[R] lost attrubute:names

[Charles C. Berry]

On Thu, 28 Aug 2008, Yuan Jian wrote:

> Hi,
> when I pick out one element from a matrix, the attribute name is kept, but when
> more than one elements are extracted, the attribute name lost;

To what attribute 'name' do you refer?

I only see 'dim' and 'dimnames' attributes:

> attributes(a)
$dim
[1] 3 3

$dimnames
$dimnames[[1]]
NULL

$dimnames[[2]]
[1] "c1" "c2" "c3"




>  
>> a<-matrix(c(1,2,3,11,12,13,45,56,76),ncol=3,dimnames=list(c(),c("c1","c2","c3")))
>> k<-a[a[,"c3"]>50,"c3"]
>> kk<-a[a[,"c3"]>60,"c3"]
>> attributes(k)
> NULL
>> attributes(kk)
> $names
> [1] "c3"
>  

Try attributes(a[a[,"c3"]>50,"c3", drop = FALSE ] ).

Try adding rownames to 'a', and see what happens to the attributes of each 
of the above forms.

"[" merely tries to do something reasonable.

Why do you think k should have any attributes, and what names do you think 
k should have???

HTH,

Chuck

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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