[R] getting column names of row-by-row sorted matrix

[David Winsemius]

On Sep 1, 2010, at 11:18 AM, <Murali.Menon at avivainvestors.com> wrote:

> Hi folks,
>
> I want to sort a matrix row-by-row and create a new matrix that  
> contains the corresponding colnames of the original matrix.
>
> E.g.
>
>> set.seed(123)
>> a <- matrix(rnorm(20), ncol=4); colnames(a) <- c("A","B","C","D")
>> a
>               A          B          C          D
> [1,] -0.56047565  1.7150650  1.2240818  1.7869131
> [2,] -0.23017749  0.4609162  0.3598138  0.4978505
> [3,]  1.55870831 -1.2650612  0.4007715 -1.9666172
> [4,]  0.07050839 -0.6868529  0.1106827  0.7013559
> [5,]  0.12928774 -0.4456620 -0.5558411 -0.4727914
>
> I want to obtain a matrix that looks like this

 > t( apply(a, 1, function(x) colnames(a)[order(x)]) )
      [,1] [,2] [,3] [,4]
[1,] "A"  "C"  "B"  "D"
[2,] "A"  "C"  "B"  "D"
[3,] "D"  "B"  "C"  "A"
[4,] "B"  "A"  "C"  "D"
[5,] "C"  "D"  "B"  "A"

(apply returns a transposed version.)
>
> A C B D
> A C B D
> D B C A
> B A C D
> C D B A
>
> How best to achieve this? I was able to do it for the max and min of  
> each row by which.min, which.max, but for the entire thing, I'm  
> stymied.


-- 
David Winsemius, MD
West Hartford, CT