# 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:

1. Use as.name() to convert the string to a variable name.
2. Use !! to tell filter() 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)