From: Marc Schwartz <marc_schwartz_at_comcast.net>

Date: Sat 20 Jan 2007 - 19:09:19 GMT

R-help@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. Received on Sun Jan 21 06:17:04 2007

Date: Sat 20 Jan 2007 - 19:09:19 GMT

On Sat, 2007-01-20 at 14:00 -0500, Robert Barber wrote:

*> Hi,
**>
*

> I apologize if there is a simple answer to this question that I've

*> missed. I did search the mailing list but I might not have used the
**> right keywords. Why does sum(A3^2) give the result of 1, but
**> sum(A3^2)==1 give the result of FALSE?
**>
**> > A3<-matrix(nrow=3,c(1/(2^.5),1/(2^.5),0))
**> > A3
**> [,1]
**> [1,] 0.7071068
**> [2,] 0.7071068
**> [3,] 0.0000000
**> > sum(A3^2)
**> [1] 1
**> > sum(A3^2)^.5
**> [1] 1
**> > sum(A3^2)==1 # here's the part I don't understand
**> [1] FALSE
**> > sum(A3^2)^.5==1 # here's the part I don't understand
**> [1] FALSE
**>
**> I realize that it has something to do with the conversion of the square
**> roots into decimals. But shouldn't it then give me some number other
**> than 1 as the result for sum(A3^2)? Are there other ways to do this
**> than what I've tried? I'm trying to confirm that A3 is a unit vector.
**>
**> Thank you for your help.
**>
**> Bob B.
*

See R FAQ:

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

> print(sum(A3^2)^.5, 20)

[1] 0.99999999999999988898

**HTH,
**
Marc Schwartz

R-help@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. Received on Sun Jan 21 06:17:04 2007

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