Question

I have three or more independent variables represented as R vectors, like so:

A <- c(1,2,3)
B <- factor(c('x','y'))
C <- c(0.1,0.5)

and I want to take the Cartesian product of all of them and put the result into a data frame, like this:

A B C
1 x 0.1
1 x 0.5
1 y 0.1
1 y 0.5
2 x 0.1
2 x 0.5
2 y 0.1
2 y 0.5
3 x 0.1
3 x 0.5
3 y 0.1
3 y 0.5

I can do this by manually writing out calls to rep:

d <- data.frame(A = rep(A, times=length(B)*length(C)),
                B = rep(B, times=length(A), each=length(C)),
                C = rep(C, each=length(A)*length(B))

but there must be a more elegant way to do it, yes? product in itertools does part of the job, but I can’t find any way to absorb the output of an iterator and put it into a data frame. Any suggestions?

p.s. The next step in this calculation looks like

d$D <- f(d$A, d$B, d$C)

so if you know a way to do both steps at once, that would also be helpful.

Answer 1

You can use expand.grid(A, B, C)

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EDIT: an alternative to using do.call to achieve the second part, is the function mdply from the package plyr:

library(plyr)

d = expand.grid(x = A, y = B, z = C)
d = mdply(d, f)

To illustrate its usage using a trivial function ‘paste’, you can try

d = mdply(d, 'paste', sep = '+');