I want to merge two data frames: X with length 10 (for example) and Y with length 3 but with similar row names. I only want to retain the length of X but with the values of B in the 3 corresponding rows and the other 10 - 3 = 7 values set to zero.

For example,

X<-data.frame(c(1,2,3,4,5,6,7,8,9,10))
rownames(X)<-c("a","b","c","d","e","f","g","h","i","j")
Y<-data.frame(c(20,30,40))
rownames(Y)<-c("d","f","h")

gives me these data frames

    X             Y
a   1         d   20
b   2         f   30
c   3         h   40
d   4
e   5
f   6
g   7
h   8
i   9
j   10

and I want this now

    Z
a   0
b   0
c   0
d   20
e   0
f   30
g   0
h   40
i   0
j   0

Can this be done easily?

We can use match to find the positions of the row names of Y that are found in X. The values of Y are put into a vector and concatenated with 0. We use the nomatch argument to fill in 0 when there is no match. This returns z as a vector:

Z <- c(unlist(Y, use.names=FALSE), 0)[match(row.names(X), row.names(Y), nomatch=4L)]
Z
[1]  0  0  0 20  0 30  0 40  0  0

To get a data.frame

Z <- data.frame(Z)

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