R function 中有条件缺少 arguments？

Question

I'm trying to work with a function in R that allows for only one of two arguments to be passed, through a series of missing(arg) calls. I cannot easily modify the function. My data is such that sometimes I supply one argument and other times the other. Is it possible to have some variant of:

myFunction(arg1 = switch(condition == T, arg1, NULL), 
           arg2 = switch(condition == F, arg2, NULL))

When I run the function like that, it counts the switch function as an argument and not a NULL object. Wrapping each switch in eval doesn't help either.

Thanks for the help!

Post script: I know this is bad practice, and I should have the call the function specified by the condition. But this is part of a magrittr -style pipeline that I'd rather not disturb.

Answer 1

You're probably much better off using @user2554330 's solution, or, as it seems from comments that you're fine with considering NULL and missing to be equivalent, use missing(arg1) || is.null(arg1) missing(arg1) || is.null(arg1) in your code.

Nevertheless If we really want to type myFunction once only and keep it as is we could do the following:

myFunction <- function(arg1, arg2) {
  if(missing(arg2)) 
    paste(arg1, "apples")
  else 
    paste(arg2, "pears")
}

arg1 <- 2
arg2 <- 3

# SOLUTION 1

# similar to what you've tried, but we build the expression first, then eval
# `substitute()` will become your missing argument here

condition <- TRUE
eval(bquote(myFunction(
  .(if(condition) arg1 else substitute()),
  .(if(!condition) arg2 else substitute())
)))
#> [1] "2 apples"

condition <- FALSE
eval(bquote(myFunction(
  .(if(condition) arg1 else substitute()),
  .(if(!condition) arg2 else substitute())
)))
#> [1] "3 pears"

# SOLUTION 2

# feed to the function the right list of arguments depending on condition

condition <- TRUE
do.call(myFunction, list(arg1 = arg1, arg2 = arg2)[xor(condition,c(FALSE, TRUE))])
#> [1] "2 apples"
condition <- FALSE
do.call(myFunction, list(arg1 = arg1, arg2 = arg2)[xor(condition,c(FALSE, TRUE))])
#> [1] "3 pears"

Answer 2

I think your description is incorrect. With the code you have, one of arg1 or arg2 will be NULL . I suspect the function is testing for missing, not testing for NULL . Neither argument is missing.

What you could do instead is run

if (condition) myFunction(arg1 = arg1) else myFunction(arg2 = arg2)

Regarding keeping it in a pipeline: you can certainly wrap this line in a function that's compatible with magrittr .