Can someone explain to me how to call the new pipe placeholder _ inside a function, inside mutate? Here is a silly example:

library(dplyr)
iris |>
  mutate(n_rows = nrow(x = _))

Error in mutate(iris, n_rows = nrow(x = "_")) : 
  invalid use of pipe placeholder

CodePudding user response：

The native pipe placeholder _ cannot be used in nested calls, that is why you are getting this error.

You could something like this to make it work:

iris |>
  nrow() |>
  transform(iris, x = _)

CodePudding user response：

You can't for 2 reasons:

1. The base pipe can only pipe to one argument. In your example, you need the data frame to appear twice: mutate(iris, n_rows = nrow(iris)). This is possible with the magrittr pipe %>%, but not the base pipe |>.

2. As Maël says, you can't use _ in a nested function call.

From the R Release News announcing _:

In a forward pipe |> expression it is now possible to use a named argument with the placeholder _ in the rhs call to specify where the lhs is to be inserted. The placeholder can only appear once on the rhs.

CodePudding user response：

You may use an anonymous function instead; gives the intended transformed data frame.

iris |>
  {\(.) transform(., n_rows=nrow(x=.))}()

or

iris |>
  {\(.) dplyr::mutate(., n_rows=nrow(x=.))}()

#     Sepal.Length Sepal.Width Petal.Length Petal.Width    Species n_rows
# 1            5.1         3.5          1.4         0.2     setosa    150
# 2            4.9         3.0          1.4         0.2     setosa    150
# 3            4.7         3.2          1.3         0.2     setosa    150
# ...