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

aggregate to the rescue:

aggregate(df, list(Group=replace(rownames(df),rownames(df) %in% c("A","C"), "A&C")), sum)
#  Group V2 V3 V4 V5
#1   A&C  7 11  5  8
#2     B  3  2  7  9
#3     D  3  2  8  9