I want to extract all pairs within groups of a data.table by two columns.
I have the following code:
dt = data.table(g1 = c(1, 1, 1, 2, 2, 2, 2, 2),
g2 = c(3, 3, 4, 5, 5, 6, 6, 6),
values = letters[1:8])
dt[, .(ind = combn(values, 2, paste0, collapse = "")), by = .(g1, g2)]
But this returns the error
Error in combn(values, 2, paste0, collapse = "") : n < m
The ideal result is some object that contains groups such as:
a,b
c
d,e
f,g
f,h
g,h
If c was not included because there is only one value, that is not a huge concern to me.
Is this possible to get using data.table?
Thanks in advance.
Note: This answer does what I want for one level. Generate All ID Pairs, by group with data.table in R
You can try the code below
> setnames(dt[, combn(values, min(2, .N), toString), by = .(g1, g2)], "V1", "ind")[]
g1 g2 ind
<num> <num> <char>
1: 1 3 a, b
2: 1 4 c
3: 2 5 d, e
4: 2 6 f, g
5: 2 6 f, h
6: 2 6 g, h
or
> dt[, .(ind = combn(values, min(2, .N), toString, FALSE)), by = .(g1, g2)][, ind := unlist(ind)][]
g1 g2 ind
<num> <num> <char>
1: 1 3 a, b
2: 1 4 c
3: 2 5 d, e
4: 2 6 f, g
5: 2 6 f, h
6: 2 6 g, h
where min(2, .N) is to rectify the number of selected elements in case that it is less than 2.