aggregate in j column to string column name, data.table

Question

I'm trying to create a summarized data.table using the j column, but assign to a name stored in a variable.

For example, I can do this:

x = data.table(c(1,2,3,4,5,6),c(2,2,2,3,3,3))

x[,.("a" = mean(V1), "b" = max(V1)),by=V2]

which returns as wanted

   V2 a b
1:  2 2 3
2:  3 5 6

Now instead of using the name "a", I would like to use a variable name:

varname = "a"

x[,.(varname = mean(V1), "b" = max(V1)), by=V2]

I'd like it to return the same output, but of course here column a is labeled as "varname". I've tried using eval, get, and others and haven't figured out the right syntax. Is this built in, or will I have to relabel the name outside of data.table?

akrun · Accepted Answer · 2018-05-18 18:06:40Z

2

We can use setnames after the aggregation

out <- x[,.( mean(V1), "b" = max(V1)), by=V2]
setnames(out, 'V1', varname)
out
#   V2 a b
#1:  2 2 3
#2:  3 5 6

Or use setNames

x[, setNames(.(mean(V1), max(V1)), c(varname, "b")), by = V2]

With tidyverse, the lhs evaluation is possible

library(tidyverse)
x %>%
   group_by(V2) %>% 
   summarise(!! varname := mean(V1))
# A tibble: 2 x 2
#     V2     a
#   <dbl> <dbl>
#1     2     2
#2     3     5

edited May 18, 2018 at 18:06

answered May 18, 2018 at 17:51

akrun

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2 Comments

Allen Wang Over a year ago

Is there a reason the eval or get syntax doesn't work on the left hand side?

akrun Over a year ago

@AllenWang It is difficult to get that to work, but in tidyverse you can do x %>% group_by(V2) %>% summarise(!! varname := mean(V1))

chinsoon12 · Accepted Answer · 2018-05-18 23:12:49Z

1

Alternatively, you can put the functions into a named list as follows:

x[, lapply(structure(list(mean, max), names=c(varname, "b")), 
        function(f) f(V1)), 
    by=V2]

answered May 18, 2018 at 23:12

chinsoon12

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