Why does !! (bang-bang) combined with as.name() give a different output compared to !! or as.name() alone?
You can wrap your expressions in the function quo() to see the result of the operation after applying the !! operator. For simplicity I will use a shorter expression for demonstration:
Preparations:
library(tidyverse)
ID <- "birth_year"
## Test without quasiquotation:
starwars %>%
filter(birth_year < 50)
Experiment 1:
quo(
starwars %>%
filter(ID < 50)
)
## result: starwars %>% filter(ID < 50)
We learn: filter() does not treat ID as variable, but "as is". So we need a mechanism to tell filter() that it should treat ID as variable, and it should use its value.
--> The !! operator can be used to tell filter() it should treat an expression as variable and substitute its value.
Experiment 2:
quo(
starwars %>%
filter(!!ID < 50)
)
## result: starwars %>% filter("birth_year" < 50)
We learn: The !! operator has indeed worked: ID was replaced with its value. But: The value of ID is the string "birth_year". Note the quotes in the result. But as you probably know, tidyverse functions don't take variable names as strings, they want the raw names, without quotes. Compare with Experiment 1: filter() takes everything "as is", so it looks for a column named "birth_year" (including the quotes!)
What does the function as.name() do?
This is a base R fuction that takes a string (or a variable containing a string) and returns the content of the string as variable name.
So if you call as.name(ID) in base R, the result is birth_year, this time without quotes - just like the tidyverse expects it. So let's try it:
Experiment 3:
quo(
starwars %>%
filter(as.name(ID) < 50)
)
## result: starwars %>% filter(as.name(ID) < 50)
We learn: This did not work, because, again, filter() takes everything "as is". So now it looks for column named as.name(ID), which does of course not exist.
--> We need to combine the two things to make it work:
- Use
as.name()to convert the string to a variable name. - Use
!!to tellfilter()it should not take things "as is", but substitute the real value.
Experiment 4:
quo(
starwars %>%
filter(!!as.name(ID) < 50)
)
## result: starwars %>% filter(birth_year < 50)
Now it works! :)
I have used filter() in my experiments, but it works exactly the same with mutate() and other tidyverse functions.
To make it easier, you can also use .data[[]] as suggested by @Lionel Henry in this comment. See also rlang 0.4.0 release notes
library(tidyverse)
ID <- "birth_year"
# Correct output
test <- starwars %>%
mutate(FootballLeague = case_when(
!!as.name(ID) < 10 ~ "U10",
!!as.name(ID) >= 10 & !!as.name(ID) < 50 ~ "U50",
!!as.name(ID) >= 50 & !!as.name(ID) < 100 ~ "U100",
!!as.name(ID) >= 100 ~ "Senior",
TRUE ~ "Others"
))
test
Using .data
test2 <- starwars %>%
mutate(FootballLeague = case_when(
.data[[ID]] < 10 ~ "U10",
.data[[ID]] >= 10 & .data[[ID]] < 50 ~ "U50",
.data[[ID]] >= 50 & .data[[ID]] < 100 ~ "U100",
.data[[ID]] >= 100 ~ "Senior",
TRUE ~ "Others"
))
test2
#> # A tibble: 87 x 15
#> name height mass hair_color skin_color eye_color
#> <chr> <int> <dbl> <chr> <chr> <chr>
#> 1 Luke Skywalker 172 77 blond fair blue
#> 2 C-3PO 167 75 <NA> gold yellow
#> 3 R2-D2 96 32 <NA> white, blue red
#> 4 Darth Vader 202 136 none white yellow
#> 5 Leia Organa 150 49 brown light brown
#> 6 Owen Lars 178 120 brown, grey light blue
#> 7 Beru Whitesun lars 165 75 brown light blue
#> 8 R5-D4 97 32 <NA> white, red red
#> 9 Biggs Darklighter 183 84 black light brown
#> 10 Obi-Wan Kenobi 182 77 auburn, white fair blue-gray
#> 11 Anakin Skywalker 188 84 blond fair blue
#> 12 Wilhuff Tarkin 180 NA auburn, grey fair blue
#> 13 Chewbacca 228 112 brown unknown blue
#> 14 Han Solo 180 80 brown fair brown
#> 15 Greedo 173 74 <NA> green black
#> birth_year sex gender homeworld species films vehicles starships
#> <dbl> <chr> <chr> <chr> <chr> <list> <list> <list>
#> 1 19 male masculine Tatooine Human <chr [5]> <chr [2]> <chr [2]>
#> 2 112 none masculine Tatooine Droid <chr [6]> <chr [0]> <chr [0]>
#> 3 33 none masculine Naboo Droid <chr [7]> <chr [0]> <chr [0]>
#> 4 41.9 male masculine Tatooine Human <chr [4]> <chr [0]> <chr [1]>
#> 5 19 female feminine Alderaan Human <chr [5]> <chr [1]> <chr [0]>
#> 6 52 male masculine Tatooine Human <chr [3]> <chr [0]> <chr [0]>
#> 7 47 female feminine Tatooine Human <chr [3]> <chr [0]> <chr [0]>
#> 8 NA none masculine Tatooine Droid <chr [1]> <chr [0]> <chr [0]>
#> 9 24 male masculine Tatooine Human <chr [1]> <chr [0]> <chr [1]>
#> 10 57 male masculine Stewjon Human <chr [6]> <chr [1]> <chr [5]>
#> 11 41.9 male masculine Tatooine Human <chr [3]> <chr [2]> <chr [3]>
#> 12 64 male masculine Eriadu Human <chr [2]> <chr [0]> <chr [0]>
#> 13 200 male masculine Kashyyyk Wookiee <chr [5]> <chr [1]> <chr [2]>
#> 14 29 male masculine Corellia Human <chr [4]> <chr [0]> <chr [2]>
#> 15 44 male masculine Rodia Rodian <chr [1]> <chr [0]> <chr [0]>
#> FootballLeague
#> <chr>
#> 1 U50
#> 2 Senior
#> 3 U50
#> 4 U50
#> 5 U50
#> 6 U100
#> 7 U50
#> 8 Others
#> 9 U50
#> 10 U100
#> 11 U50
#> 12 U100
#> 13 Senior
#> 14 U50
#> 15 U50
#> # ... with 72 more rows
Check if they are the same
identical(test, test2)
#> [1] TRUE
Created on 2020-11-26 by the reprex package (v0.3.0)