
> roots[sort(Im(roots),index.return=T)$ix]
[1]  0.287815-1.416093i -1.287815-0.857897i
[3] -1.287815+0.857897i  0.287815+1.416093i

Sort the complex numbers by their imaginary parts and return their indexes, then use the indexes to refer the corresponding complex value in the sorted order.