Tidyselect in custom function

Question

I want to make a custom function that I can apply to a range of variables in a data frame using the var1:var20 tidyselect syntax, but I'm running into an error. A basic example below:

library(dplyr)
mtcars
test_func <- function(start_var,end_var) {
  mutate(across(start_var:end_var, ~ifelse(.x!=0,.x+1,.x)))
}
mtcars %>% test_func(mpg,hp)

The intended result (without using the function) is:

mtcars %>% 
  mutate(across(mpg:hp, ~ifelse(.x!=0,.x+1,.x)))

Using the function results in the following error message:

error in test_func(., mpg, hp) : unused argument (hp)

Answer 1

Here's a potential solution:

library(dplyr)

test_func <- function(data, start_var, end_var) {
  data %>% 
    mutate(across({{start_var}}:{{end_var}}, ~ifelse(.x!=0,.x+1000,.x)))
}

mtcars %>% 
  test_func(mpg, hp)

I changed your "else" arg to 1000 to highlight the change:

                     mpg  cyl disp   hp drat    wt  qsec vs am gear carb
Mazda RX4         1021.0 1006 1160 1110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     1021.0 1006 1160 1110 3.90 2.875 17.02  0  1    4    4
Datsun 710        1022.8 1004 1108 1093 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    1021.4 1006 1258 1110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 1018.7 1008 1360 1175 3.15 3.440 17.02  0  0    3    2

More info about why this works can be found here

Answer 2

You can make the following adjustments to your function to get it to work:

test_func <- function(data, start_var, end_var) { # add data source to the function
  data %>% mutate(across({{start_var}}:{{end_var}}, ~ifelse(.x!=0,.x+1,.x))) # add {{}}
}

test_func(mtcars, mpg, hp)