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, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UKFax: +44 1865 272595
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