create data.table with column name from string variable

Question

i want to create a data.table and I want to use dynamic column names, by the way this is not about adding columns to an existing data.table.

Here is an example

outer.name <- "outer.column"
outer.members <- c("I", "II")
outer.members.cnt <- length(outer.members)
dt.outerinner <- data.table(outer = outer.members)
setnames(dt.outerinner, outer.name)

Now I want to know, if it's possible to pass the variable "outer.name" directly to data.table(... = outer.members) and omit setnames(...)?

Any hint is appreciated

Tom

Can you explain why you are not happy with what you have? It's efficient and very readable code. — Roland
– Roland, Commented Sep 9, 2014 at 8:28
I doubt if it possible, although an interesting question. You could do the whole thing in one line if it helps, something like setnames(dt.outerinner <- data.table(outer.name = outer.members), outer.name) — David Arenburg
– David Arenburg, Commented Sep 9, 2014 at 9:01
I love chain!! library(magrittr); data.table(outer.members) %>% setnames(outer.name) -> dt.outerinner — kohske
– kohske, Commented Sep 9, 2014 at 9:25
@DavidArenburg Thanks for your input. My question stems from the fact, that I'm very flexible using .SD and .SDcols when a data.table already exists. So I was just wandering if it's possible to use a variable as a column name (maybe a matter of complete control without the additional setnames function). I'm very happy with the solution, just wanted to make sure that there is no thing missing. — Tom Martens
– Tom Martens, Commented Sep 10, 2014 at 8:59

Francesco Grossetti · Accepted Answer · 2020-03-31 17:23:45Z

2

Could this be what you want?

outer.name <- "outer.column"
outer.members <- c("I", "II")
dtfinal = data.table()
dtfinal[ , (outer.name) := outer.members]

First create a placeholder for you data.table, then update it by reference using the usual := operator. Use parenthesis to pass in a column name programmatically.

answered Mar 31, 2020 at 17:23

Francesco Grossetti

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