Defining optional arguments in R when more complex function

Question

I've looked through various postings on this, but haven't found them to be practical for more complex situations when not adding two numbers ("Correct" way to specifiy optional arguments in R functions).

If I wanted to define an optional argument (in my function to select a variable from the dataset), most responses point to doing something like below. However, when you have a longer function, it just seems impractical, and then you're essentially repeating things twice, which is the whole point of using a function in the first place.

select_fun_cars = function(df, var){
  if(missing(var)){
  df%>%
  select(hp, cyl)
  }
  df%>%
  select({{var}}, hp, cyl)
}

select_fun_cars(mtcars, mpg)

For instance, in my select statement, I am wanting to rename a variable and then select other variables. I would have to do this (as far as I know). It seems like there should be an easier way in this instance.

tab_output_fun_sd <- function(df, df_ov, df_74, df_75, Domains){
  if(missing(Domains)){
  return(df%>%
      select(
        !!paste("Intake", "(", "N=", max(df_ov$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := initial_overall, 
           !!paste("Intake", "(", "N=", max(df_ov$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := last_overall, 
           !!paste("Intake", "(", "N=", max(df_74$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := `initial_4274`, 
           !!paste("Intake", "(", "N=", max(df_74$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := `last_4274`,
           !!paste("Intake", "(", "N=", max(df_75$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := `initial_4275`,
           !!paste("Intake", "(", "N=", max(df_75$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := `last_4275`)%>%
        gt() %>%
  opt_stylize(style= 2, color = "blue")%>%
  as_raw_html())
  }
  
 return(df%>%
    select(Domains = label,
           !!paste("Intake", "(", "N=", max(df_ov$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := initial_overall, 
           !!paste("Intake", "(", "N=", max(df_ov$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := last_overall, 
           !!paste("Intake", "(", "N=", max(df_74$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := `initial_4274`, 
           !!paste("Intake", "(", "N=", max(df_74$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := `last_4274`,
           !!paste("Intake", "(", "N=", max(df_75$`new_timing_of_assessment = First.N.Valid`), ")","† Mean (Std. Deviation)") := `initial_4275`,
           !!paste("Intake", "(", "N=", max(df_75$`new_timing_of_assessment = Last.N.Valid`), ")","† Mean (Std. Deviation)") := `last_4275`)%>%
  gt() %>%
  opt_stylize(style= 2, color = "blue")%>%
  as_raw_html())
}

Answer 1

This will pass the var argument column to select and rename it using the string in the label variable.

library(dplyr)

select_fun_cars <- function(df, var, label = "X") {
  df %>% select("{label}" := {{var}}, hp, cyl) %>% head(2)
}

select_fun_cars(mtcars, mpg)
           X  hp cyl
## Mazda RX4     21 110   6
## Mazda RX4 Wag 21 110   6

select_fun_cars(mtcars)
##                hp cyl
## Mazda RX4     110   6
## Mazda RX4 Wag 110   6