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

Here's a solution using rowSums()

sapply( list(1:2,3:4) , function(i) rowSums(testM[,i]) )

if the number of columns should be arbitrary, it gets more complicated:

li <- split( 1:ncol(testM) , rep(1:(ncol(testM)/2), times=1 , each=2))

sapply( li , function(i) rowSums(testM[,i]) )