Dynamically add column names to data.table when aggregating

Question

I know we can dynamically add column names when creating columns by reference (using :=), as described e.g. here: Dynamic column names in data.table.

However, I'm looking to dynamically add column names when we aggregate. Can you help with this?

test_dtb <- data.table(a = sample(1:100, 100), b = sample(1:100, 100), id = rep(1:10, 10))
m = "blah"
test_dtb[ , list((m) = mean(b)), by = id]

The error I get is

Error: unexpected '=' in "test_dtb[ , list((m) =

I'm programmatically creating the m variable and its length can vary from 1 to 5. I think it would be hard to define which index to setnames from. — ArunK
– ArunK, Commented Dec 20, 2016 at 10:03
What about m = c("blah", "foo");test_dtb[,setNames(list(mean(b),median(b)), m),by = id]? — lukeA
– lukeA, Commented Dec 20, 2016 at 10:24
works with multiple values also: m = c("blah", "foo"); test_dtb[,eval(m):=list(mean(b),median(b)),by = id,verbose=TRUE] gives: a b id blah foo columns — Tensibai
– Tensibai, Commented Dec 20, 2016 at 17:29

Henrik · Accepted Answer · 2018-06-18 21:20:40Z

5

As mentioned in the comments by lukeA, setNames can be used:

m <- c("blah", "foo")
test_dtb[ , setNames(list(mean(b), median(b)), m), by = id]

edited Jun 18, 2018 at 21:20

Henrik

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answered Feb 7, 2017 at 15:43

ArunK

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Community · Accepted Answer · 2020-06-20 09:12:55Z

-2

Just make sure data.table knows "m" is not a variable name.

Parentheses or "c" will do.

test_dtb[, (m)  := mean(b),by = id]
test_dtb[, c(m) := mean(b),by = id]

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 21, 2016 at 0:03

rflores

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2 Comments

ArunK Over a year ago

As I mentioned in the question, its while I'm aggregating. The answers in the comments help with that.

rflores Over a year ago

Woops! My mistake. In this case, the solution suggested by lukeA is an excellent one.

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Dynamically add column names to data.table when aggregating

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Just make sure data.table knows "m" is not a variable name.

Parentheses or "c" will do.

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Just make sure data.table knows "m" is not a variable name.

Parentheses or "c" will do.

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