Count variable on a Variable R [duplicate]

Answer 1

With base R

as.data.frame(table(unique(Data)$Tid))
#   Var1 Freq
# 1    1    1
# 2    2    2
# 3    3    1
# 4    4    2

Or (though the column name is less informative)

aggregate(Uid ~ Tid, unique(Data), length)
#   Tid Uid
# 1   1   1
# 2   2   2
# 3   3   1
# 4   4   2

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The basic idea here is to only operate on the unique combinations of Tid/Uid and then count the different Tid instances

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Edit: per @nicolas comment, we can add tapply too here as a possible solution

as.data.frame.table(tapply(Data$Uid, Data$Tid, function(x) length(unique(x))))
#   Var1 Freq
# 1    1    1
# 2    2    2
# 3    3    1
# 4    4    2