选择组

# example data
DT = data.table(Titanic)

假设我们只想看第二堂课：

DT[ Class == "2nd" ]

#    Class    Sex   Age Survived   N
# 1:   2nd   Male Child       No   0
# 2:   2nd Female Child       No   0
# 3:   2nd   Male Adult       No 154
# 4:   2nd Female Adult       No  13
# 5:   2nd   Male Child      Yes  11
# 6:   2nd Female Child      Yes  13
# 7:   2nd   Male Adult      Yes  14
# 8:   2nd Female Adult      Yes  80

在这里，我们简单地使用 i（where 子句）对数据进行子集化。