CODEWITHSUNDEEP
rtidyversetidyrtidyeval
Wooheon
Wooheon

Reputation: 339

tidy evaluation in tidyr

This is a question about the codes in vignette("in-packages")

The dataset is below.

(mini_iris <- iris %>% 
    as_tibble() %>% 
    .[c(1, 2, 51, 52, 101, 102), ])
#> # A tibble: 6 x 5
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species   
#>          <dbl>       <dbl>        <dbl>       <dbl> <fct>     
#> 1          5.1         3.5          1.4         0.2 setosa    
#> 2          4.9         3            1.4         0.2 setosa    
#> 3          7           3.2          4.7         1.4 versicolor
#> 4          6.4         3.2          4.5         1.5 versicolor
#> 5          6.3         3.3          6           2.5 virginica 
#> 6          5.8         2.7          5.1         1.9 virginica

If the column names are in a character vector (possibly from a function call), you can provide that to one_of()

nest_egg <- function(data, cols) {
  nest(data, egg = one_of(cols))
}

nest_egg(mini_iris, c("Petal.Length", "Petal.Width", "Sepal.Length", "Sepal.Width"))
#> # A tibble: 3 x 2
#>   Species               egg
#>   <fct>      <list<df[,4]>>
#> 1 setosa            [2 × 4]
#> 2 versicolor        [2 × 4]
#> 3 virginica         [2 × 4]

And then, vignette describes that

The use of one_of() here is important; if you don’t use it, and data contains a column named cols, nest() will nest it instead of the columns named in cols.

I think it can be solved in using tidy evaluation.

library(rlang)
nest_egg2 <- function(data, cols) {
  cols <- enexprs(cols)
  nest(data, egg = !!!cols)
}
nest_egg2(mini_iris, c("Petal.Length", "Petal.Width", "Sepal.Length", "Sepal.Width"))

but it shows error

Error: `!!!` can't be supplied with a name. Only the operand's names are retained.

In next section, Vignette describes that

To provide an interface that works like the tidyr function that you’re wrapping, you should pass the argument along using {{ arg }}. {{ }} is a special tidy eval operator that captures the expression supplied by the user and forwards it to another tidy eval-enabled function.

nest_egg <- function(df, cols) {
  nest(df, egg = {{ cols }})
}

nest_egg(mini_iris, -Species)

But I wonder what my nest_egg2 has problem in

Upvotes: 1

Views: 355

Answers (2)

Lionel Henry
Lionel Henry

Reputation: 6803

In selection contexts (not in action contexts like in mutate()), you can use !!! inside c() if you need to give a name:

nest_egg <- function(df, cols) {
  nest(df, egg = c(!!!cols))
}

Since tidyselect also supports vectors of names, you can also simply unquote it, both approaches are valid:

nest_egg <- function(df, cols) {
  nest(df, egg = !!cols)
}

The unquoting makes sure the env-variable cols is chosen over the data-variable cols, for the case that df has a column named `cols.

But really the proper way in selection contexts is to simply use one_of() (or all_of() in the next version).

Upvotes: 1

Nareman Darwish
Nareman Darwish

Reputation: 1261

So there are two things here;

  1. nest function takes a vector however enexprs returns a list
  2. You do not need (bang bang/eval) for nest. Mostly evaluation is needed is the function calls the name of a column without quotations "", however in your case because cols is already a vector of a string it will work fine without evaluation.

So this should work;

library(dplyr)
library(tidyr)

mini_iris <- 
  iris %>% 
  as_tibble() %>% 
  .[c(1, 2, 51, 52, 101, 102), ]

nest_egg2 <- function(data, cols) {
  nest(data, egg = cols)
}

nest_egg2(mini_iris, c("Petal.Length", "Petal.Width", "Sepal.Length", "Sepal.Width"))

# # A tibble: 3 x 2
# Species               egg
# <fct>      <list<df[,4]>>
# setosa            [2 x 4]
# versicolor        [2 x 4]
# virginica         [2 x 4]

Upvotes: 0

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