How to do cross join in R?

Problem

We want to calculate a cross join, the Cartesian product of rows from tables —or elements of a vector— in the join. In other words, we want to combine each row from the first table with each row from the second table.

Solution

Two vectors

• expand.grid

expand.grid(1:5, 1:5)

• merge

merge(1:5, 1:5)

• Cross join with sqldf

library(sqldf)
df1 <- data.frame(a = 1:5)
df2 <- df1
sqldf("SELECT df1.a, df2.a FROM df1 
      CROSS JOIN df2")

• Results

   x y
1  1 1
2  2 1
3  3 1
4  4 1
5  5 1
6  1 2
7  2 2
8  3 2
9  4 2
10 5 2
11 1 3
12 2 3
13 3 3
14 4 3
15 5 3
16 1 4
17 2 4
18 3 4
19 4 4
20 5 4
21 1 5
22 2 5
23 3 5
24 4 5
25 5 5

More than two vectors

• expand.grid.

a <- c('europe', 'asia')
b <- c('co2', 'temperature')
c <- c('min', 'max')
expand.grid(a, b, c)

    Var1        Var2 Var3
1 europe         co2  min
2   asia         co2  min
3 europe temperature  min
4   asia temperature  min
5 europe         co2  max
6   asia         co2  max
7 europe temperature  max
8   asia temperature  max

References

• Merge Two Data Frames
• Create a Data Frame from All Combinations of Factor Variables
• Generating Cartesian product of strings in R
• Getting all combinations in R, repetition allowed