Consider the array a:

> a <- array(c(1:9, 1:9), c(3,3,2))
> a
, , 1

     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

, , 2

     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

How do we efficiently compute the row sums of the matrices indexed by the third dimension, such that the result is:

     [,1] [,2]
[1,]   12   12
[2,]   15   15
[3,]   18   18

??

The column sums are easy via the 'dims' argument of colSums():

> colSums(a, dims = 1)

but I cannot find a way to use rowSums() on the array to achieve the desired result, as it has a different interpretation of 'dims' to that of colSums().

It is simple to compute the desired row sums using:

> apply(a, 3, rowSums)
     [,1] [,2]
[1,]   12   12
[2,]   15   15
[3,]   18   18

but that is just hiding the loop. Are there other efficient, truly vectorised, ways of computing the required row sums?