[R] Matrix Multiplication, Floating-Point, etc.
Doran, Harold
HDoran at air.org
Mon Jul 30 16:43:59 CEST 2007
This is giving you exactly what you are asking for. The operator * does
element by element multiplication. So, .48 + -.48 =0, right? Is there
another mathematical possibility you were expecting?
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Talbot Katz
> Sent: Friday, July 27, 2007 6:31 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Matrix Multiplication, Floating-Point, etc.
>
> Hi.
>
> I recently tried the following in R 2.5.1 on Windows XP:
>
> >ev2<-c(0.8,-0.6)
> >ev1<-c(0.6,0.8)
> >ev1%*%ev2
> [,1]
> [1,] -2.664427e-17
> >sum(ev1*ev2)
> [1] 0
> >
>
> (I got the same result with R 2.4.1 on a different Windows XP
> machine.)
>
> I expect this issue is very familiar and probably has been
> discussed in this forum before. Can someone please point me
> to some documentation or discussion about this? Is there
> some standard way to get the "correct"
> answer from %*%?
>
> Thanks!
>
> -- TMK --
> 212-460-5430 home
> 917-656-5351 cell
>
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