find *all* duplicated records in data.table (not all-but-one)

Question

if I understand correctly, duplicated() function for data.table returns a logical vector which doesn\'t contain first occurrence of duplicated reco

Answer 1

As of data.table version 1.9.8, the solution by eddi needs to be modified to be:

dups = duplicated(myDT, by = key(myDT));
myDT[, fD := dups | c(tail(dups, -1), FALSE)]

since:

Changes in v1.9.8 (on CRAN 25 Nov 2016)

POTENTIALLY BREAKING CHANGES

By default all columns are now used by unique(), duplicated() and uniqueN() data.table methods, #1284 and #1841. To restore old behaviour: options(datatable.old.unique.by.key=TRUE). In 1 year this option to restore the old default will be deprecated with warning. In 2 years the option will be removed. Please explicitly pass by=key(DT) for clarity. Only code that relies on the default is affected. 266 CRAN and Bioconductor packages using data.table were checked before release. 9 needed to change and were notified. Any lines of code without test coverage will have been missed by these checks. Any packages not on CRAN or Bioconductor were not checked.