How to combine two rows in R?

Question

I would like to combine/sum two rows based on rownames to make one row in R. The best route might be to create a new row and sum the two rows together.

Example df:<

Answer 1

matrix multiply?

> A <- matrix(c(1,0,0,0,1,0,1,0,0,0,0,1), 3)
> A                                        
     [,1] [,2] [,3] [,4]
[1,]    1    0    1    0
[2,]    0    1    0    0
[3,]    0    0    0    1
> A %*% X
     V2 V3 V4 V5
[1,]  7 11  5  8
[2,]  3  2  7  9
[3,]  3  2  8  9

Or using the Matrix package for sparse matrices:

fac2sparse(factor(c(1,2,1,4))) %*% X