r - 按逻辑列子集 data.table

Question

我有一个 data.table带有逻辑列。为什么逻辑列的名称不能直接用于i争论？请参阅示例。

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]

Answer 1

来自 ?data.table

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

所以 dt[x]将尝试评估 x在调用范围内(在这种情况下是全局环境)

你可以通过使用 ( 来解决这个问题。或 {或 force

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