[R] colSums and rowSums with arrays - different classes and dim ?

[Ernesto Jardim]

Prof Brian Ripley wrote:
> 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.
> 


Ok,

Thanks for the enlightment.

EJ