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

We can do a matrix multiplication:

M <- matrix(c(1,1,0,0, 0,0,1,1), 4, 2)
testM %*% M

another solution with tapply():

g <- gl(ncol(testM)/2, 2)
t(apply(testM, 1, FUN=tapply, INDEX=g, sum))