Re: [R] WIERD: Basic computing in R

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Tue, 01 Jul 2008 13:43:28 -0500

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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