How to pairs the elements of two vectors in R

Question

I would like to pair elements of two vectors in R. The order is important.

For example, 

    X= c(1:3)
    Y= c(1:3)
I expect to have:

1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3 

I would like to use 'lapply'. My data is more complicated but the idea is the same. I just need to pair each element of the first vector with all element of the second vector.

Answer 1

If "order is important" means that the "monotonic increase" is important, then simply

expand.grid(X, Y)
#   Var1 Var2
# 1    1    1
# 2    2    1
# 3    3    1
# 4    1    2
# 5    2    2
# 6    3    2
# 7    1    3
# 8    2    3
# 9    3    3

However, if column-order (second column increments before first), then just reverse the order of arguments (inferring that the real problem has differing numbers) and reorder them post-function:

expand.grid(Y, X)[2:1]
#   Var2 Var1
# 1    1    1
# 2    1    2
# 3    1    3
# 4    2    1
# 5    2    2
# 6    2    3
# 7    3    1
# 8    3    2
# 9    3    3

The column names are inferred from the arguments, so expand.grid(X=X, Y=Y) will name them X and Y instead of Var# .

Answer 2

In tidyverse , we can use expand_grid

library(tidyr)
expand_grid(Y, X)

---

Or use crossing

crossing(X, Y)

Answer 3

Another base R option is outer (but not as straightforward as expand.grid )

> do.call(rbind, c(t(outer(X, Y, Vectorize(function(x, y) list(c(x, y)))))))
      [,1] [,2]
 [1,]    1    1
 [2,]    1    2
 [3,]    1    3
 [4,]    2    1
 [5,]    2    2
 [6,]    2    3
 [7,]    3    1
 [8,]    3    2
 [9,]    3    3

---

Or we can use lapply like below

> unname(do.call(rbind, lapply(X, cbind, Y)))
      [,1] [,2]
 [1,]    1    1
 [2,]    1    2
 [3,]    1    3
 [4,]    2    1
 [5,]    2    2
 [6,]    2    3
 [7,]    3    1
 [8,]    3    2
 [9,]    3    3

Answer 4

Here is a data table solution:

library(data.table)
CJ(X, Y, unique = TRUE)

Output:

   X Y
1: 1 1
2: 1 2
3: 1 3
4: 2 1
5: 2 2
6: 2 3
7: 3 1
8: 3 2
9: 3 3