Variable as name in aggregate list of data.table

Question

I'm aggregating an R/data.table (v1.12.2) and I need to use a variable as the name of the aggregated column. E.g.:

library(data.table)

DT <- data.table(x= 1:5, y= c('A', 'A', 'B', 'B', 'B'))

aggname <- 'max_x'  ## 'max_x' should be the name of the aggregated column

DT2 <- DT[, list(aggname= max(x)), by= y]
DT2
   y aggname  <- This should be 'max_x' not 'aggname'!
1: A       2
2: B       5

I can rename the column(s) afterwards with something like:

setnames(DT2, 'aggname', aggname)
DT2
   y max_x
1: A     2
2: B     5

But I would have to check that the string 'aggname' doesn't create duplicate names first. Is there any better way of doing it?

akrun · Accepted Answer · 2020-05-09 14:49:05Z

1

We can use setNames on the list column

DT[, setNames(.(max(x)), aggname), by = y]
#    y max_x
#1: A     2
#2: B     5

aggname2 <- 'min_x'
DT[, setNames(.(max(x), min(x)), c(aggname, aggname2)), by = y]
#   y max_x min_x
#1: A     2     1
#2: B     5     3

Or another option is lst from dplyr

library(dplyr)
DT[, lst(!! aggname := max(x)), by = y]
#    y max_x
#1: A     2
#2: B     5


DT[, lst(!! aggname := max(x), !! aggname2 := min(x)), by = y]
#   y max_x min_x
#1: A     2     1
#2: B     5     3

edited May 9, 2020 at 14:49

answered May 9, 2020 at 14:37

akrun

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3 Comments

dariober Over a year ago

Thanks - How can I use setNames with more than one column? LIke DT[, list(aggname= max(x), aggname2= min(x)), by = y]

dariober Over a year ago

Nevermind - I think I got it: DT[, setNames(c(.(max(x)), .(min(x))), c(aggname, aggname2)), by = y] - Thanks again I didn't know about setNames I'm still figuring out the menaing of that leading dot.

akrun Over a year ago

@dariober i did update the post. The .( is just a concise way for list(

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