Subsetting columns works on data.frame but not on data.table

问题

I can select a few columns from a data.frame:

> z[c("events","users")]
     events  users
1  26246016 201816
2    942767 158793
3  29211295 137205
4  30797086 124314

but not from a data.table:

> best[c("events","users")]
Starting binary search ...Error in `[.data.table`(best, c("events", "users")) : 
  typeof x.pixel_id (integer) != typeof i.V1 (character)
Calls: [ -> [.data.table

What do I do? Is there a better way than to turn the data.table back into a data.frame?

回答1:

Column subsetting should be done in j, not in i. Do instead:

DT[, c("x", "y"), with=FALSE]

The with=FALSE makes sure that j is not evaluated as an expression. This is basically the idiomatic way for column subset if you're looking for similar means as one would subset on a data.frame.

Check this presentation (slide 4) to get an idea of how to read a data.table syntax (more like SQL). That'll help convince you that it makes more sense for providing columns in j - equivalent of SELECT in SQL.

回答2:

Given that you're looking for a data.table back you should use list rather than c in the j part of the call.

z[, list(events,users)]    # first comma is important

Note that you don't need the quotes around the column names.

来源：https://stackoverflow.com/questions/21170030/subsetting-columns-works-on-data-frame-but-not-on-data-table