CODEWITHSUNDEEP
rdata.table
Wet Feet
Wet Feet

Reputation: 4555

Number of unique rows for each unique element in column A

I am trying to find the number of unique rows in a data.table, for each unique element in "A". Here's what I did:

DT <- data.table(A = rep(1:3, each=4), B = rep(1:4, each=3), C = rep(1:2, 6), key = "A")

unique(DT,by=names(DT)) #Gives me each unique row in DT
#    A B C
# 1: 1 1 1
# 2: 1 1 2
# 3: 1 2 2
# 4: 2 2 1
# 5: 2 2 2
# 6: 2 3 1
# 7: 2 3 2
# 8: 3 3 1
# 9: 3 4 2
#10: 3 4 1
nrow(unique(DT,by=names(DT))) #Gives me the number of unique rows in DT
# [1] 10

DT[,nrow(unique(DT,by=names(DT))),by=A] #Doesn't give me the number of unique rows for each unique DT$A.
#   A V1
# 1: 1 10
# 2: 2 10
# 3: 3 10

Can anyone see what am I doing wrong here?

Upvotes: 1

Views: 417

Answers (2)

Troy
Troy

Reputation: 8691

I think you want to use .SD (the sub table for each group)

DT[,nrow(unique(.SD)),by=A]

#   A V1
#1: 1  3
#2: 2  4
#3: 3  3

Upvotes: 3

JeremyS
JeremyS

Reputation: 3525

because nrow(unique(DT,by=names(DT)) is 10 you are basically saying DT[,10,by=A]

Upvotes: 2

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