The following code should sort a column, hp, in a descending order, but it fails.
Could someone please point out the problem?
data(mtcars)
result = dplyr::arrange( mtcars, !! rlang::expr("desc(hp)") )
head(result)
mpg cyl disp hp drat wt qsec vs am gear carb
Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
Using str2lang() instead of rlang::expr() works fine. Could someone explain the reason?
data(mtcars)
result = dplyr::arrange( mtcars, !! str2lang("desc(hp)") )
mpg cyl disp hp drat wt qsec vs am gear carb
Maserati Bora 15.0 8 301 335 3.54 3.570 14.60 0 1 5 8
Ford Pantera L 15.8 8 351 264 4.22 3.170 14.50 0 1 5 4
Duster 360 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4
Camaro Z28 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4
Chrysler Imperial 14.7 8 440 230 3.23 5.345 17.42 0 0 3 4
Lincoln Continental 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4
To explain the problem, we need to look at what rlang::expr does.
It captures an expression without evaluating it. So when you write rlang::expr("desc(hp)")
the result is actually a string:
# setup:
mtcars <- dplyr::as_tibble(mtcars)
eval(rlang::expr("desc(hp)"))
#> [1] "desc(hp)"
and by this no different than:
dplyr::arrange(mtcars, "desc(hp)")
#> # A tibble: 32 x 11
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 21 6 160 110 3.9 2.62 16.5 0 1 4 4
#> 2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
#> 3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
#> 4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
#> 5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
#> 6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
#> 7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2
#> 9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2
#> 10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4
#> # ... with 22 more rows
to actually make it work with rlang::expr we need to use an actual expression and not a string as argument:
dplyr::arrange(mtcars, !! rlang::expr(desc(hp)))
#> # A tibble: 32 x 11
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 15 8 301 335 3.54 3.57 14.6 0 1 5 8
#> 2 15.8 8 351 264 4.22 3.17 14.5 0 1 5 4
#> 3 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 4 13.3 8 350 245 3.73 3.84 15.4 0 0 3 4
#> 5 14.7 8 440 230 3.23 5.34 17.4 0 0 3 4
#> 6 10.4 8 460 215 3 5.42 17.8 0 0 3 4
#> 7 10.4 8 472 205 2.93 5.25 18.0 0 0 3 4
#> 8 16.4 8 276. 180 3.07 4.07 17.4 0 0 3 3
#> 9 17.3 8 276. 180 3.07 3.73 17.6 0 0 3 3
#> 10 15.2 8 276. 180 3.07 3.78 18 0 0 3 3
#> # ... with 22 more rows
Since you want to use a string we could use rlang::parse_expr
as you already posted in your own answer:
dplyr::arrange( mtcars, !! rlang::parse_expr("desc(hp)"))
#> # A tibble: 32 x 11
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 15 8 301 335 3.54 3.57 14.6 0 1 5 8
#> 2 15.8 8 351 264 4.22 3.17 14.5 0 1 5 4
#> 3 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 4 13.3 8 350 245 3.73 3.84 15.4 0 0 3 4
#> 5 14.7 8 440 230 3.23 5.34 17.4 0 0 3 4
#> 6 10.4 8 460 215 3 5.42 17.8 0 0 3 4
#> 7 10.4 8 472 205 2.93 5.25 18.0 0 0 3 4
#> 8 16.4 8 276. 180 3.07 4.07 17.4 0 0 3 3
#> 9 17.3 8 276. 180 3.07 3.73 17.6 0 0 3 3
#> 10 15.2 8 276. 180 3.07 3.78 18 0 0 3 3
#> # ... with 22 more rows
Created on 2021-11-03 by the reprex package (v2.0.1)