[R] R: to the power

[Duncan Murdoch]

On 7/12/2005 9:53 AM, allan_sta_staff_sci_main_uct at mail.uct.ac.za wrote:
> hi all
> 
> i simply wanted to work with real numbers and thought that (-8)^(1/3) should
> work.

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



> 
> sorry for not making the question clearer.
> 
> /
> allan
> 
> Quoting Duncan Murdoch <murdoch at 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
>>
> 
> 
> 
> 
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