Feng Chen - 6 months ago 44

R Question

I need to do a cartesian product of two data frames. For example,

`A = id weight type`

10 20 a

10 30 b

25 10 c

B = date report

2007 y

2008 n

then C would be like after doing cartesian product of A and B

`C = id weight type date report`

10 20 a 2007 y

10 20 a 2008 n

10 30 b 2007 y

10 30 b 2008 n

25 10 c 2007 y

25 10 c 2008 n

as some ids are the same in A, so I cannot use a way like

`C <- merge(A$id,B$date)`

C <- merge(C,A,by="id")

C <- merge(C,B,by="date")

This way will generate more rows. Could anyone help me out of here? Thanks

Answer

`merge(A, B)`

, provided there are no columns linking the two, should do this by default, no?

From `?merge`

(emphasis mine):

If by or both by.x and by.y are of length 0 (a length zero vector or NULL), the result, r, is the

Cartesian productof x and y, i.e., dim(r) = c(nrow(x)*nrow(y), ncol(x) + ncol(y)).

Admittedly, this does require one to know to look in `?merge`

. Context-based searching in R is severely lacking; even rseek doesn't immediately provide this.