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

To sum consecutive sets of n elements from each row, you just need to write a function that does the summing and apply it to each row:

n <- 3
t(apply(y, 1, function(x) tapply(x, ceiling(seq_along(x)/n), sum)))
#       1  2  3
# [1,] 12 39 66
# [2,] 15 42 69
# [3,] 18 45 72