install.packages('sos')# if you don't have it already
library(sos)
rs <- ???roots
# 216 matches
summary(rs)
# in 106 packages
rs # opens a web browser with all 216 matched in a table
# listing the package with the most matches first.
# This included roots{signal}, which referenced polyroot{base},
# which led me to the following:
> polyroot(c(4, 0, 0, 1))
[1] 0.793701+1.37473i -1.587401+0.00000i 0.793701-1.37473i
> polyroot(c(4, 0, 1))
[1] 0+2i 0-2i
Hope this helps.
Spencer
# please excuse: I'm the lead author of "sos". In my not-so-humble
opinion, it's the fastest way to do a literature search for anything
statistical. If your search with "writeFindFn2lxs" does NOT answer your
question in a very few minutes, it's OK to look elsewhere. (Please see
the vignette for more details if you are not familiar with it.)
On 10/26/2010 4:34 PM, bill.venab...@csiro.au wrote:
To take it one step further:
x<- as.complex(-4)
cx<- x^(1/3)
r<- complex(modulus = Mod(cx), argument = Arg(cx)*c(1,3,5))
r
[1] 0.793701+1.37473i -1.587401+0.00000i 0.793701-1.37473i
r^3
[1] -4+0i -4+0i -4+0i
So when you ask for "the" cube root of -4, R has a choice of three possible
answers it can give you.
It is no surprise that this does not work when working in the real domain, except
"by fluke" with something like
-4^(1/3)
[1] -1.587401
where the precedence of the operators is not what you might expect. Now that
could be considered a bug, since apparently
-4^(1/2)
[1] -2
which comes as rather a surprise!
Bill.
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf Of Kjetil Halvorsen
Sent: Wednesday, 27 October 2010 9:17 AM
To: Gregory Ryslik
Cc: r-help Help
Subject: Re: [R] cube root of a negative number
Look at this:
x<- as.complex(-4)
x
[1] -4+0i
x^(1/3)
[1] 0.793701+1.37473i
(-4)^(1/3)
[1] NaN
It seems that R gives you the principal root, which is complex, and
not the real root.
Kjetil
On Tue, Oct 26, 2010 at 8:05 PM, Gregory Ryslik<rsa...@comcast.net> wrote:
Hi,
This might be me missing something painfully obvious but why does the cube root
of the following produce an NaN?
(-4)^(1/3)
[1] NaN
As we can see:
(-1.587401)^3
[1] -4
Thanks!
Greg
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