Porting set operations from R's data frames to data tables: How to identify duplicated rows?

Question

[Update 1: As Matthew Dowle noted, I\'m using data.table version 1.6.7 on R-Forge, not CRAN. You won\'t see the same behavior with an earlier version of

Answer 1

duplicated.data.table needs the same fix unique.data.table got [EDIT: Now done in v1.7.2]. Please raise another bug report: bug.report(package="data.table"). For the benefit of others watching, you're already using v1.6.7 from R-Forge, not 1.6.6 on CRAN.

But, on Note 1, there's a 'not join' idiom :

x[-x[y,which=TRUE]]

See also FR#1384 (New 'not' and 'whichna' arguments?) to make that easier for users, and that links to the keys that don't match thread which goes into more detail.

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Update. Now in v1.8.3, not-join has been implemented.

DT[-DT["a",which=TRUE,nomatch=0],...]   # old idiom
DT[!"a",...]                            # same result, now preferred.