R help - Apr 2005 - colSums and rowSums with arrays - different classes and dim ?

Hi,

I'm using colSums and rowSums to sum the first dimensions of arrays. It 
works ok but the resulting object is different. See

 > a3d <- array(rnorm(120, mean=2), dim=c(20,6,1))
 > dim(colSums(a3d))
[1] 6 1
 > dim(rowSums(a3d))
NULL
 > class(colSums(a3d))
[1] "matrix"
 > class(rowSums(a3d))
[1] "numeric"

I was expecting rowSums to preserve the array class and the relevant 
dimensions (1,20).

The main problem is with arrays where the third dimension (or higher) is 
 > 1. colSums preserve the array but rowSums concatenate the results:

 > a3d <- array(rnorm(120, mean=2), dim=c(20,3,2))
 > rowSums(a3d)
  [1]  8.894178 11.932361 15.231601 12.374629 11.823671 10.564709  9.065166
  [8] 13.900264 13.331756  9.351242 11.989821  7.643745  9.923288  8.169997
[15] 12.124624 16.711742 11.414150 15.221880 12.053734 13.368988
 > colSums(a3d)
          [,1]     [,2]
[1,] 44.80941 29.49216
[2,] 42.18339 39.81121
[3,] 39.90528 38.89010

Is this on purpose ?

Regards

EJ

On Thu, 21 Apr 2005, Ernesto Jardim wrote:
> I'm using colSums and rowSums to sum the first dimensions of arrays. It
works
> ok but the resulting object is different. See
>
>> a3d <- array(rnorm(120, mean=2), dim=c(20,6,1))
>> dim(colSums(a3d))
> [1] 6 1
>> dim(rowSums(a3d))
> NULL
>> class(colSums(a3d))
> [1] "matrix"
>> class(rowSums(a3d))
> [1] "numeric"
>
> I was expecting rowSums to preserve the array class and the relevant 
> dimensions (1,20).
>
> The main problem is with arrays where the third dimension (or higher) is
> 1.
> colSums preserve the array but rowSums concatenate the results:
No, it acts as documented.
>> a3d <- array(rnorm(120, mean=2), dim=c(20,3,2))
>> rowSums(a3d)
> [1]  8.894178 11.932361 15.231601 12.374629 11.823671 10.564709  9.065166
> [8] 13.900264 13.331756  9.351242 11.989821  7.643745  9.923288  8.169997
> [15] 12.124624 16.711742 11.414150 15.221880 12.053734 13.368988
>> colSums(a3d)
>         [,1]     [,2]
> [1,] 44.80941 29.49216
> [2,] 42.18339 39.81121
> [3,] 39.90528 38.89010
>
> Is this on purpose ?
Yes, and documented.  rowSums(a3d) is a vector of length 20, by

     dims: Which dimensions are regarded as "rows" or
"columns" to sum
           over.  For 'row*', the sum or mean is over dimensions
           'dims+1, ...'; for 'col*' it is over dimensions
'1:dims'.

whereas colSums(a3d) is an array of dims c(6, 1).

Don't be confused by the class: there is no S3 class here.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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