Unique combination of all elements from two (or more) vectors

Question

I am trying to create a unique combination of all elements from two vectors of different size in R.

For example, the first vector is

a <- c("ABC", "DEF", "GHI")

and the second one is dates stored as strings currently

b <- c("2012-05-01", "2012-05-02", "2012-05-03", "2012-05-04", "2012-05-05")

I need to create a data frame with two columns like this

> data
    a          b
1  ABC 2012-05-01
2  ABC 2012-05-02
3  ABC 2012-05-03
4  ABC 2012-05-04
5  ABC 2012-05-05
6  DEF 2012-05-01
7  DEF 2012-05-02
8  DEF 2012-05-03
9  DEF 2012-05-04
10 DEF 2012-05-05
11 GHI 2012-05-01
12 GHI 2012-05-02
13 GHI 2012-05-03
14 GHI 2012-05-04
15 GHI 2012-05-05

So basically, I am looking for a unique combination by considering all the elements of one vector (a) juxtaposed with all the elements of the second vector (b).

An ideal solution would generalize to more input vectors.

---

See also:
How to generate a matrix of combinations

Answer 1

this maybe what you are after

> expand.grid(a,b)
   Var1       Var2
1   ABC 2012-05-01
2   DEF 2012-05-01
3   GHI 2012-05-01
4   ABC 2012-05-02
5   DEF 2012-05-02
6   GHI 2012-05-02
7   ABC 2012-05-03
8   DEF 2012-05-03
9   GHI 2012-05-03
10  ABC 2012-05-04
11  DEF 2012-05-04
12  GHI 2012-05-04
13  ABC 2012-05-05
14  DEF 2012-05-05
15  GHI 2012-05-05

If the resulting order isn't what you want, you can sort afterwards. If you name the arguments to expand.grid, they will become column names:

df = expand.grid(a = a, b = b)
df[order(df$a), ]

And expand.grid generalizes to any number of input columns.