How many unique keys does my data.table have?

Question

Given a data.table, how do I find the number of unique keys it contains?

library(data.table)
z <- data.table(id=c(1,2,1,3),key=\"id\")
length(uni         


        

Answer 1

Maybe this:

sum(Negate(duplicated)(z$id))

z$id remains sorted, so duplicated can work faster on it:

bigVec <- sample(1:100000, 30000000, replace=TRUE)
system.time( sum(Negate(duplicated)(bigVec)) )
   user  system elapsed 
  8.161   0.475   8.690 

bigVec <- sort(bigVec)
system.time( sum(Negate(duplicated)(bigVec)) )
   user  system elapsed 
   0.00    2.09    2.10 

But I just checked and length(unique()) works faster on sorted vectors as well...

So maybe there is some kind of checking if the vector is sorted going on (which can be done in a linear time). To me this doesn't look to be quadratic:

system.time( length(unique(bigVec)) )
   user  system elapsed 
  0.000   0.583   0.664 

bigVec <- sort(sample(1:100000, 20000000, replace=TRUE))
system.time( length(unique(bigVec)) )
   user  system elapsed 
  0.000   1.290   1.242 

bigVec <- sort(sample(1:100000, 30000000, replace=TRUE))
system.time( length(unique(bigVec)) )
   user  system elapsed 
  0.000   1.655   1.715