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Branimir K. Hackenberger

2011-Apr-17 05:51 UTC

[R] cube root

This is some interesting:

 
> -8^(1/3)
[1] -2
> x=(-8:8)
> y=x^(1/3)
> y
 [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN      NaN
0.000000 1.000000

[11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000

 

So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
-8^(1/3)=-2?)

 

Thx!!!

 

 


	[[alternative HTML version deleted]]

Niels Richard Hansen

2011-Apr-17 05:58 UTC

head link

[R] cube root

?Syntax for precedence of operators.

On 16/04/11 22.51, Branimir K. Hackenberger wrote:> This is some interesting:
>
>
>
>> -8^(1/3)
>
> [1] -2
>
>> x=(-8:8)
>
>> y=x^(1/3)
>
>> y
>
>   [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN     
NaN
> 0.000000 1.000000
>
> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>
>
>
> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
> -8^(1/3)=-2?)
>
>
>
> Thx!!!
>
>
>
>
>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
-- 
Niels Richard Hansen                     Web:   www.math.ku.dk/~richard	
Associate Professor                      Email: Niels.R.Hansen at math.ku.dk
Department of Mathematical Sciences             nielsrichardhansen at gmail.com
University of Copenhagen                 Skype: nielsrichardhansen.dk	
Universitetsparken 5                     Phone: +1 510 502 8161	
2100 Copenhagen ?
Denmark

Berend Hasselman

2011-Apr-17 06:00 UTC

head link

[R] cube root

Branimir K. Hackenberger wrote:> 
> This is some interesting:
> 
>> -8^(1/3)
> 
> [1] -2
> 
>> x=(-8:8)
> 
>> y=x^(1/3)
> 
>> y
> 
>  [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN     
> NaN
> 0.000000 1.000000
> 
> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
> -8^(1/3)=-2?)
> 
Operator precedence.

See R FAQ 7.33 Why are powers of negative numbers wrong?


Berend


--
View this message in context:
http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
Sent from the R help mailing list archive at Nabble.com.

Tsjerk Wassenaar

2011-Apr-17 07:46 UTC

head link

[R] cube root

Hey :)

In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
fractional power of a negative number is complex. And this can be
obtained properly in R:
> (-8+0i)^(1/3)[1] 1+1.732051i

Cheers,

Tsjerk
>> This is some interesting:
>>
>>> -8^(1/3)
>>
>> [1] -2
>>
>>> x=(-8:8)
>>
>>> y=x^(1/3)
>>
>>> y
>>
>> ?[1] ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN
>> NaN
>> 0.000000 1.000000
>>
>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>> -8^(1/3)=-2?)
>>
>
> Operator precedence.
>
> See R FAQ 7.33 Why are powers of negative numbers wrong?
>
>
> Berend
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Tsjerk A. Wassenaar, Ph.D.

post-doctoral researcher
Molecular Dynamics Group
* Groningen Institute for Biomolecular Research and Biotechnology
* Zernike Institute for Advanced Materials
University of Groningen
The Netherlands

David Winsemius

2011-Apr-17 08:25 UTC

head link

[R] cube root

Hey :)

In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
fractional power of a negative number is complex. 

_Not_ in addition. The citation of FAQ 7.33 did not address the correct
issue since the sequence was enclosed in parens. Yours is the only correct
answer. For further discussion about various approaches to fractional powers
of negative numbers one can get further approaches by searching the
archives.

-- 
David.

And this can be
obtained properly in R:
> (-8+0i)^(1/3)[1] 1+1.732051i

Cheers,

Tsjerk
>> This is some interesting:
>>
>>> -8^(1/3)
>>
>> [1] -2
>>
>>> x=(-8:8)
>>
>>> y=x^(1/3)
>>
>>> y
>>
>> ?[1] ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN
>> NaN
>> 0.000000 1.000000
>>
>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>> -8^(1/3)=-2?)
>>
>
> Operator precedence.
>
> See R FAQ 7.33 Why are powers of negative numbers wrong?
>
>
> Berend
>
Tsjerk Wassenaar wrote:> 
> Hey :)
> 
> In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
> fractional power of a negative number is complex. And this can be
> obtained properly in R:
> 
>> (-8+0i)^(1/3)
> [1] 1+1.732051i
> 
> Cheers,
> 
> Tsjerk
> 
>>> This is some interesting:
>>>
>>>> -8^(1/3)
>>>
>>> [1] -2
>>>
>>>> x=(-8:8)
>>>
>>>> y=x^(1/3)
>>>
>>>> y
>>>
>>> ?[1] ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN
>>> NaN
>>> 0.000000 1.000000
>>>
>>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>>> -8^(1/3)=-2?)
>>>
>>
>> Operator precedence.
>>
>> See R FAQ 7.33 Why are powers of negative numbers wrong?
>>
>>
>> Berend
>>
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
> 
> 
> 
> -- 
> Tsjerk A. Wassenaar, Ph.D.
> 
> post-doctoral researcher
> Molecular Dynamics Group
> * Groningen Institute for Biomolecular Research and Biotechnology
> * Zernike Institute for Advanced Materials
> University of Groningen
> The Netherlands
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
Tsjerk Wassenaar wrote:> 
> Hey :)
> 
> In addition, although (-2)^3 = -8, (-8)^(1/3) != -2, mathematically. A
> fractional power of a negative number is complex. And this can be
> obtained properly in R:
> 
>> (-8+0i)^(1/3)
> [1] 1+1.732051i
> 
> Cheers,
> 
> Tsjerk
> 
>>> This is some interesting:
>>>
>>>> -8^(1/3)
>>>
>>> [1] -2
>>>
>>>> x=(-8:8)
>>>
>>>> y=x^(1/3)
>>>
>>>> y
>>>
>>> ?[1] ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN ? ? ?NaN
>>> NaN
>>> 0.000000 1.000000
>>>
>>> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>>> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
>>> -8^(1/3)=-2?)
>>>
>>
>> Operator precedence.
>>
>> See R FAQ 7.33 Why are powers of negative numbers wrong?
>>
>>
>> Berend
>>
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/cube-root-tp3455020p3455027.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
> 
> 
> 
> -- 
> Tsjerk A. Wassenaar, Ph.D.
> 
> post-doctoral researcher
> Molecular Dynamics Group
> * Groningen Institute for Biomolecular Research and Biotechnology
> * Zernike Institute for Advanced Materials
> University of Groningen
> The Netherlands
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 

--
View this message in context:
http://r.789695.n4.nabble.com/cube-root-tp3455020p3455118.html
Sent from the R help mailing list archive at Nabble.com.

Sascha Vieweg

2011-Apr-17 09:38 UTC

head link

[R] cube root

On 11-04-17 07:51, Branimir K. Hackenberger wrote:
> This is some interesting:
>
>> -8^(1/3)
>
> [1] -2
>
>> x=(-8:8)
>
>> y=x^(1/3)
>
>> y
>
> [1]      NaN      NaN      NaN      NaN      NaN      NaN      NaN      NaN
> 0.000000 1.000000
>
> [11] 1.259921 1.442250 1.587401 1.709976 1.817121 1.912931 2.000000
>
> So, can anybody explain this?! (Why is x[1]^(1/3)=y[1]=NaN, but
> -8^(1/3)=-2?)
-8^(1/3) == (-8)^(1/3) # NA
-8^(1/3) == -(8^(1/3)) # TRUE
> Thx!!!
You're welcome.

-- 
Sascha Vieweg, saschaview at gmail.com

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