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

[Charles C. Berry]

7.31 Why doesn't R think these numbers are equal?

On Fri, 27 Jul 2007, Talbot Katz wrote:

> 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
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901