Extract a column by reference from a data.table as a vector

Question

I would like to extract the column values from a data.table object specifying the column name using a variable. For example:

DT <- data.table(x = c(1, 2), y = c(3, 4), z = c(5, 6))
col <- "z"

then

> is.vector(DT[, col, with = F])
[1] FALSE

because it returns a data.table object instead.

I have tried also: is.vector(DT[, (col), with = F]) and also: is.vector(DT[, ..col]) with the same result. I have tried other possibilities that produce an error.

Using directly the variable name works:

> is.vector(DT[, z])
[1] TRUE

I found this post, that solves it using column position but not using a name by reference:

> is.vector(DT[[3]])
[1] TRUE

I didn't find an explicit reference about this particular case in the data.table documentation. I am sure it is an easy way to do it, but I didn't find how.

@Roland this solution is by index position, now I realized that for name reference it also works but it was not mentioned explicitly. — David Leal
– David Leal, Commented May 20, 2017 at 18:42
Now I found this solution was addressed by @Frank answer at this post, the question was not specific for this particular problem, but his solution solves my problem too. — David Leal
– David Leal, Commented May 20, 2017 at 18:46
That answer explains how the index solution works. Extraction by column name works exactly the same. — Roland
– Roland, Commented May 20, 2017 at 18:48
The first several questions in the FAQ go over the options here (and the FAQ is considered part of the documentation). Type vignette("datatable-faq"). (Btw, the FAQ on the website is out of date, I think.) — Frank
– Frank, Commented May 20, 2017 at 21:48

akrun · Accepted Answer · 2017-05-20 18:14:09Z

4

We can use the [[ to extract the column as vector

is.vector(DT[[col]])
#[1] TRUE

answered May 20, 2017 at 18:14

akrun

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David Leal Over a year ago

Now I found this solution was addressed by @Frank answer at this post

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