How can I pass NULL as an argument in R?

Question

I have a vector arguments, where some values are NA. I would like to pass these arguments sequentially to a function like this:

myFunction(argument = ifelse(!is.na(arguments[i]),
           arguments[i], NULL))

so that it will take the value in arguments[i] whenever it's not NA, and take the default NULL otherwise. But this generates an error.

If it makes any difference, the function in question is match_on(), from the optmatch package. The argument in question is caliper, because I would like to provide a caliper only when one is available (i.e. when the value in the vector of calipers is not NA). And the error message is this:

Error in ans[!test & ok] <- rep(no,  length.out = length(ans))[!test &  : 
replacement has length zero
In addition: Warning message:
In rep(no, length.out = length(ans)) :'x' is NULL so the result will be NULL 

Answer 1

You can use ?switch() instead of ifelse -

myFunction(argument = switch(is.na(arguments[i]) + 1, arguments[i], NULL))

Here's the help doc for switch -

switch(EXPR, ...)

Arguments

EXPR an expression evaluating to a number or a character string.

... the list of alternatives. If it is intended that EXPR has a character-string value these will be named, perhaps except for one alternative to be used as a ‘default’ value.

Details

switch works in two distinct ways depending whether the first argument evaluates to a character string or a number.

If the value of EXPR is not a character string it is coerced to integer. If the integer is between 1 and nargs()-1 then the corresponding element of ... is evaluated and the result returned: thus if the first argument is 3 then the fourth argument is evaluated and returned

Basically, when argument is NA then EXPR evaluates to 2 which returns NULL and when it is not NA then EXPR evaluates to 1 and returns arguments[i].