# Value matching in R

I have three data frames A, B and C:

set.seed(0)
N <- 5
A<-data.frame(cbind(date=c(2,3,5,1), x=NA, id=sample(letters[1:2], 4, replace=T)), stringsAsFactors = F)
B<-data.frame(cbind(date=1:N, y=runif(N)), stringsAsFactors = F)
C<-data.frame(cbind(date=1:N, z=100+sample(N), id=rep(letters[1:2], N, replace=T)), stringsAsFactors = F)
C$z<-as.numeric(C$z)


and they look like this:

A
B
C
> A
date    x id
1    2 <NA>  b
2    3 <NA>  a
3    5 <NA>  a
4    1 <NA>  b
> B
date      y
1    1 0.9082
2    2 0.2017
3    3 0.8984
4    4 0.9447
5    5 0.6608
> C
date   z id
1     1 104  a
2     2 101  b
3     3 105  a
4     4 103  b
5     5 102  a
6     1 104  b
7     2 101  a
8     3 105  b
9     4 103  a
10    5 102  b


I would like to fill in A$x with a function of y and z, let's say, for instance, the product of B$y*C$z for the corresponding dates and ids, like this: for (i in 1:length(A$x)){
A$x[i] <- B$y[A$date[i] == B$date] * C$z[A$date[i] == C$date & A$id[i] == C\$id]
}
> A
date                x id
1    2  20.369875034783  b
2    3 94.3309169216082  a
3    5 67.4013748336583  a
4    1 94.4536101594567  b


This is a very bad idea for a data set with many elements (obviously), as it is slow. I also tried with match() and which(), but there isn't any significant speed up, I believe. Maybe I could use dcast(), after merging everything into one data frame, but I would prefer not to merge the data frames at all (if this can be avoided).

Is it possible to do it more efficiently?

Although you mention that you don't want to merge the data frames, your for loop could be replaced with this single line using merge:

with(merge(A, merge(B, C)), data.frame(date, x=y * z, id))


Given your example of A, B, C, this is returns a data frame:

  date        x id
1    1 94.45361  b
2    2 20.36988  b
3    3 94.33092  a
4    5 67.40137  a


The problem with the for loop is that it's discouraged in R because it's inefficient. Using merge should be fast. I don't think you can get around merging, as the meaning of your logic is in fact merging.