Access data.table columns with strings

Question

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!

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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 1

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"]]