Passing missing argument from function to function in R

Question

I’ve learned that it’s common practice to use optional arguments in function and check them with missing() (e.g. as discussed in SO 22024082)

In this example round0 is the optional argument (I know, round0 could be defined as logical).

foo = function(a, round0) {
    a = a * pi
    if(!missing(round0)) round(a)
    else a
}

But what if I call this function from another function, how can I pass “missing”?

bar = function(b) {
    if(b > 10) round1=T
    foo(b, round1)
}

If b < 10 then round1 in bar() is not defined, but is passed to foo anyway. If I modify foo():

foo = function(a, round0) {
    a = a * pi
    print(missing(round0))
    print(round0)
    if(!missing(round0)) round(a)
    else a
}

and run bar(9) the output is:

bar(9)
[1] FALSE
Error in print(round0) : object 'round1' not found
Called from: print(round0)

That means: round0 is not missing, but can’t be accessed either?

I don’t want to use different function calls in bar(), if there are several optional arguments in foo(), I would have to write a function call for every missing/not missing - combination of all optional arguments.

Is it possible to pass "missing", or what other solution would apply for this problem?

Answer 1

In your example, round0 isn't missing, it's set to round1 which is undefined (as opposed to missing).

The best way in general of doing this is to use a default value, in your case FALSE:

foo = function(a, round0 = FALSE) {
    a = a * pi
    if (!round0) round(a)
    else a
}

bar = function(b) {
    round1 <- FALSE
    if (b > 10) round1=TRUE
    foo(b, round1)
}

or where the default value cannot easily be expressed in the parameter list:

foo = function(a, round0 = NULL) {
    a = a * pi
    if(!is.null(round0)) round(a)
    else a
}

bar = function(b) {
    round1 <- NULL
    if (b > 10) round1=TRUE
    foo(b, round1)
}

Note in both cases you need to set the parameter to be the default value manually in your calling function.

You could also call your foo function with or without an argument if needed within your if statement:

bar = function(b) {
    if (b > 10) foo(b, TRUE) else foo(b)
}

An alternative approach that shows how to generate a missing value is shown by @moody_mudskipper’s answer.