CODEWITHSUNDEEP
rdata.table
David Leal
David Leal

Reputation: 6759

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

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.

Upvotes: 3

Views: 3165

Answers (1)

akrun
akrun

Reputation: 887118

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

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

Upvotes: 4

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