From: <davidr_at_rhotrading.com>

Date: Wed 13 Jul 2005 - 00:16:55 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Wed Jul 13 00:20:43 2005

Date: Wed 13 Jul 2005 - 00:16:55 EST

In general, x^y is evaluated as exp(y*log(x)). In your case, x is
negative, so log(x) is NaN. Note also that 1/3 is not represented
exactly in your computer anyway, so you would not get an exact cube root
this way; e.g.:

R> format((1234567891112^3)^(1/3),digits=16) [1] "1234567891112.001"

(Probably a bad example, but you get the idea.)

In general, sign(x)*(abs(x)^(1/3)) is the way to go for cube roots.

David L. Reiner

> -----Original Message-----

*> From: r-help-bounces@stat.math.ethz.ch [mailto:r-help-
**> bounces@stat.math.ethz.ch] On Behalf Of
**> allan_sta_staff_sci_main_uct@mail.uct.ac.za
**> Sent: Tuesday, July 12, 2005 8:54 AM
**> To: Duncan Murdoch
**> Cc: r-help@stat.math.ethz.ch
**> Subject: Re: [R] R: to the power
**>
**> hi all
**>
**> i simply wanted to work with real numbers and thought that (-8)^(1/3)
**> should
**> work.
**>
**> sorry for not making the question clearer.
**>
**> /
**> allan
**>
**> Quoting Duncan Murdoch <murdoch@stats.uwo.ca>:
**>
**> > On 7/12/2005 9:29 AM, Robin Hankin wrote:
**> > > Hi
**> > >
**> > > I find that one often needs to keep reals real and complexes
*

complex.

*> > >
*

> > > Try this:

*> > >
**> > > "cuberooti" <-
**> > > function (x)
**> > > {
**> > > if (is.complex(x)) {
**> > > return(sqrt(x + (0+0i)))
**> > > }
**> > > sign(x)* abs(x)^(1/3)
**> > > }
**> > >
**> > >
**> > > best wishes
**> > >
**> > > [see that (0+0i) sitting there!]
**> >
**> > I don't understand this.
**> >
**> > 1. I don't think you meant to use sqrt() there, did you??
**> >
**> > 2. What effect does the 0+0i have? x has already been determined
*

to be

*> > complex.
**> >
*

> > Duncan Murdoch

*> >
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide! http://www.R-project.org/posting-
**> guide.html
*

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Wed Jul 13 00:20:43 2005

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