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Is it possible to select a column of a data.table and get back a vector? In base R, the argument drop=TRUE would do the trick. For example,

library(data.table)
dat <- as.data.table(iris)
dat[,"Species"] # returns data.table
dat[,"Species", drop=TRUE] # same
iris[, "Species", drop=TRUE] # a factor, wanted result

Is there a way to do this with data.table?

EDIT: the dat[,Species] method is fine, however I need a method where I can pass the column name in a variable:

x <- "Species"
dat[,x, drop=TRUE]
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  • 2
    In newer data.table versions you can just say dat[, Species]. You get a factor here because you started with a factor from the iris data set. (I didn't downvote.) Commented Nov 29, 2019 at 20:24
  • 1
    Try dat[,get(x), drop=TRUE] (not my downvote either) Commented Nov 29, 2019 at 20:36
  • 1
    @Karsten It helps if you start with the real question. Try colnam <- "Species" followed by dat[, ..colnam][[1]]. All this is, methinks, in the data.table FAQ ... Commented Nov 29, 2019 at 20:41

3 Answers 3

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With data.frame, the default is drop = TRUE and in data.table, it is the opposite while it is done internally. According to ?data.table

drop - Never used by data.table. Do not use. It needs to be here because data.table inherits from data.frame.

In order to get the same behavior, we can use [[ to extract the column by passing a string

identical(dat[["Species"]], iris[, "Species"])
#[1] TRUE

Or 

dat$Species

By using [[ or $, it extracts as a vector while also bypass the data.table overhead

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Comments

1

In data.table, if you use column name to select columns, it depends how you write the name.

library(data.table)
dat <- as.data.table(iris)
dat[,"Species"] # returns a data.table
dat[["Species"]] # returns a vector, like selection of list elements.
dat[, Species] # returns a vector, use it like a variable.

To note, if you need a vector with some filters:

dat[petal.Length > 1, Species] # dat[["Species"]] can not use condition in i expression.
dat[petal.Length > 1, `#Species`] # for special name, use `` to quote such as `name@#$%^`
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0

See data.table FAQ #1.1. This comes as a feature since 2006.

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