按条件选择组

# example data
DT = data.table(Titanic)

假设我们只想在多数人幸存的情况下才能看到每个类：

DT[, if (sum(N[Survived=="Yes"]) > sum(N[Survived=="No"]) ) .SD, by=Class]

#    Class    Sex   Age Survived   N
# 1:   1st   Male Child       No   0
# 2:   1st Female Child       No   0
# 3:   1st   Male Adult       No 118
# 4:   1st Female Adult       No   4
# 5:   1st   Male Child      Yes   5
# 6:   1st Female Child      Yes   1
# 7:   1st   Male Adult      Yes  57
# 8:   1st Female Adult      Yes 140

在这里，只有满足条件，我们才会返回数据子集 .SD。另一种选择是

DT[, .SD[ sum(N[Survived=="Yes"]) > sum(N[Survived=="No"]) ) ], by=Class]

但这有时被证明是较慢的。