Is it possible to use a variable containing column names as strings in expressions for creating new-CodePudding

Is there an easy way to use a variable containing a data.table column name (string) in the right hand size of an expression that creates a new data.table column?

library(data.table)
mtcars = as.data.table(mtcars)

# Say I have the name of a data.table column stored in a variable
colname = "mpg"


# How could i use this variable to create new columns
# My initial attempt:
mtcars[, `:=` (xMPGBiggerThan20 = colname > 20)]

# I think result is based on "mpg" > 20, rather than using the column values
head(mtcars)
#>     mpg cyl disp  hp drat    wt  qsec vs am gear carb xMPGBiggerThan20
#> 1: 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4             TRUE
#> 2: 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4             TRUE
#> 3: 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1             TRUE
#> 4: 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1             TRUE
#> 5: 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2             TRUE
#> 6: 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1             TRUE


# How would I make the command equivalent to if I wrote
mtcars[, `:=` (xMPGBiggerThan20 = mpg > 20)]

# Desired result
head(mtcars)
#>     mpg cyl disp  hp drat    wt  qsec vs am gear carb xMPGBiggerThan20
#> 1: 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4             TRUE
#> 2: 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4             TRUE
#> 3: 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1             TRUE
#> 4: 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1             TRUE
#> 5: 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2            FALSE
#> 6: 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1            FALSE

^{Created on 2022-02-14 by the reprex package (v2.0.1)}

CodePudding user response：

You can try (data.table 1.14.3)

mtcars[, xMPGBiggerThan20 := colname > 20, env = list(colname = "mpg")][]

mtcars[, `:=` (xMPGBiggerThan20 = colname > 20), env = list(colname = "mpg")][]

If you don't want to update to the development version, you can consider to use

mtcars[, `:=` (xMPGBiggerThan20 = get(colname) > 20)]

PS: You can update to the newest version with data.table::update.dev.pkg()

Ref: https://rdatatable.gitlab.io/data.table/articles/datatable-programming.html

CodePudding user response：

Here is some code that should work

mtcars[["xMPGBiggerThan20"]] <- mtcars[[colname]] > 20

The following also produces an identical result

mtcars$xMPGBiggerThan20 <- mtcars[[colname]] > 20