R: How to sum pairs in a Matrix by row?

Question

Probably this would be easy. I have a Matrix:

testM <- matrix(1:40, ncol = 4, byrow = FALSE)
testM
      [,1] [,2] [,3] [,4]
 [1,]    1   11   21   31
 [2,]           


        

Answer 1

a solution around your initial idea:

sapply(seq(2, ncol(testM), 2), function(x) apply(testM[, (x-1):x], 1, sum))