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)
``````

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_]
}
``````
Source (Stackoverflow)