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

[Talbot Katz]

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!

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