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

I would do something similar to the OP -- apply rowSums on subsets of the matrix:

n  = 3
ng = ncol(y)/n
sapply( 1:ng, function(jg) rowSums(y[, (jg-1)*n + 1:n ]))

#      [,1] [,2] [,3]
# [1,]   12   39   66
# [2,]   15   42   69
# [3,]   18   45   72