Nils Gudat Nils Gudat - 2 months ago 13
R Question

Access data.table columns with strings

Apologies for a question that probably makes it obvious that I usually work in Python/pandas, but I'm stuck with this. How do I select a

data.table
column using a string?

dt$"string"
dt$as.name("string")
dt$get("string")


I'm sure this is super simple, but I'm not getting it. Any help is greatly appreciated!




-------------- EDITED TO ADD ----------------------



After some of the helpful comments and tips below, I think I've narrowed down the problem a bit and have a reproducible example. Consider:

dt = data.table(ID = c("a","a","a","b","b","b"), col1=rnorm(6), col2=rnorm(6)*100)


And assume we want to assign the values in
col2
to
col1
. As I've learned below, the
data.table
syntax for this would be
dt[,col1:=col2]
, clean and simple. The problems start when one (or both) of the variables in the
j
argument are strings. I found the following:

dt[, "col1":=col2]
works as expected

dt[, "col1":="col2"]
fails as expected (tries to assign the character
col2
to the double vector
col1


dt[, "col1":=get("col2")]
works as expected

dt[, get("col1")]
returns
col1
as expected

but:
dt[, get("col1"):=col2]
or any other assignment fails.

Some context: the reason for doing this is that I'm constructing strings in a loop, to access a larger number of columns that are all named
colname_colnumber
, i.e. I loop over
colname
and
colnumber
to then access column
paste0(colname,colnumber)
.

Answer

You can use get() as the j argument using single brackets:

library(data.table)
dt <- data.table(iris)
dt[, get("Species")]

The result:

[1] setosa     setosa     setosa     setosa     setosa     setosa .....

You can also use a string directly inside the double bracket operator, like this:

dt[["Species"]]