Subset data.table by logical column

问题:

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]

回答1:

From ?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.

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

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

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

回答2:

x is not defined in the global environment. If you try this,

> with(dt, dt[x])
      x y
1: TRUE 1
2: TRUE 2
3: TRUE 4

It would work. Or this:

> attach(dt)
> dt[!x]
       x y
1: FALSE 3

EDIT:

according to the documentation the j parameter takes column name, in fact:

> dt[x]
Error in eval(expr, envir, enclos) : object 'x' not found
> dt[j = x]
[1]  TRUE  TRUE FALSE  TRUE

then, the i parameter takes either numerical or logical expression (like x itself should be), however it seems it (data.table) can't see x as logical without this:

> dt[i = x]
Error in eval(expr, envir, enclos) : object 'x' not found
> dt[i = as.logical(x)]
      x y
1: TRUE 1
2: TRUE 2
3: TRUE 4

回答3:

This should also work and is arguably more natural:

setkey(dt, x)
dt[J(TRUE)]
dt[J(FALSE)]