按逻辑列子集 data.table [英] Subset data.table by logical column

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?请参阅示例.

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]

推荐答案

From ?data.table

高级:当 i 是单个变量名时，它不被视为列名的表达式，而是在调用范围内进行评估.

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] 将尝试在调用范围(在本例中为全局环境)评估 x

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

您可以通过使用 ( 或 { 或 force

You can get around this by using ( or { or force

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

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