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]) )

Answer 2

a solution around your initial idea:

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

Answer 3

How about:

matrix(c(testM[, 1] + testM[, 2], testM[, 2] + testM[, 4]), nrow = 10)

Answer 4

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

Answer 5

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))