How to merge and sum two data frames

Question

Here is my issue:

df1 <- data.frame(x = 1:5, y = 2:6, z = 3:7)
rownames(df1) <- LETTERS[1:5]
df1
  x y z
A 1 2 3
B 2 3 4
C 3 4 5
D 4 5 6
E 5 6 7

df2 <- data.frame(x = 1:5, y = 2:6, z = 3:7)
rownames(df2) <- LETTERS[3:7]
df2
  x y z
C 1 2 3
D 2 3 4
E 3 4 5
F 4 5 6
G 5 6 7

what I wanted is:

  x y z
A 1 2 3
B 2 3 4
C 4 6 8
D 6 8 10
E 8 10 12
F 4 5 6
G 5 6 7

where duplicated rows were added up by same variable.

Answer 1

A solution with base R:

# create a new variable from the rownames
df1$rn <- rownames(df1)
df2$rn <- rownames(df2)

# bind the two dataframes together by row and aggregate
res <- aggregate(cbind(x,y,z) ~ rn, rbind(df1,df2), sum)
# or (thx to @alistaire for reminding me):
res <- aggregate(. ~ rn, rbind(df1,df2), sum)

# assign the rownames again
rownames(res) <- res$rn

# get rid of the 'rn' column
res <- res[, -1]

which gives:

> res
  x  y  z
A 1  2  3
B 2  3  4
C 4  6  8
D 6  8 10
E 8 10 12
F 4  5  6
G 5  6  7