[R] WIERD: Basic computing in R
Marc Schwartz
marc_schwartz at comcast.net
Tue Jul 1 20:43:28 CEST 2008
on 07/01/2008 01:15 PM poolloopus at yahoo.com wrote:
> Can someone please enlighten me as to why the following happens?
>> -2.7^8.6
> [1] -5125.407
>
>> p<- -2.7 q<- 8.6 p^q
> [1] NaN
> R seems perfectly able to calculate -2.7^8.6, but fails when
> the exact same values are assigned to variables and then the
> computation is repeated. Thanks in advance for any suggetsions. Kris.
You are not seeing what you think you are seeing in the first result.
> -2.7^8.6
[1] -5125.407
is parsed in the same way as:
> -(2.7^8.6)
[1] -5125.407
In other words, it is parsed as:
> 2.7^8.6
[1] 5125.407
and then negated.
If you were to 'properly' define the precedence of operation, you would use:
> (-2.7)^8.6
[1] NaN
which is the same result you get when you use the vectors.
See R FAQ 7.33 Why are powers of negative numbers wrong?:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-are-powers-of-negative-numbers-wrong_003f
HTH,
Marc Schwartz
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