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I want to merge two data.tables that don’t have a common column, so I would end up with N1*N2 rows, where N1 and N2 are the number of rows in each dataframe.
Doing this with base R works:
A <- data.frame(id = 1:6, value = 19:24)
B <- data.frame(value2 = c(25, 25, 26, 26), value3 = 4:5)
A
#> id value
#> 1 1 19
#> 2 2 20
#> 3 3 21
#> 4 4 22
#> 5 5 23
#> 6 6 24
B
#> value2 value3
#> 1 25 4
#> 2 25 5
#> 3 26 4
#> 4 26 5
merge(A, B, all = TRUE)
#> id value value2 value3
#> 1 1 19 25 4
#> 2 2 20 25 4
#> 3 3 21 25 4
#> 4 4 22 25 4
#> 5 5 23 25 4
#> 6 6 24 25 4
#> 7 1 19 25 5
#> 8 2 20 25 5
#> 9 3 21 25 5
#> 10 4 22 25 5
#> 11 5 23 25 5
#> 12 6 24 25 5
#> 13 1 19 26 4
#> 14 2 20 26 4
#> 15 3 21 26 4
#> 16 4 22 26 4
#> 17 5 23 26 4
#> 18 6 24 26 4
#> 19 1 19 26 5
#> 20 2 20 26 5
#> 21 3 21 26 5
#> 22 4 22 26 5
#> 23 5 23 26 5
#> 24 6 24 26 5
But if I now have two data.tables and not dataframes anymore, it errors:
library(data.table)
A <- data.table(id = 1:6, value = 19:24)
B <- data.table(value2 = c(25, 25, 26, 26), value3 = 4:5)
merge(A, B, all = TRUE)
#> Error in merge.data.table(A, B, all = TRUE): A non-empty vector of column names for `by` is required.
How can I reproduce the base R behavior with data.table (without necessarily using merge())?
>Solution :
You are looking for a cross-join. In data.table, there is a CJ function but it only works with one data set, otherwise you can do:
res <- setkey(A[, c(k=1, .SD)], k)[B[, c(k=1, .SD)], allow.cartesian = TRUE][, k := NULL]
res
id value value2 value3
1: 1 19 25 4
2: 2 20 25 4
3: 3 21 25 4
4: 4 22 25 4
5: 5 23 25 4
6: 6 24 25 4
7: 1 19 25 5
8: 2 20 25 5
9: 3 21 25 5
10: 4 22 25 5
11: 5 23 25 5
12: 6 24 25 5
13: 1 19 26 4
14: 2 20 26 4
15: 3 21 26 4
16: 4 22 26 4
17: 5 23 26 4
18: 6 24 26 4
19: 1 19 26 5
20: 2 20 26 5
21: 3 21 26 5
22: 4 22 26 5
23: 5 23 26 5
24: 6 24 26 5
id value value2 value3
Note the alternative dplyr solution:
dplyr::cross_join(A, B)