[R] how to get row name of matrix when result is a vector

[David Winsemius]

On Sep 3, 2010, at 9:08 AM, syrvn wrote:

>
> Hi,
>
> the R code:
>
> a <-  
> matrix 
> (c(1,5,4,3,7,10,34,4,3,8,6,5,12,17,45,3,2,45,46,47,3,4,22,12,21),
> nrow=5)
> rownames(a) <- c("a","b","c","d","e")
> a
> a[which(a[,3] < 8), ]
> a[which(a[,3] < 6), ]

It wasn't exactly clear and possible that you wanted only rownames  
that satisfied those criteria
 > rownames(a)[which(a[,3] < 8)]
[1] "a" "b"
 > rownames(a)[which(a[,3] < 6)]
[1] "b"
 > rownames(a)[which(a[,3] < 4)]
character(0)

Sometimes which() is needed, and others it is not. This appears to be  
one where it is not needed:

 > rownames(a)[a[,3] < 4]
character(0)
 > rownames(a)[a[,3] < 6]
[1] "b"
 > rownames(a)[a[,3] < 8]
[1] "a" "b"

-- 
David.
>
>
> produces the following output:
>
>> a
>  [,1] [,2] [,3] [,4] [,5]
> a    1   10    6    3    3
> b    5   34    5    2    4
> c    4    4   12   45   22
> d    3    3   17   46   12
> e    7    8   45   47   21
>
>> a[which(a[,3] < 8), ]
>  [,1] [,2] [,3] [,4] [,5]
> a    1   10    6    3    3
> b    5   34    5    2    4
>
>> a[which(a[,3] < 6), ]
> [1]  5 34  5  2  4
>
>
> I actually thought it must be rather easy to somehow extract the row  
> name
> when the result is just a vector as in the second example and not a  
> matrix.
> I tried several ways but without any success.
>
> Any advice?
> Best,
> syrvn
>
>
> -- 
> View this message in context: http://r.789695.n4.nabble.com/how-to-get-row-name-of-matrix-when-result-is-a-vector-tp2525631p2525631.html
> Sent from the R help mailing list archive at Nabble.com.
>
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David Winsemius, MD
West Hartford, CT