Sum Every N Values in Matrix

Question

So I have taken a look at this question posted before which was used for summing every 2 values in each row in a matrix. Here is the link: sum specific columns among rows. I als

Answer 1

Transform the matrix to an array and use colSums (as suggested by @nongkrong):

y <- matrix(1:27, nrow = 3)
n <- 3

a <- y
dim(a) <- c(nrow(a), ncol(a)/n, n)
b <- aperm(a, c(2,1,3))
colSums(b)
#     [,1] [,2] [,3]
#[1,]   12   39   66
#[2,]   15   42   69
#[3,]   18   45   72

Of course this assumes that ncol(y) is divisible by n.

PS: You can of course avoid creating so many intermediate objects. They are there for didactic purposes.