R Sum every k columns in matrix

Question

I have a matrix temp1 (dimensions Nx16) (generally, NxM)

I would like to sum every k columns in each row to one value.

Here is what I got to so far:

Answer 1

If the dimensions are equal for the sub matrices, you could change the dimensions to an array and then do the rowSums

 m1 <- as.matrix(temp1)
 n <- 4
 dim(m1) <- c(nrow(m1), ncol(m1)/n, n)
 res <- matrix(rowSums(apply(m1, 2, I)), ncol=n)
 identical(res[,1],rowSums(temp1[,1:4]))
 #[1] TRUE

Or if the dimensions are unequal

  t(sapply(seq(1,ncol(temp2), by=4), function(i) {
                  indx <- i:(i+3)
               rowSums(temp2[indx[indx <= ncol(temp2)]])}))

data

set.seed(24)
temp1 <- as.data.frame(matrix(sample(1:20, 16*4, replace=TRUE), ncol=16))

set.seed(35)
temp2 <- as.data.frame(matrix(sample(1:20, 17*4, replace=TRUE), ncol=17))