如何沿第 3 维求和数组

Question

What is the canonical way to sum a 3D array along dimension 3 (thereby yielding a matrix)?

I know I can apply(A,c(1,2),sum) but (wrongly or rightly) I got the impression from somewhere that using apply is no better than using for loops.

I could probably aperm the array, colSum it, then un aperm it again, but that wouldn't be very readable.

Is there a better way?

Answer 1

Use rowSums . It offers a dims parameter which specifies "[w]hich dimensions are regarded as 'rows' or 'columns' to sum over. For row*, the sum or mean is over dimensions dims+1".

a <- array(1:16, c(2, 2, 2))
#, , 1
#
#     [,1] [,2]
#[1,]    1    3
#[2,]    2    4
#
#, , 2
#
#     [,1] [,2]
#[1,]    5    7
#[2,]    6    8

rowSums(a, dims = 2)
#     [,1] [,2]
#[1,]    6   10
#[2,]    8   12