R help - Sep 2010 - getting column names of row-by-row sorted matrix

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

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.

Thanks,

Murali

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

On Sep 1, 2010, at 11:34 AM, David Winsemius wrote:
>
> snipped
> > 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.)
That is over-broad.  apply() only returns a "transposed" result of a  
matrix when the second argument is "1". It is (as I understand it  
anyway) supplying results for building another matrix in the default,  
for R,  column-major order. If you start with rows uou gt columns. If  
your start with columns you get columns. And someday I will figure out  
how this generalizes to arrays.

apply(mtx, 1, I) #transposes
apply(mtx, 2, I)  #does not "transpose"

-- 
David Winsemius, MD
West Hartford, CT