CronAcronis CronAcronis - 2 months ago 26
R Question

Non standard evaluation of by in data.table

I am lost with evaluation of

by
in
data.table
. What will be correct way to merge functionality of
LJ
and
LJ2
into one function?


LJ <- function(dt_x_, dt_y_, by_)
{
merge(
dt_x_,
dt_y_,
by = eval(substitute(by_)), all.x = TRUE, sort = FALSE)
}
LJ2 <- function(dt_x_, dt_y_, by_)
{
merge(
dt_x_,
dt_y_,
by = deparse(substitute(by_)), all.x = TRUE, sort = FALSE)
}
LJ(
data.table(A = c(1,2,3)),
data.table(A = c(1,2,3), B = c(11,12,13)),
"A")
LJ2(
data.table(A = c(1,2,3)),
data.table(A = c(1,2,3), B = c(11,12,13)),
A)

Answer

I consider this a bad idea. Have the user always pass a character value. You could do this:

LJ3 <- function(dt_x_, dt_y_, by_)
{ 
  by_ <- gsub('\"', "", deparse(substitute(by_)), fixed = TRUE)
  dt_y_[dt_x_, on = by_] 
}

LJ3(
  data.table(A = c(4,1,2,3)),
  data.table(A = c(1,2,3), B = c(11,12,13)), 
  A)
#   A  B
#1: 4 NA
#2: 1 11
#3: 2 12
#4: 3 13

LJ3(
  data.table(A = c(4,1,2,3)),
  data.table(A = c(1,2,3), B = c(11,12,13)), 
  "A")
#   A  B
#1: 4 NA
#2: 1 11
#3: 2 12
#4: 3 13

This question is not related to data.table. The by parameter in merge.data.table always expects a character value, as does on.

Edit: @eddi points out that the above will fail if you have column names with actual " in them (something you should avoid in general, but may happen if you fread some input files prepared by others).

An alternative that can handle such edge cases would be:

LJ4 <- function(dt_x_, dt_y_, by_)
{ 
  by_ <- substitute(by_)
  if (!is.character(by_)) by_ <- deparse(by_)
  dt_y_[dt_x_, on = by_] 
}
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