can not understand Curly Curly in tidyeavl in r

Question

I used {{}} to bulid a self-function in r.

I don't understand why this happend.

test.data = data.frame(pm10 = 1:5)
test.data

new.col.name = 'lag_pm10'
col = 'pm10'

test.data %>% # error
  mutate({{new.col.name}} := lag({{col}}, 2))

test.data %>% # works
  mutate({{new.col.name}} := lag(get(col), 2))

In my view, new.col.name and col would be parsed when I put them into {{}}.

But the situation is that {{new.col.name}} works fine and {{col}} don't return what I expected.

Then I change {{col}} into get(col) and it works.

I don't know why.

In my opinion, the two objects' (new.col.name and col) value are all characters.

It should be the same when I use {{}} to new.col.name and col.

I guess maybe {{new.col.name}} become a variable name and {{col}} become a parameter in mutate().

If that's the case. Am I allowed to say that I can use {{}}for any variable name and get() for all arguments ?

Answer 1

You can write this as a function passing unquoted variables and not as a string using the curly-curly {{ operator like this:

my_function <- function(test.data, new.col.name, col) {
  test.data %>% # error
    mutate({{new.col.name}} := lag({{col}}, 2))
}

my_function(test.data, lag_pm10, pm10)

Output:

  pm10 lag_pm10
1    1       NA
2    2       NA
3    3        1
4    4        2
5    5        3