[R] Matrix Multiplication, Floating-Point, etc.

[Doran, Harold]

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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