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

testM[,c(T,F)]+testM[,c(F,T)];
##       [,1] [,2]
##  [1,]   12   52
##  [2,]   14   54
##  [3,]   16   56
##  [4,]   18   58
##  [5,]   20   60
##  [6,]   22   62
##  [7,]   24   64
##  [8,]   26   66
##  [9,]   28   68
## [10,]   30   70