Q:

sum specific columns among rows

I need to sum every two columns
for example
x1  x2  x3  x4
12  2   3   7
1   4   6   5

I need 
X1     X2
14     10
5      11

I tried apply function
and I tried this function 
mat is a matrix
mat1=matrix()

for(i in 1:nrow(mat)){
for(j in 1:ncol(mat)){
mat1[i,j]=mat[j,i]+mat[j,i+1]
}}

A:

A generalization of this problem (for a data.frame) might be something like:
sapply(split.default(mydf, 0:(length(mydf)-1) %/% 2), rowSums)
#       0  1
# [1,] 14 10
# [2,]  5 11

Replace the "2" in %/% 2 with the number of sets of columns you would like to "aggregate". 

A:

For example: 
 mat[,c(TRUE,FALSE)]+mat[,c(FALSE,TRUE)]

  x1 x3
1 14 10
2  5 11

