子集数据表由逻辑列

问题描述：

我有一个带有逻辑列的 data.table 。为什么逻辑列的名称不能直接用于 i 参数？参见示例。

I have a data.table with a logical column. Why the name of the logical column can not be used directly for the i argument? See the example.

dt <- data.table(x = c(T, T, F, T), y = 1:4)

# Works
dt[dt$x]
dt[!dt$x]

# Works
dt[x == T]
dt[x == F]

# Does not work
dt[x]
dt[!x]

答

从？data.table

高级：当i是单个变量名时，不会被视为列名的
表达式，

Advanced: When i is a single variable name, it is not considered an expression of column names and is instead evaluated in calling scope.

因此 dt [x] 会尝试在调用范围（在这种情况下是全局环境）中 x 进行求值

So dt[x] will try to evaluate x in the calling scope (in this case the global environment)

（或 {或 force

dt[(x)]
dt[{x}]
dt[force(x)]