Subset data.table by logical column

Question

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] 

Answer 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)] 

Answer 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