Given a data.table (vith version 1.9.5)
TEST <- data.table(1:20,rep(1:5,each=4, times=1))
If I run this:
TEST[unique(V2)]
I get this result:
V1 V2
1: 1 1
2: 2 1
3: 3 1
4: 4 1
5: 5 2
Is it really the intended beahaviour or a bug? Or I'm just not using it properly?
I was reading the "R book" and in an example they use TEST[unique(Vegetation),] and say it's intended to select a subset of rows unique for the vegetation.
I expected to get something like
V1 V2
1: 1 1
2: 5 2
3: 9 3
4: 13 4
5: 16 5
Though I understand that would need to specify an aggregation criteria.
TEST[,unique(V2)] gives [1] 1 2 3 4 5. Since TEST[1:5] is supposed to give you the first 5 rows and that's what you get, there is no bug.
To get your expected result, you can do this:
TEST[!duplicated(V2)]
# V1 V2
#1: 1 1
#2: 5 2
#3: 9 3
#4: 13 4
#5: 17 5
or this:
TEST[, V1[1], by = V2]
# V2 V1
#1: 1 1
#2: 2 5
#3: 3 9
#4: 4 13
#5: 5 17
or as @Arun reminds me there is now a data.table method for unique:
unique(TEST, by="V2")
# V1 V2
#1: 1 1
#2: 5 2
#3: 9 3
#4: 13 4
#5: 17 5