Inside some functions as dplyr::mutate_at or purrr::map, it seems that one can use tilde operator ~ to build anonymous functions.
For example, one can do as in the linked question: map(iris, ~length(unique(.)))
Or also: mtcars %>% mutate_all(~.*2)
I tried to imitate this inside sapply, to avoid sapply(list, function(item) {something_with_item}). I was writing sapply(list, ~ something_with_.) but I get an error
Error in match.fun(FUN) :
'~ something_with_.' is not a function, character or symbol
A reproducible example:
> sapply(names(mtcars),function(item) mean(mtcars[[item]]))
mpg cyl disp hp drat wt qsec
20.090625 6.187500 230.721875 146.687500 3.596563 3.217250 17.848750
vs am gear carb
0.437500 0.406250 3.687500 2.812500
> sapply(names(mtcars),~mean(mtcars[[.]]))
Error in match.fun(FUN) :
'~mean(mtcars[[.]])' is not a function, character or symbol
Why? Understanding this syntaxis as a function is just a behaviour of some packages as dplyr and purrr? Is it understood by base R for some special cases?
Thank you!
As caldwellst says above, the tilda ~ is used to create a formula object, which is a bit of unevaluated code that can be used later. Formulae are not the same as functions, and the purrr functions work with formulae because they make a call to rlang::as_function (via purrr::as_mapper) in the background. The *apply functions obviously don't do this, though you can imitate this behavior by calling one of the above functions in your own modified *apply function (it's better to just use the map functions, the following is just to illustrate the point):
# Write function with `as_mapper()`.
fapply <- function(x, func) {
sapply(x, purrr::as_mapper(func))
}
# Now we can use formulae.
fapply(names(mtcars), ~ mean(mtcars[[.]]))
#### OUTPUT ####
mpg cyl disp hp drat wt qsec vs am gear carb
20.090625 6.187500 230.721875 146.687500 3.596563 3.217250 17.848750 0.437500 0.406250 3.687500 2.812500