From: Duncan Murdoch <murdoch_at_stats.uwo.ca>

Date: Wed 13 Jul 2005 - 00:07:22 EST

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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 00:09:23 2005

Date: Wed 13 Jul 2005 - 00:07:22 EST

On 7/12/2005 9:53 AM, allan_sta_staff_sci_main_uct@mail.uct.ac.za wrote:

> hi all

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**> i simply wanted to work with real numbers and thought that (-8)^(1/3) should
**> work.
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It might work in an ideal world, but not in the R floating point world. There's no way to express (1/3) exactly. Since (-8)^(1/3 + epsilon) (restricted to the reals) is not defined for epsilon near zero but not exactly zero, it's really hopeless to expect R to give you what you wanted.

*You* know that you're taking an odd root of a negative number, but R doesn't. You need to use your knowledge to rewrite that mathematical expression as the mathematically equivalent -(8^(1/3)) and then things will be fine.

Duncan Murdoch

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> sorry for not making the question clearer.

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**> /
**> allan
**>
**> Quoting Duncan Murdoch <murdoch@stats.uwo.ca>:
**>
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>> 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 >>

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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 00:09:23 2005

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