r - naming a list when returning values from data.table -


when returning items data.table, nice if automatically took on names of variables. how 1 this? mean:

require(data.table) x = data.table(a=1:10, id=1:2) x[,{s = sum(a); p=prod(a); y = sqrt(abs(s*p)); z = y+1; list(y, z)},by=id]  #   id v1   v2 #1:  1 25  945 #2:  2 30 3840

instead of v1 , v2 nice if columsn labeled s , p. it's no big thing here if have 20 columns becomes real pain. ideas on how this?

edit: changed question make clear why don't list(name = value)

forgive me if i'm missing something... isn't standard list syntax data.table you're looking for? more concise , clearer imho.

x[,    list(s = sum(a),        p = prod(a)),   by=id]   #    id  s    p # 1:  1 25  945 # 2:  2 30 3840

you can build list expression , eval it.

foo <- expression(list(s=sum(a), p=prod(a)))  x[, eval(foo), by=id]

this can extended function (using as.quoted plyr instead cause handy):

expression_maker <- function(funs, cols, names) {    require(plyr)    list_contents <- paste0(names, '=', funs, '(', cols, ')', collapse=',')    as.quoted(paste('list(', list_contents, ')'))[[1]] }  output <- expression_maker(funs=c('sum', 'prod'), cols=c('a', 'a'), names=c('s', 'p')) x[, eval(output), by=id]

... there dragons!

per op's edit:

x[,{s = sum(a); p=prod(a); y = sqrt(abs(s*p)); z = y+1; list(y, z)},by=id]

i in function , return data.table directly:

yourfun <- function(a) {   s <- sum(a)   p <- prod(a)   y <- sqrt(abs(s*p))   z <- y+1   data.table(y, z) }  x[, yourfun(a), by=id]

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