Closed
Description
iis <- data.table(iris)
iis[, letts := .(list(sample(letters[1:4], size = 2))), 1:nrow(iris)]
iis[, .N, .(Species, letts)]
Error in `[.data.table`(iis, , .N, .(Species, letts)) :
column or expression 2 of 'by' or 'keyby' is type list. Do not quote column names. Usage: DT[,sum(colC),by=list(colA,month(colB))]
"Do not quote column names" seems out of place.