R help - May 2009 - max.col weirdness

Hi,
I think there's some rounding issue with returning the max column.  
(running 2.9.0 on an Apple, but my buddy found it on his PC)

 > x <- matrix(c(1234.568,1234.569,1234.567),1)
 > max.col(x)
[1] 2
 > x <- matrix(c(12345.568,12345.569,12345.567),1)
 > max.col(x)
[1] 3
 > x <- matrix(c(112345.568,112345.569,112345.567),1)
 > max.col(x)
[1] 3
 > max.col(-x)
[1] 1

Thanks, Daryl

Hi,
Sorry, I was too hasty.  I see that the tolerance is in the 
documentation (and that "random" is the default tie-breaker).

Would it be possible to allow tolerance to be a parameter?

thanks, Daryl



Daryl Morris wrote:
> Hi,
> I think there's some rounding issue with returning the max column.  
> (running 2.9.0 on an Apple, but my buddy found it on his PC)
>
> > x <- matrix(c(1234.568,1234.569,1234.567),1)
> > max.col(x)
> [1] 2
> > x <- matrix(c(12345.568,12345.569,12345.567),1)
> > max.col(x)
> [1] 3
> > x <- matrix(c(112345.568,112345.569,112345.567),1)
> > max.col(x)
> [1] 3
> > max.col(-x)
> [1] 1
>
> Thanks, Daryl
>
>
>

Try reading the man page, which says:

Details

When ties.method = "random", as per default, ties are broken at
random. In
this case, the determination of a tie assumes that the entries are
probabilities: there is a relative tolerance of 1e-5, relative to the
largest (in magnitude, omitting infinity) entry in the row.


Bert Gunter
Genentech Nonclinical Biostatistics

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
Behalf Of Daryl Morris
Sent: Thursday, May 28, 2009 1:47 PM
To: r-help at r-project.org
Subject: [R] max.col weirdness

Hi,
I think there's some rounding issue with returning the max column.  
(running 2.9.0 on an Apple, but my buddy found it on his PC)

 > x <- matrix(c(1234.568,1234.569,1234.567),1)
 > max.col(x)
[1] 2
 > x <- matrix(c(12345.568,12345.569,12345.567),1)
 > max.col(x)
[1] 3
 > x <- matrix(c(112345.568,112345.569,112345.567),1)
 > max.col(x)
[1] 3
 > max.col(-x)
[1] 1

Thanks, Daryl

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