I get similar timings to arun, with the data.table call being a lot slower than the other timings. If data.table is not optimized for that .SD expression, perhaps that is okay because, as Arun pointed out, there are alternatives.. I can't guess why it would perform differently on different hardware, though...
# alternatives: a <- dt1[dt1[, .I[.N], by='Date']$V1] b <- dt1[J(unique(Date)),,mult='last'] # a little slower d <- dt1[, .SD[.N], by='Date'] # 600x slower; it would take ages to benchmark identical(a,b) # true identical(a,d) # false identical(as.data.frame(d),as.data.frame(a)) # true --Frank
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