R help - Mar 2012 - Why aren't row names in a data frame unique?

I expected the row names to be unique but a data frame appears to be able to
hold duplicate row names. This makes me thing that there must be circumstances
when it is necessary. However, I cannot think of any. Please enlighten me.

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On 03/06/2012 06:42 PM, Ajay Askoolum wrote:
> I expected the row names to be unique but a data frame appears to be able
to hold duplicate row names. This makes me thing that there must be
circumstances when it is necessary. However, I cannot think of any. Please
enlighten me.
>
Hi Ajay,
An example of how you managed to do this would be helpful. I tried it:

dodo<-data.frame(a=1:3,b=4:6)
dodo
   a b
1 1 4
2 2 5
3 3 6
rownames(dodo)<-c("s","t","s")
Error in `row.names<-.data.frame`(`*tmp*`, value = value) :
   duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique value when setting 'row.names': ?s?

Jim

S+ does allow duplicates in the row names if you ask for them
to be allowed (with data.frame(..., dup.row.names=TRUE) or
attr(df, "dup.row.names") <- TRUE).

They were allowed because bootstrappers were sampling the
rows of data.frames with replacement and it turned out that
a fair bit of time was being used calling make.unique() on
the row names of the sample.

There may be better solutions to that problem, but that is
why S+ allows them.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at
r-project.org] On Behalf Of Ajay Askoolum
> Sent: Monday, March 05, 2012 11:42 PM
> To: R General Forum
> Subject: [R] Why aren't row names in a data frame unique?
> 
> I expected the row names to be unique but a data frame appears to be able
to hold duplicate row names.
> This makes me thing that there must be circumstances when it is necessary.
However, I cannot think of
> any. Please enlighten me.
> 
> 	[[alternative HTML version deleted]]
> 
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