If auto.index is TRUE, the RHS of := is evaluated even if there are zero rows in the subset.
packageVersion('data.table')
#[1] ‘1.9.5’
options(datatable.auto.index=TRUE)
DT <- data.table(a=1:2)
DT[a==3, b:=d+1]
# Error in eval(expr, envir, enclos) : object 'd' not foundSince DT[a==3] above returns a zero row data.table, there is nothing to assign in the j expression, so it should not be evaluated. This did not throw an exception in 1.9.3 and does not throw an exception if auto.index is turned off.
options(datatable.auto.index=FALSE)
DT[a==3, b:=d+1]
If auto.index is TRUE, the RHS of
:=is evaluated even if there are zero rows in the subset.Since
DT[a==3]above returns a zero rowdata.table, there is nothing to assign in thejexpression, so it should not be evaluated. This did not throw an exception in 1.9.3 and does not throw an exception if auto.index is turned off.