How to perform cartesian join for two data.table objects in R?
The cartesian join is the joining of two objects that creates the combination of each value in object with all the values in the other object. For example, if we have a vector x that contains 1, 2, 3 and the other object y contains a, b, c then the cartesian join will be 1a, 2a, 3a, 1b, 2b, 3b, 1c, 2c, and 3c. Check out the below examples to understand how it can be done.
Example
> library(data.table)
> DT1<-data.table(x=1:4)
> DT1
Output
x
1: 1
2: 2
3: 3
4: 4
Example
> DT2<-data.table(y=25:30)
> DT2
Output
y
1: 25
2: 26
3: 27
4: 28
5: 29
6: 30
Performing cartesian join of DT1 and DT2:
Example
> DT1[,as.list(DT2),by=DT1]
Output
x y
1: 1 25
2: 1 26
3: 1 27
4: 1 28
5: 1 29
6: 1 30
7: 2 25
8: 2 26
9: 2 27
10: 2 28
11: 2 29
12: 2 30
13: 3 25
14: 3 26
15: 3 27
16: 3 28
17: 3 29
18: 3 30
19: 4 25
20: 4 26
21: 4 27
22: 4 28
23: 4 29
24: 4 30
x y
Let’s have a look at another example:
Example
> DT_G<-data.table(Group=LETTERS[1:5])
> DT_G
Output
Group
1: A
2: B
3: C
4: D
5: E
Example
> DT_Count<-data.table(sample(21:50,5))
> DT_Count
Output
V1
1: 22
2: 28
3: 45
4: 31
5: 47
Performing cartesian join of DT_G and DT_Count:
Example
> DT_G[,as.list(DT_Count),by=DT_G]
Output
Group V1
1: A 22
2: A 28
3: A 45
4: A 31
5: A 47
6: B 22
7: B 28
8: B 45
9: B 31
10: B 47
11: C 22
12: C 28
13: C 45
14: C 31
15: C 47
16: D 22
17: D 28
18: D 45
19: D 31
20: D 47
21: E 22
22: E 28
23: E 45
24: E 31
25: E 47
Group V1
Published on 19-Nov-2020 07:27:04
